Plumularia polymorpha Billard, 1913 (pro parte): 24, figs 14A, 15. – Van Soest, 1976: 89.

non Plumularia polymorpha Billard, 1913 (pro parte): 24, fig. 14B, C.

Thecocarpus polymorphus – Bedot, 1921: 9. – Von Schenck, 1965: 928.

Heterotheca polymorpha – Stechow, 1923: 15.

Halopteris polymorpha – (?) Pennycuik, 1959: 178. – Schuchert, 1997 (pro parte): 64, fig. 20A, C-F. – Ryland & Gibbons, 1991: 530, fig. 4. – Di Camillo et al., 2008: 1592.

non Halopteris polymorpha – Vervoort, 1966: 132, fig. 35. – Millard & Bouillon (pro parte), 1973: 83, fig. 10F-H, J; 1974: 9. – Millard, 1975: 354, fig. 112G-L; 1977: 107; 1978: 193; 1980: 132. – Hirohito, 1983: 62, fig. 31. – Bouillon et al., 1995: 49. – Schuchert, 1997 (pro parte): 64, figs 20B, 21–23. – Watson, 2000: 46, fig. 35. – Ansín Agís et al., 2001: 167, fig. 70. – Preker, 2001: 154. – Kirkendale & Calder, 2003: 169. – Preker, 2005: 49. – Preker & Lawn, 2005: 342. – Ansín Agís et al., 2009: 53. – Preker & Lawn, 2010: 120; 2012: 45, fig. 7.

non Antennella polymorpha – Vervoort, 1941: 218.

non Antennella secundaria – Vervoort, 1967: 42, fig. 12 [not Antennella secundaria (Gmelin, 1791)].

non Plumularia nuttingi Billard, 1911: 66, fig. 8 – Van Soest, 1976: 89.

non Plumularia buskii – Thornely, 1904: 120. – Ritchie, 1910: 832. – Thornely, 1916: 150. – Nutting, 1927: 221.

non Plumularia buski – Hartlaub, 1901: 374, pl. 22 figs 22, 32, 36. – Billard, 1913: 21, fig. 11, pl. 1 fig. 15. – Redier, 1966: 90, pl. 2 figs 1 & 3, pl. 3 fig. 1 (incorrect subsequent spelling). non Heterotheca buski – Hirohito, 1974: 30, fig. 14 (incorrect subsequent spelling).

non Halopteris buskii – Vervoort & Vasseur, 1977: 72, fig. 31. – Rees & Vervoort, 1987 (pro parte): 119, fig. 25A-B. – Ryland & Gibbons, 1991: 527, fig. 2. – Bouillon et al., 1995: 49. – Migotto, 1996: 48, fig. 9F-H. – Preker, 2001: 154; 2005: 48.

non Halopteris buski – Rees & Thursfield, 1965: 160. – Hirohito, 1983: 61; 1995: 244, fig. 82 (incorrect subsequent spelling).

Material examined: MHNG-INVE-97937; Indonesia, Bali, Padangbai, Jepun shipwreck, -8.52819° 115.51478°, 20 m, coll. H. R. Galea; 06.10.2016; several sterile plumes, up to 3.7 cm high; 16S sequence MF784530. – MNHG-INVE-97951; Indonesia, Bunaken National Marine Park, Manado Tua I., Negeri, 1.61684° 124.70140°, 10 m, coll. G. Allard; 22.11.2010; several plumes, up to 3.7 cm high, some bearing female gonothecae. – HRG-0421; Indonesia, Bunaken National Marine Park, off Bunaken I., Leukan 2, 1.59989° 124.76697°, 10 m, coll. G. Allard; 21.11.2010; 6 stems up to 3 cm high, one of which bears female gonothecae, and three others carrying male gonothecae. – CDC002; Indonesia, Bali, Tulamben, Liberty shipwreck, -8.27417° 115.59265°, 18 m, coll. C. G. Di Camillo; 21.10.2008; two sterile cormoids, 14 and 21 mm high. – CDC003; Indonesia, Ambon, Laha I., -3.69221° 128.12310°, 10–15 m, coll. C. G. Di Camillo; 14.10.2008; a 17 mm high cormoid and a 12 mm high fragment, both infertile. – CDC004; Indonesia, Bunaken National Marine Park, Raymond's Point, 1.62713° 124.73363°, 40–50 m, coll. C. G. Di Camillo; 01.09.2003; two sterile fragments, 7 and 16 mm high, likely from different cormoids. – CDC005; Indonesia, Bunaken National Marine Park, Mandolin's Point, 1.61095° 124.73257°, 20 m, coll. C. G. Di Camillo; 12.02.2005; a 27 mm high cormoid bearing a gonotheca, probably male.

Additional material: MNHN H.L.1309; Indonesia, Rote Island, Buka Bay, -10.87333 123.01833, 34 m, Siboga Stn. 299; a 1.1 cm high sterile cormoid belonging to the syntype of Plumularia polymorpha Billard, 1913.

Diagnosis: Halopteris with cormoids reaching heights of up to 3.7 cm, with monosiphonic, unbranched stems, divided homomerously into rather long internodes bearing a hydrotheca, a lateral apophysis, and up to 9 nematothecae [1 mesial, a pair of laterals, an axillar one, and generally 2–3 (though up to 5 possibly present) above hydrotheca]. Cladia alternately arranged along stem, heteromerously divided into internodes; hydrothecate internodes slightly shorter than their ahydrothecate counterparts, carrying a hydrotheca and its up to 4 associated nematothecae (1 mesial, a pair of laterals, and occasionally an axillar one); ahydrothecate internodes with 1–2 nematothecae. Hydrothecae conical and shallow; lateral nematothecae with either lowered, emarginated or sinuated margin adaxially, not surpassing hydrothecal rim, borne on inconspicuous apophyses. Female gonotheca broadly ovoid, with apical, large, rounded aperture perpendicular to long axis of the theca, and closed by glass-watch-shaped operculum; 2–3 basal nematothecae. Male gonotheca smaller than female, ovoid, without noticeable aperture, with 2 basal nematothecae. Cormoids yellow throughout in life.

Description: Colonies composed of a varied number of cormoids arising from creeping, branching stolon, not carrying nematothecae. Cormoids erect (though flaccid when out of liquid), up to 3.7 cm high. Cauli simple, monosiphonic (Figs 1A, 2A), composed of an up to 1 cm long, ahydrothecate, proximal part above origin from stolon, and a much longer, distal part carrying both hydrothecae and hydrocladia. Basal part usually divided by up to 4 transverse nodes into segments of varied length, bearing a number of nematothecae (up to 34 observed) arranged into two parallel, closely-set rows; distalmost segment delimited from the remainder of caulus through a deeply-cut, oblique node. Stem above basal part longer, homomerously segmented into up to 37 internodes trough oblique constrictions of the perisarc (Fig. 3A); internodes long, bearing one hydrotheca in their lower third, a number of nematothecae, and an apophysis lateral to the hydrotheca supporting a cladium (two apophyses are usually present in the proximal most internode, and these support a pair of cladia) (Fig. 3B); nematothecae: one mesial, a pair of laterals, one axillar, as well as usually 2–3 (though occasionally 1–5 possibly present) above the hydrotheca, arranged in two closely-set rows. Cladia, except for the proximal most that can be paired, alternately-arranged along caulus; up to 5 mm long, usually less so; each composed of a short, proximal, athecate, quadrangular segment, followed by a succession of ahydrothecate and hydrothecate internodes, delimited through a heteromerous segmentation; ahydrothecate internodes with proximal node transverse and distal node oblique; the reverse in hydrothecate internodes. First ahydrothecate internode quite long, and longer than its subsequent counterparts, bearing constantly two superior nematothecae in a single row; remaining ahydrothecate internodes provided with commonly 1, or rarely 2, nematothecae. Hydrothecate internodes, up to 7 (usually 4–6) per cladium, relatively short, with a hydrotheca confined to most of their length (Fig. 3B), and up to four nematothecae: one mesial, a pair of laterals and, occasionally, an axillar one (Fig. 3E). Hydrothecae cup-shaped and shallow, walls slightly divergent, rim circular, entire (Fig. 3D, F). All nematothecae of the colony bithalamic and movable; mesial ones short, triangular in frontal view, rim of upper chamber with deep, adaxial emargination (Fig. 3G^{3, 4}); laterals short, not surpassing the hydrothecal rim (Fig. 3D, F), and mounted on very short apophyses, conical in shape, aperture wide, margin of upper chamber of variable shape: variously lowered to emarginated to sigmoid adaxially (Fig. 3G^5–9); cauline (Fig. 3G¹) and cladial (Fig. 3G²) nematothecae long, with tall basal and shallow upper chambers, rim scooped adaxially; axillar nematothecae conical to broadly ovoid, rather inconspicuous due to their comparatively smaller size and thinner perisarc (Fig. 3G^{10, 11}). Hydranths with 15–16 filiform tentacles; in life, whole colony of a distinctive yellow tinge (Fig. 1A). Colonies dioecious. Gonothecae borne on both stems and cladia, inserted singly beside the base of a hydrotheca through a short, lateral apophysis, and mounted on single quadrangular pedicel; female large, broadly ovoid, tapering below, and there provided with 2–3 basal nematothecae; aperture distal, perpendicular to long axis of the theca, large and circular, with conspicuously thickened rim, closed by a watch-glass-shaped operculum (Fig. 3H, I); male comparatively smaller than female, ovoid, tapering below, without distinct aperture, provided basally with a couple of nematothecae (Fig. 3J). Cnidome (Fig. 3K) composed of 3 types of microbasic mastigophores: large, elongated-ovoid [(21.9–22.6) × (8.2–8.5) μm, in nematophores, as well as scattered in the coenosarc]; small, banana-shaped [(6.9–7.5) × ca. 2.4 μm, in tentacles]; small, ovoid capsules [(4.5–5.1) × (2.8–2.9) μm, scattered in the coenosarc].

Dimensions: See Table 1.

Remarks: The ordinary cauline internodes bear usually 2–3 nematothecae above their corresponding hydrothecae (60.8% and 34.8%, respectively, n=115), though exceptionally as few as 1, or as much as 4 or even 5, may occur (1%, 1.7% and 1.7%, respectively, n=115). The basalmost cauline internodes, supporting pairs of cladia, bear an increased number of nematothecae, usually 4–5. The ordinary cladial ahydrothecate internodes bear generally 1 and, less frequently, 2 nematothecae (92% and 8%, respectively, n=110).

Behind each cauline hydrotheca, there is large foramen for the passage of a nematophore, itself protected by a bithalamic nematotheca; the latter is occasionally lost, but the constant presence of the foramen in all internodes indicates the pre-existence of an axillar nematotheca. Conversely, the cladial hydrothecae bear only occasionally single axillar nematothecae, but their basal foramina are inconspicuous.

Terminal stolonization is quite common in the available samples, but no branched cladia have been observed.

The bulk of the syntype material of H. polymorpha (Billard, 1913) is housed in the collection of NBC but, due to important ongoing renovation works, could not be re-examined for the purpose of the present study (Koos van Egmond, pers. comm.).

However, the lectotype (from Siboga Stn. 80) – designated and well-illustrated by Schuchert (1997) – is distinctive through its long cauline and cladial ahydrothecate internodes, and its rather shallow hydrothecae, provided with short, conical lateral nematothecae borne on inconspicuous apophyses. The present material fully agrees with these, allowing an updated and more comprehensive account on H. polymorpha to be done.

A microslide (H.L.1309), stored in MNHN and containing a sterile cormoid from Siboga Stn. 299, was re-examined. The caulus is homomerously segmented, with the exception of its distalmost part, where transverse nodes intervene; each internode is moderately long and comprises a hydrotheca in its lower half, a lateral apophysis and up to 7 nematothecae (1 mesial, a pair of laterals, a pair of axillar, as well as 1–2 superior ones, a certain distance one above the other, and slightly displaced laterally to one another) (Fig. 3L). The hydrocladia are heteromerously segmented; the 1st ahydrothecate internode is longer than its subsequent counterparts, and bears generally 2 nematothecae (although 3 were noted in one instance); the ordinary ahydrothecate internodes are of a rather varied length and carry 1–2 nematothecae (Fig. 3M); the hydrothecate internodes bear a hydrotheca and its 4 associated nematothecae (1 mesial, a pair of laterals, and an axillar one) (Fig. 3N). All nematothecae, including the axillar ones, are bithalamic. The hydrothecae are deep and almost cylindrical, and their lateral nematothecae are mounted on well-developed apophyses, and scarcely reach the hydrothecal rim (Fig. 3O). Its gonothecae remain to be discovered.

Accordingly, it results that the material from Siboga Stn. 299 is morphologically different from the lectotype of H. polymorpha (compare Fig. 3A-F and 3L-O; see also Table 1), a finding that contrasts with earlier views expressed by both Billard (1913) and Schuchert (1997). The former material obviously belongs to a different, possibly an as yet unnamed species, whose comprehensive description requires additional, fertile material.

The 3rd morphotype belonging to the syntype of H. polymorpha, from Siboga Stn. 77, is presently the less documented (the available data are summarized in Table 4 herein). According to Billard (1913), its cladia, besides the proximal most, quadrangular segment, begin with an ahydrothecate internode provided with only one nematotheca, followed by a succession of “mostly” homomerously-segmented internodes bearing “most often” a single nematotheca. The rather deep hydrothecae, provided with lateral nematothecae borne on well-developed apophyses, combined with the structure of the hydrocladia, suggest – with little doubt – that the material from Siboga Stn. 77 is specifically different from both the lectotype of H. polymorpha and the material in hand described above, thus contradicting – again – the opinions expressed by both Billard (1913) and Schuchert (1997).

Halopteris nuttingi (Billard, 1911) was synonymized with H. polymorpha by Schuchert (1997), an opinion not shared here. However, we agree with him that its inclusion in the synonymy of H. buskii (Bale, 1884), proposed by Billard (1913), is not justified as they have morphologically different gonothecae. According to Billard (1913), H. nuttingi [as Plumularia buski (sic!)] and H. polymorpha could be confidently distinguished through the shape of the upper chamber of their lateral nematothecae: globular with distinctly emarginated ad- and abaxial walls in the former, and conical with slight adaxial emargination in the latter (it was stated above that the complete panoply of shapes displayed by the latter also include a sinuated rim or an adaxial emargination). In Schuchert's (1997) view, this character is unreliable, arguing that in other halopteridids, e.g. Antennella quadriaurita Ritchie, 1909, “it is notoriously variable”. However, since then, it has been suggested that the former concept of A. quadriaurita likely includes a complex of species (Galea, 2013: 29), and it has been demonstrated, for instance, that at least one “morphotype” represents a distinct, well-characterized species (Galea & Ferry, 2015: 237). Moreover, besides the distinctive shape of the lateral nematothecae, the number and position of their counterparts confined to the cauline internodes distal to hydrotheca is different in H. nuttingi (see Billard, 1913). Last but not least, according to both Billard (1911, fig. 8; 1913, fig. 11) and Schuchert (1997, fig. 21C), this nominal species has shorter cladial ahydrothecate internodes and deeper hydrothecae compared to H. polymorpha.

Additionally, it should be stressed that H. nuttingi was created based on a syntype, as it results from Billard's [1913, as Plumularia buski (sic!) Bale, 1884] work, but not from his original account (Billard, 1911). Moreover, according to the former publication, it is very likely that the syntype contains a mix of species, as Billard mentions (p. 22) ahydrothecate cladial internodes either short or long, and provided with one or two nematothecae. However, a neotype (sic!) for H. nuttingi has been designated (Coel. 5241) by Schuchert (1997: 64), who also provided reliable illustrations of it (N.B.: This material should be best referred to as the lectotype). Besides Billard's (1911, 1913) record of H. nuttingi, at least two others seem to occur in the literature, viz. Redier [1966: 90, pl. 2 figs 1 & 3, pl. 3 fig. 1; as P. buski (sic!)] and Watson (2000: 46, fig. 35C, E; pro parte as H. polymorpha).

Literature records of H. polymorpha are a matter of debate, due to several main factors: 1) the lack of formal descriptions, or descriptions too succinct, sometimes not accompanied by illustrations, a situation mainly occurring in older literature; 2) only sterile material was available, thus generating confusion with H. buskii (Bale, 1884); 3) the artificial inclusion in the synonymy of Billard's (1913) species of a variety of hydroids displaying a large panoply of morphological features, the specific name “polymorpha” being obviously misleading. Schuchert (1997) and Ansín Agis et al. (2001, 2009) provided extensive lists of synonyms for this taxon (a compilation is given in the synonymy above), though only a few prove reliable in light of the present observations.

For instance, the material studied by Di Camillo et al. (2008) and re-examined herein, belongs to the present species. In addition, the Fijian record by Ryland & Gibbons (1991: 530) is also in agreement with it, since it displays the distinctively long cladial ahydrothecate internodes, relatively shallow hydrothecae, lateral nematothecae (with flared upper chamber) borne on inconspicuous apophyses, as well as the occasional presence of axillar nematothecae behind the cladial hydrothecae (it is assumed that their cauline counterparts were overlooked by the authors). Although neither formally described, nor illustrated, the Queensland record by Pennycuik (1959) is reportedly said similar with Billard's fig. 14A, presently known as representing the lectotype of H. polymorpha. Besides these few records, many others clearly deviate morphologically from the lectotype. Among them, there are morphotypes characteristically forming either tall (> 4 cm high) or small-sized (< 2 cm high) cormoids. Specimens with tall stems were described, for instance, in materials from South Africa (Millard, 1975), Zanzibar (Rees & Vervoort, 1987; as H. buskii), and the Seychelles (Millard & Bouillon, 1973). Millard's (1975) material is, obviously, a mix of species: one with very deep, almost tubular hydrothecae (her fig. 112K), while the other (her fig. 112L) corresponds morphologically to the redescription of the lectotype of H. buskii provided by Schuchert (1997). Conversely, the specimens from Zanzibar and the Seychelles (part of the latter reexamined herein) belong to an as yet undescribed species, Halopteris millardae (see below).

On the other hand, among the materials with small-sized cormoids, several morphological groups could be distinguished. First, there are specimens whose hydrothecae distinctly display sinuated margins (Vervoort & Vasseur, 1977), and these belong to the new species, Halopteris australis, described below. Second, there are materials whose hydrothecae possess an even rim, but further divide into a subgroup with homomerously-segmented cladia (e.g. Vervoort, 1966; Hirohito, 1983) and another one displaying a heteromerous division into internodes [e.g. Vervoort (1967), as Antennella secundaria; Hirohito (1974), as Heterotheca buski (sic!); Ryland & Gibbons (1991), as H. buskii; Hirohito (1995), as H. buski (sic!); Preker & Lawn (2010, 2012)]. Some specimens, among the materials with heteromerous cladia, are thought to belong to H. vervoorti Galea, 2008 (see below under this species), while the taxonomic status of the remaining ones is uncertain in light of the available data (see Tables 2 and 4). The reexamination of extant specimens, the availability of newly-collected materials, as well as modern, molecular approaches are expected to gradually solve the intricacies of this species group.

A sample provisionally identified as H. polymorpha, originating from the Mediterranean, was first used in a molecular phylogeny by Leclère et al. (2007). The voucher specimen used in that work (MHNG-INVE-30117, data in Appendix 2) was re-examined for the purpose this study. The single cormoid is sterile and thus not reliably identifiable. It resembles H. vervoorti, notably in having pairs of axillar nemathothecae associated to the cauline hydrothecae. More and especially fertile material is needed for a correct identification, as it likely belongs to an as yet undescribed species, according to the 16S data (Fig. 9; DQ855922).

Halopteris polymorpha, as presently understood, can be separated from its congeners [see list in Schuchert (2015)] through a series of morphological features. The following hydroids can be excluded a priori from the comparison, on the account of a series of diagnostic traits which separate them easily from the species discussed here:

As to the remaining species, their differences to H. polymorpha are summarized in Appendix 1.

Among them, according to the phylogenetic tree shown in Fig. 9, H. polymorpha comes close to H. platygonotheca Schuchert, 1997. Besides notable differences in their respective female gonothecae (pear-shaped in the former, and conspicuously laterally-flattened in the latter), their trophosomes display several common characters: 1) their stem internodes are long and provided with several nematothecae distal to the hydrothecae [commonly 2–3 (but up to 5 possible) in H. polymorpha, and from 1 (sample MHNG-INVE-97943) to 1–3 (sample HRG-1288) in H. platygonotheca]; 2) the occurrence (regular in the former and occasional in the latter) of an axillar nematotheca behind the cauline hydrothecae; 3) their cladial ahydrothecate internodes are long; 4) the apophyses supporting their lateral nematothecae are inconspicuous, and the thecae themselves do not reach the hydrothecal rim.

Taken together, the previous supposed morphological variability of, and the implicit difficulty in establishing a specific limitation in H. polymorpha, are now solved through the discovery of additional records in perfect agreement with the lectotype designated by Schuchert (1997). It is concluded that Billard's (1913) species is well-characterized and morphologically homogenous, as illustrated with the present material belonging to various geographically-distant Indonesian populations.

Distribution: Scattered records from Indonesia, viz. off Kalimantan (Billard, 1913, Siboga Stn. 80), Bali and Ambon (present study), Bunaken National Park (Di Camillo et al., 2008; present study). Also occurring in Fiji (Ryland & Gibbons, 1991). A doubtful record from Queensland, Australia (Pennycuik, 1959).

Plumularia polymorpha var. sibogae Billard, 1913: 25, fig. 16. – Van Soest, 1976: 89.

Thecocaulus polymorphus var. sibogae – Bedot, 1921: 9. – Stechow, 1925: 497.

Halopteris polymorpha var. sibogae – Millard & Bouillon, 1973: 84, fig. 10K. – (?) Hirohito, 1983: 62; 1995: 244.

Halopteris polymorpha – Schuchert, 1997 (pro parte): 66, 69, fig. 22E [non Halopteris polymorpha (Billard, 1913)].

Material examined: MHNG-INVE-97926; Indonesia, Bali, Tulamben, Liberty shipwreck, -8.27417° 115.59265°, 22 m, coll. H. R. Galea; 29.09.2016; numerous plumes up to 23 mm high, of which many bear male gonothecae; 16S sequence MF784526. – MHNG-INVE-97938; Indonesia, Bali, Padangbai, Jepun shipwreck, -8.52812° 115.51478°, 20 m, coll. H. R. Galea; 06.10.2016; numerous plumes up to 25 mm high, of which 2 bear female gonothecae; 16S sequence MF784531. – HRG-0991; Indonesia, Tukang Besi Archipelago, reef north off Hoga I., -5.44633° 123.76417°, 20 m, coll. G. Allard; 27.09.2011; several infertile plumes up to 20 mm high.

Diagnosis: Halopteris with cormoids reaching heights of up to 2.5 cm, with monosiphonic, unbranched stems, divided homomerously into short internodes bearing a hydrotheca, a lateral apophysis, and up to 8 nematothecae (1 mesial, a pair of laterals, 1–2 axillary ones, and commonly 2–3 above hydrotheca). Cladia alternately arranged along stem, heteromerously divided into internodes; hydrothecate internodes longer than their ahydrothecate counterparts, carrying a hydrotheca and its up to 5 associated nematothecae (1 mesial, a pair of laterals, and 1–2 axillary ones); ahydrothecate internodes short, provided with 1–2 nematothecae. Hydrothecae deep, almost tubular. Lateral nematothecae borne on well-developed apophyses; basal chamber tall, whole nematotheca greatly surpassing hydrothecal margin; rim of upper chamber with adaxial emargination. Female gonotheca broadly ovoid, with large, rounded, apical aperture perpendicular to long axis of the theca, and closed by glass-watch-shaped operculum; 3 basal nematothecae. Male gonotheca smaller than female, ovoid, without noticeable aperture, with 2 basal nematothecae. In life, cauline polyps yellow, contrasting with their purely white cladial counterparts.

Description: Colonies composed of numerous cormoids arising from tubular, creeping, branching hydrorhiza lacking nematothecae. Cormoids erect (though flaccid when out of liquid), up to 2.5 cm high. Cauli simple, monosiphonic (Figs 1B-C, 2B), from straight in their lower halves to distinctly geniculate in their upper halves; basal parts of varied length (1–11 mm long), rarely entire, generally subdivided into segments by a number of transverse nodes (up to 4 observed); segments ahydrothecate, but carrying a total of 0–43 nematothecae arranged in two distinct, longitudinal, closely-set rows; distal end of last segment marked by deeply-cut, oblique node. Remainder of stem longer, with strict homomerous segmentation; each cauline internode (Fig. 4A) moderately-long, bearing a hydrotheca in its lower half, a number of nematothecae, as well as at least a short, lateral apophysis supporting a cladium; up to 33 successive internodes observed. The proximal most internode bears always two opposite apophyses supporting a pair of cladia, but there may be up to 3 consecutive segments displaying this feature; all are demarcated by deeply-incised, oblique nodes, both proximally and distally. Above, the segments are separated by less-marked, oblique constrictions of the perisarc, and each bears but a single apophysis originating laterally next to the hydrotheca; apophyses alternate along the stem. There are 6–9 nematothecae per internode, of which 4–5 are associated to the hydrotheca: 1 mesial, a pair of laterals, 1–2 axillar, as well as 2–4 superior ones arranged in two parallel rows (Fig. 4A); all cauline nematothecae, exclusive of those associated to the hydrothecae, tightly and backwardly appressed against stem (turned posteriad), at least in preserved material. Cladia up to 3 mm long; composed of a very short, proximal, quadrangular, athecate segment, followed by a succession of alternating ahydrothecate and hydrothecate internodes (Fig. 4A, C); ahydrothecate internodes with proximal transverse node and oblique distal node; the reverse in hydrothecate internodes. First ahydrothecate internode comparatively longer than following counterparts, and always carrying 2 nematothecae in a row; remaining internodes with either 1 or 2 nematothecae in a row. Hydrothecate internodes, up to 8 per cladium, relatively short, accommodating a hydrotheca in their lower 3/4th and its 4–5 associated nematothecae: 1 mesial, a pair of laterals, and 1–2 axillar (Fig. 4B, D). Hydrothecae cup-shaped and relatively deep, adnate for half their length to corresponding internode, free adaxial and abaxial walls parallel, flaring slightly to margin; the latter circular in apical view, and slightly sigmoid laterally, with imperceptible scoop between side facing the lateral nematothecae and abaxial wall (Fig. 4C, E). All nematothecae, including the axillar ones, bithalamic and movable. Mesial ones triangular in frontal view, upper chamber with conspicuously lowered margin on adaxial side (Fig. 4F^{5, 6}). Lateral nematothecae mounted on well-developed apophyses (Fig. 4F⁷); long, greatly surpassing the hydrothecal rim (Fig. 4C, E), lower chamber high, upper one relatively shallow, and with adaxial emargination. Axillar nematothecae cornucopia-shaped, adaxial wall of upper chamber with significant emargination (Fig. 4F^{8, 9}). Remaining nematothecae from both caulus (not shown) and cladia (Fig. 4F^1–4) similar to the lateral ones, though comparatively shorter. Hydranths with 13–15 filiform tentacles; in life, all belonging to the stem characteristically yellow, contrasting with those from the cladia that are uniformly white (Fig. 1C). Colonies dioecious. Gonothecae arising laterally from below the stem hydrothecae through a short, lateral apophysis; mounted on short, quadrangular pedicel, broadly ovoid, though sexually dimorphic, with the males comparatively smaller than females; the latter provided basally with 3 nematothecae similar to the laterals of the hydrotheca, and a large, watch-glass-shaped apical lid closing a rounded aperture with conspicuously thickened perisarc at margin (Fig. 4G); male gonothecae bearing two basal nematothecae and no noticeable aperture; a globular mass of sperm cells encircles a central, digitiform blastostyle (Fig. 4H); all female gonothecae observed ripe and empty, with the exception of basal remains of the blastostyle. Cnidome (Fig. 4I) composed of 3 types of microbasic mastigophores: large, elongated-ovoid [(18.1–19.8) × (6.6–7.3) μm, in nematophores, as well as scattered in the coenosarc]; small, banana-shaped [(6.1–6.6) × (2.1–2.3) μm, in tentacles]; small, ovoid capsules [(5.1–5.4) × (2.9–3.1) μm, scattered in the coenosarc].

Dimensions: See Table 1.

Remarks: In most stems, the proximal most hydrothecate internode gives rise to a pair of cladia; occasionally, two successive internodes are involved; in one instance, three proximal pairs occurred, followed by an internode provided with a single apophysis, and by another one giving rise to a pair of cladia. In such internodes, there are always 3 or even 4 nematothecae above the hydrotheca. The remaining stem internodes are randomly provided with either 2 or 3 nematothecae in two parallel rows. Both cauline and cladial hydrothecae bear either one (70% and 80% of cases, respectively, n=50 examined hydrothecae) or two (30% and 20% of cases, respectively) axillar nematothecae. Terminal stolonization occurs occasionally from the distal ends of both stems and cladia. Rarely, aberrant branching of cladia occurs: a short apophysis is given off laterally from either one or both apophyses supporting the lateral nematothecae; the structure of these 2nd order branchlets (not exceeding two hydrothecate internodes) is similar to that of a normal cladia, and begins with a short, quadrangular, athecate segment, followed by the first ahydrothecate intersegment carrying but a single nematotheca, instead of two as in the 1st order cladia. Sterile cormoids, similar to the present ones, with hydrothecae provided with exceedingly long lateral nematothecae were already reported notably from Indonesia by Billard (1913) and the Seychelles by Millard & Bouillon (1973) (both as Halopteris polymorpha var. sibogae). According to the former author, this character mainly distinguishes his so-called variety from the nominal species.

Similarly to the present observations, Millard & Bouillon indicate that “Intermediate athecate internodes [are] absent on stem but invariably present on hydrocladia”, although Billard observed that, on cladia, the ahydrothecate internodes may be occasionally fused to their preceding hydrothecate counterparts. Among the quite rich material studied here, only one case of fusion (Fig. 4B, after the proximal most hydrotheca) was noted upon the careful examination of 10 cormoids from sample MHNG-INVE-97938.

As noted above, the most prominent morphological difference between H. polymorpha and its so-called variety sibogae is to be found in the shape and size of their lateral nematothecae, including their corresponding apophyses (compare Fig. 3D-F and 4B-E). Another noteworthy difference relies in the comparatively shallower hydrothecae (compare Fig. 3D & E and 4C & E), and shorter cauline (compare Fig. 4A and 3A-B) and cladial ahydrothecate (compare Fig. 4C and 3B) internodes in the former. Moreover, only noticeable in living material, the bicolor aspect of the cormoids of the latter is striking (Fig. 1B, C), compared to the wholly yellow tinge observed in the former (Fig. 1A), especially when both species occur in sympatry. Other morphological and morphometrical differences observed in the material studied herein are listed in Table 1. In light of these differences, we advocate here that the so-called variety sibogae must be recognized as a full species, different from H. polymorpha. The 16S data clearly corroborated this (Fig. 9).

Raising Plumularia polymorpha var. sibogae Billard, 1913 to full species is not threatened by Plumularia sibogae Billard, 1911, because the former is to be correctly placed in the genus Halopteris Allman, 1877 [as H. sibogae (Billard, 1913)], while the latter is accommodated in Antennella Allman, 1877.

Halopteris sibogae differs from its congeners through a series of characters. A number of species with “peculiarities” (see under H. polymorpha) should be excluded from the comparison, while the remaining ones are compared in Appendix 1.

Among them, and in accordance with the phylogenetic tree, H. sibogae comes close to H. vervoorti Galea, 2008 (see below an account on the latter). Besides its exceedingly long, and thus very distinctive, lateral nematothecae, H. sibogae shares the following features with H. vervoorti: 1) their cauline internodes are rather short and bear 1–2 nematothecae distal to each hydrotheca; 2) there are 1–2 axillar nematothecae associated to both cauline and cladial hydrothecae; 3) their cladia are heteromerously-segmented, their ahydrothecate internodes are short compared to their hydrothecate counterparts, and bear single nematothecae; 4) their female gonothecae are indistinguishable morphologically (compare Fig. 4G and 5H, I, P).

Distribution: Indonesia [between Misool I. and West Papua (Billard, 1913), Bali (present study), Tukang Besi Archipelago (present study)], Seychelles (Millard & Bouillon, 1973), and questionably Japan (Hirohito, 1983).

Halopteris vervoorti Galea, 2008: 42, fig. 9; 2010: 3, 4; 2013: 50.

Halopteris polymorpha – Ansín Agís et al., 2001: 167, fig. 70. – (?) Preker & Lawn, 2010: 120 [non Plumularia polymorpha Billard, 1913 (pro parte): 24, fig. 14B, C].

Heterotheca buski – (?) Hirohito, 1974: 30, fig. 14 (incorrect subsequent spelling).

Halopteris buskii – (?) Ryland & Gibbons, 1991: 527, fig. 2.

Halopteris buski – (?) Hirohito, 1995: 244 fig. 82 (incorrect subsequent spelling).

non Plumularia buskii Bale, 1884: 125, pl. 10 fig. 3, pl. 19 figs 34–35.

Antennella secundaria – (?) Vervoort, 1967: 42, fig. 12 [non Antennella secundaria (Gmelin, 1791)].

Material examined: MHNG-INVE-98635; Republic of Maldives, Faafu Atoll, Magoodhoo Island, 3.07606° 72.96159°, 3 m, coll. D. Maggioni and S. Montano; 29.01.2016; many stems, up to 1.3 cm high, some of them bearing female gonothecae; 16S sequence MF773743. – MHNG-INVE-98636; Republic of Maldives, Faafu Atoll, 3.07478° 72.96628°, 10 m, coll. D. Maggioni and S. Montano; 13.04.2016; 4 stems, 5–7 mm high, of which one bears two young female gonothecae; 16S sequence MF773742. – MHNGINVE-97952; Indonesia, Bali, Pemuteran, -8.14338° 114.65805°, 0.5 m, coll. H.R. Galea; 04.10.2015; several sterile cormoids, up to 1.2 cm high. – MHNGINVE-98633; Madagascar, Bay of Toliara, Dimadimatsy reef, -23.48234° 43.73285°, 1–2 m, coll. N. GravierBonnet; 13.09.1969; fertile (monoecious) colony on alga, stems up to 0.8 cm high. – HRG-0337; France, Guadeloupe, Basse-Terre, Petite Anse, 16.09639° -61.77139°, 0–1 m, coll. H.R. Galea; 22.01.2008; numerous cormoids, up to 1.2 cm high, some bearing male gonothecae. – HRG-0897; France, Martinique, Case-Pilote, 14.64311° -61.14171°, 5–7 m, coll. H.R. Galea; 27.02.2014; several infertile plumes, up to 1.3 cm high. – HRG-1339; France, Martinique, Case-Pilote, 14.64337° -61.14199°, 1–2.5 m, coll. R. Ferry; 24.07.2017; numerous cormoids, up to 1.9 cm high, many bearing male gonothecae, and one their female counterpart; 16S sequence MF773741. – DM&SM-CU005; Dutch Caribbean, Curaçao I., Piscadera Bay, 12.12139° -68.96917°, 11 m, coll. S. Montano; 09.06.2017; two sterile stems, ca. 15 mm high; 16S sequence MF773740.

Diagnosis: Small-sized (up to 2.0 cm high) Halopteris with homomerously-divided stems; internodes rather short, comprising a hydrotheca in their lower 2/3rd part, a well-developed lateral apophysis, and up to 7 nematothecae (1 mesial, a pair of laterals, 2 axillar, and 1–2 superior ones in a median row). Cladia alternate, divided heteromerously; hydrothecate internodes comparatively longer than their ahydrothecate counterparts, bearing a centrally-placed hydrotheca and up to 5 nematothecae (1 mesial, a pair of laterals and generally 1, exceptionally 2, axillar nematothecae); ahydrothecate internodes very short, carrying single nematotheca in their lower halves. Hydrotheca deep, tubular, slightly flaring below aperture, rim circular, slightly scooped in lateral view. Lateral nematothecae barely surpassing hydrothecal rim, borne on well-developed apophyses; conical, walls of lower chamber gradually thickening distally, apical chamber shallow, rim from lowered to sinuated adaxially. Male gonothecae elongated-ovoid, borne on short, quadrangular pedicel; 2 basal nematothecae; distally a narrow, circular aperture. Female gonothecae comparatively larger, borne on short, quadrangular pedicel; broadly ovoid, tapering below, with large apical aperture with thickened rim, closed by glass-watch-shaped operculum; basally 2–3 nematothecae.

Description: Colonies composed of varied number of cormoids arising from tubular, creeping, branching hydrorhiza, devoid of nematothecae. Stems simple, erect (up to 13 mm high in present material), monosiphonic (Figs 1D, 2C-E), composed of a basal, ahydrothecate part, and a much longer, distal part bearing hydrothecae and cladia. Basal part of varied length, almost straight, irregularly divided into successive segments of unequal length through up to 6 transverse nodes, bearing a total of up to 18 nematothecae arranged in two parallel, closely-set, longitudinal rows; distalmost segment separated from hydrothecate part of caulus through deeply-incised, oblique node. Upper part of caulus almost collinear proximally, becoming decidedly geniculate distally, divided into up to 15 regular internodes by means of oblique nodes (Fig. 5A, M); distally, the segmentation may occasionally change to heteromerous through the insertion of transverse nodes. Each stem internode composed of a hydrotheca situated in its proximal half, a lateral apophysis (or a pair in the proximal most internode), and up to 7 nematothecae (a mesial, a pair of laterals, a pair of axillar, as well as 1 or, occasionally, 2 superior ones in a median row (Fig. 5C); superior nematothecae confined to a separate, ahydrothecate internode, where caulus segmentation becomes heteromerous; often a pair of laterally-displaced superior nematothecae, inserted at same level, on proximal most stem internode bearing the pair of cladia); internodes slightly bent above hydrotheca in opposite direction to corresponding cladium. Apophyses, exclusive of those of proximal most internode (that are paired), alternate along stem, each supporting a cladium. Cladia composed of a proximal, short, athecate, quadrangular internode, followed by a succession of a- and hydrothecate internodes resulting through a heteromerous segmentation (Fig. 5B). Ahydrothecate internodes with transverse proximal node and oblique distal node, carrying proximally a single nematotheca; internodes very short, though proximal most one slightly longer than subsequent counterparts (Fig. 5D). Hydrothecate internodes, up to 4 per cladium, with oblique proximal node and transverse distal node, rather short, accommodating a centrally-placed hydrotheca and its up to 5 associated nematothecae (1 mesial, a pair of laterals, as well as 1, or exceptionally 2, axillar nematothecae) (Fig. 5E, N). All hydrothecae deep, conical, adnate for about half their adaxial length; free adaxial wall straight, abaxial wall slightly sigmoid (basally imperceptibly concave, distally slightly everted below rim); aperture wide, circular, rim slightly scooped in lateral view (Fig. 5F, O); hydranths with ca. 14 filiform tentacles. Gonothecae borne on both caulus and cladia through short, lateral apophyses arising midway between hydrothecal bases and mesial nematothecae. Female gonothecae large, piriform, borne on short quadrangular pedicels; basally a couple of nematothecae; distally a large, ovoid, transverse aperture, with conspicuously thickened rim, closed by a glass-watch-shaped operculum (Fig. 5H, I, P). Male gonothecae borne on single, quadrangular pedicels; fusiform to elongated-ovoid, with couple of basal nematothecae, and a distal, narrow, circular, transverse aperture, closed by thin perisarc sheet (Fig. 5J, K, Q). All nematothecae bithalamic, including the axillar ones; mesial triangular in frontal view, upper chamber slightly shallower than basal one, with conspicuously lowered rim on adaxial side (Fig. 5G³); laterals borne on well-developed apophyses, conical, moderately-long (Fig. 5G^{4, 5}), not always reaching the hydrothecal rim (Fig. 5D, F, O); basal chamber narrow, tubular, rather tall; upper chamber shallow, wide, margin either lowered or sigmoid on adaxial side; cauline (Fig. 5G¹) and cladial (Fig. 5G²) nematothecae, other than those associated to the hydrothecae, as well as those borne by the gonothecae (Fig. 5G^{8, 9}), long conical, with tall basal chamber and comparatively shallow upper chamber, with rim slightly lowered adaxially; axillar nematothecae (Fig. 5G^{6, 7}) with much thinner perisarc than their exposed counterparts, short, with chambers of nearly the same depth, wall of upper chamber facing adaxial hydrothecal wall distinctly lowered. Cnidome (Fig. 5L, R) composed of 3 types of microbasic mastigophores: large, elongated-ovoid [(16.8–18.2) × (6.5–6.8) μm, in nematophores, as well as scattered in the coenosarc]; small, banana-shaped [(5.6–5.8) × (1.9–2.1) μm, in tentacles]; small, ovoid capsules [(4.5–4.9) × (2.8–2.9) μm, scattered in the coenosarc]. Color in life from pale yellow to brownish.

Remarks: The segmentation of cauli is generally homomerous, although the insertion of transverse node towards their distal parts introduces a heteromerous segmentation. The cladia are generally divided heteromerously, though an occasional absence of transverse nodes could be noted in material MHNGINVE-98635, this not supposing a homomerous segmentation throughout the involved cladium. The cauline hydrothecae generally display a pair of axillar nematothecae, though only one, placed medially, may occasionally occur in distalmost hydrothecae. In contrast, generally a single axillar nematotheca is associated to each cladial hydrotheca though, exceptionally, a couple may occur. The stems internodes bear generally 1, occasionally 2, median superior nematothecae (ca. 95% and 5%, respectively, n=150); these are arranged in a single row in ordinary internodes, while they are shifted laterally and occur at the same level in the basalmost internodes giving rise to pairs of cladia. The first cladial internodes bear generally but a single nematotheca; rarely, two nematothecae were observed in some cormoids from MHNG-INVE-98635. The material MHNG-INVE-9833 comprises monoecious stems, while only female gonothecae seem to occur in former.

The trophosomes of the present materials from the Indian Ocean were compared with those of specimens from the Lesser Antilles, and all appear indistinguishable, both morphologically and morphometrically (compare Fig. 5A, B, D and 5M, O). In addition, the as yet unknown female gonothecae of the Caribbean population, were found only recently (sample HRG-1339), and do not differ from those of the Indian Ocean counterparts (compare Fig. 5P and 5H & I; Table 3). Moreover, the molecular study supports the conclusions based on the morphological observations (Fig. 9).

Several literature records of hydroids conform, through the segmentation of both the stem and cladia, and the number and position of the nematothecae they carry, to the morphology of the present species. However, minor discrepancies could be occasionally noted, although they are thought to reflect rather overlooked details. Thus, Vervoort's (1967) material from the Red Sea, obviously erroneously assigned to Antennella secundaria (Gmelin, 1791), is reportedly said to carry single axillar nematothecae behind the cauline hydrothecae, a situation – indeed – met with only distally in the stems of the present species. Moreover, it should be stated that one nematotheca belonging to a pair, namely that given off on the same side as the corresponding cladium, may be difficult to notice as it is hidden (in frontal view) by the apophysis supporting the lateral nematotheca originating on the same side (see Fig. 5C – lateral nematotheca on same side as the cladium expressly shown).

The “second pair of lateral nematothecae […] mesial to and much smaller than normal pair”, observed by Ryland & Gibbons (1991) in their Fijian specimens assigned to H. buskii, is nothing else than the couple of cauline axillar nematothecae. Similarly so, besides the commonly seen single cladial axillar nematothecae, “two mediosuperior nematothecae are often present behind the free adcauline wall of the hydrotheca”. These, combined to the heteromerous segmentation of cladia, the shape and size of both hydro-and lateral nematothecae, and the morphology of the female gonotheca, suggest that their specimens are, most probably, conspecific with H. vervoorti.

The Japanese materials assigned to both Heterotheca and Halopteris buski (sic!) by Hirohito (1974, 1995, respectively) are morphologically close to the present species, as their cauli and cladia display the same segmentation, the same number and position of nematothecae, and the monoecious condition of their cormoids. However, a pending genetic study is expected to clarify their relationships undoubtedly.

The presence of H. vervoorti, originally described from the Caribbean (Galea, 2008), in the Indian Ocean is not entirely surprising. The reverse situation is, indeed, more common at present, with many examples of hydroids, primarily known to occur in the Indo-Pacific that were subsequently recorded from the Caribbean, viz. Sertularia rugosissima Thornely, 1904 (Galea, 2008), S. tongensis (Stechow, 1919) (Galea, 2010), S. hattorii Leloup, 1940 (Galea & Ferry, 2015), Aglaophenia postdentata Billard, 1913 (Galea, 2013), and Clytia edentula Gibbons & Ryland, 1989 (Galea & Ferry, 2015). Whether these species primarily occurred in one geographical area and spread afterwards elsewhere, could not be established at this stage, but it is obvious that only the frequency of the collecting efforts first revealed their presence in one area but not in the other.

Halopteris vervoorti comes close to a number of nominal species through the occurrence of paired axillar nematothecae behind the cauline hydrothecae and the heteromerous segmentation of its cladia, viz. H. australis sp. nov. (see below), H. brasiliensis (see below), H. liechtensternii (Marktanner-Turneretscher, 1890), and H. sibogae (Billard, 1913). However, unlike H. australis, it does not possess hydrothecae with distinctly sinuated margins or exceedingly-long lateral nematothecae, respectively. On the other hand, H. brasiliensis is a species forming taller cormoids, with thicker stems and cladia, broader hydrothecae, and cauline superior nematothecae distinctly arranged into two longitudinal rows; unlike in H. vervoorti, its cauline hydrothecae occupy nearly the whole length of the internode. Halopteris liechtensternii is a species with comparatively longer stem internodes (and, implicitly, more widely-spaced cladia), provided regularly with 2–3 superior nematothecae arranged in two distinct rows, its female gonothecae are much longer and almost cylindrical, while its males appear dwarfed compared to those of the present species. Halopteris sibogae, besides many common features shared with H. vervoorti (see under the former), is immediately distinguishable through its exceedingly long lateral nematothecae. Additional differences to other congeners are summarized in Appendix 1.

Distribution: Lesser Antilles (Galea, 2008, 2010, 2013), Cape Verde (Ansín Agís et al., 2001, as H. polymorpha), (?) Red Sea (Vervoort, 1967; as Antennella secundaria), Madagascar, the Maldives, Indonesia (present study), (?) Australia (Preker & Lawn, 2010, as H. polymorpha), (?) Japan [Hirohito (1974), as Heterotheca buski (sic!); Hirohito (1995), as Halopteris buski (sic!)], (?) Fiji (Ryland & Gibbons, 1991, as Halopteris buskii).

Halopteris buskii – Vervoort & Vasseur, 1977: 72, fig. 31 [non Halopteris buskii (Bale, 1884)].

Halopteris polymorpha – Schuchert, 1997 (pro parte): 64, fig. 23 [non Halopteris polymorpha (Billard, 1913)].

Type material: MHNG-INVE-82742; France, New Caledonia, Nouméa, south of N’Géa islet, -22.296° 166.489°, 9 m, coll. E. Tardy; 06.10.2012; fertile (female) colony in alcohol and two microslides (H20/37–38), with stems up to 1.8 cm high, growing on algae.

Diagnosis: Small-sized Halopteris, with cormoids not exceeding 2.5 cm high. Stems monosiphonic, homomerously-segmented, internodes rather long, bearing a hydrotheca in their proximal half, a lateral apophysis, and up to 8 nematothecae (1 mesial, a pair of laterals, a pair of axillar, as well as up to 3 superiors in two parallel rows). Cladia alternate, heteromerously-segmented; 1st ahydrothecate internode with 1, exceptionally 2, nematothecae in a row; ordinary ahydrothecate internodes with 1 nematotheca; hydrothecate internodes with a hydrotheca and its up to 5 associated nematothecae (1 mesial, a pair of laterals, and 1, or rarely 2, axillar); hydrotheca cup-shaped, deep, adnate for 2/3rd its adaxial wall, swollen basally, rim distinctly sinuated in lateral view, aperture circular, lateral nematothecae long, borne on well-developed apophyses, surpassing hydrothecal rim. Female gonothecae given off from caulus; piriform, borne on quadrangular pedicel, 2 basal nematothecae, aperture large, circular, perpendicular to long axis of theca, rim thickened, a watch-glass-shaped operculum.

Etymology: From the Latin austrālis, meaning “of the south” with reference to its occurrence in the South Pacific.

Description: Colonies composed of a varied number of upright cormoids, up to 2 cm high in present material, arising from creeping, branching, anastomosing hydrorhiza. Stems erect, simple, monosiphonic (Figs 1E, 2F), composed of a basal, ahydrothecate part, irregularly divided into a few segments (up to 5) by means of transverse constrictions of the perisarc, and carrying a varied number of nematothecae arranged in two parallel rows; above, remainder of stem comprising a much longer, hydrothecate part, homomerously-segmented into regular internodes by means of oblique nodes (Fig. 6A), slightly marked proximally to more conspicuous distally; occasionally, transverse nodes can be inserted towards the distal end of cauli, creating a heteromerous segmentation; a hinge joint between the two parts of the stem. Up to 26 relatively long cauline internodes, almost collinear proximally, gradually becoming geniculate towards distal end; each provided with a hydrotheca in its proximal half, a well-developed, lateral apophysis, as well as up to 8 nematothecae: 1 mesial, a pair of laterals, a pair of axillar (only one of these subsisting distally on stem) (Fig. 6B), as well as 1–3 superior ones in two parallel, closely-set rows (when only two of these are present, they may occur at the same level, or one above the other; they are confined to a separate ahydrothecate segment when transverse nodes intervene towards distal end of caulus). Cladia up to 2.5 mm long, alternate, except in the proximal most cauline internodes, where they are opposite; composed of a short, quadrangular segment proximally, followed by a heteromerous division into alternating ahydrothecate and hydrothecate internodes; the former with proximal node transverse and distal node oblique; the reverse in hydrothecate internodes (Fig. 6D); 1st ahydrothecate internode comparatively longer than subsequent ones, and carrying single nematotheca (exceptionally 2); ordinary ahydrothecate internodes short, and provided with one nematotheca; hydrothecate internodes up to 5 per cladium, comparatively longer than their ahydrothecate counterparts, and carrying a centrally-placed hydrotheca and its up to 5 associated nematothecae: a mesial, a pair of laterals, and generally one axillar, although a pair could be occasionally noted (Fig. 6C). Hydrothecae cup-shaped, rather deep, about 1/3rd adnate; abaxial wall slightly sigmoid, rounded proximally and slightly flaring below the rim; aperture circular in apical view, but distinctly sigmoid when seen laterally, due to broad (though shallow) abaxial emargination, giving the impression that an abaxial cusps occurs (Fig. 6A, D). All nematothecae, including the axillar ones, bithalamic and movable; mesial nematothecae triangular in frontal view, with deep adaxial emargination (Fig. 6E³); laterals relatively long, surpassing the hydrothecal rim (Fig. 6D), and borne on well-developed apophyses; basal chamber high and narrow, upper chamber shallow, with sigmoid adaxial wall (Fig. 6E^4–7); axillar nematothecae (Fig. 6E^{6, 7}) small, with wall of upper chamber lowered on side facing adaxial wall of hydrotheca; both cladial (Fig. 6E²) and cauline (Fig. 6E¹) nematothecae characteristically turned posteriad, tall, conical, with rim of apical chamber lowered adaxially. Female gonothecae borne on cauli by means of short, lateral apophyses given off from below the hydrothecae, followed by a pedicel composed of a quadrangular segment; large, piriform, with 2 basal nematothecae, and a large, apical aperture with thickened rim, perpendicular to long axis of the theca, and closed by a watch-glass-shaped operculum (Fig. 6F). Male gonothecae elongated-ovoid, curved proximally and provided there with single nematotheca; aperture distal, small, circular. Hydranths badly preserved, tentacle number could not be ascertained. Cnidome (Fig. 6G): only large capsules (ca. 17.5 × 6.3 μm), likely pseudostenoteles, observed in the material in hand. Color in life: milky white (Fig. 1E).

Dimensions: See Table 5.

Remarks: There is little doubt that the present material from New Caledonia is conspecific with the specimens with distinctly sinuated hydrothecal margins from French Polynesia examined by Vervoort & Vasseur (1977, as H. buskii) and, subsequently, by Schuchert (1997, provisionally assigned to H. polymorpha). Their main morphological and morphometrical features are compared in Table 5. Unfortunately, it was not possible to obtain the material of Vervoort & Vassueur for a re-examination, due to ongoing renovation of NBC.

The cauline internodes bear commonly 1 (52%) or 2 (44%) superior nematothecae, although up to 3 may occasionally (4%, n = 75 internodes belonging to 5 different stems) be present in the material in hand. Schuchert (1997), however, reports up to 5 of these. The number of axillar nematothecae was apparently overlooked by Vervoort & Vasseur (1977):

Only female gonothecae occur in the present material (Fig. 6F), although those of both sexes were illustrated by Schuchert (1997, fig. 23B, right).

Halopteris australis, through the presence of pairs of axillar nematothecae especially on the stem, and the heteromerous segmentation of its cladia, resembles H. brasiliensis sp. nov., H. liechtensternii, and H. vervoorti. However, any of these is provided with the distinctively sinuated hydrothecal margin displayed by it. In addition, H. brasiliensis has thicker stems and cladia, its lateral nematothecae are relatively short and conical, and do not surpass the hydrothecal rim. Halopteris liechtensternii is readily distinguished through the shape and size of its gonothecae, the females being long and almost tubular, while the males are dwarfed. Halopteris vervoorti forms comparatively shorter cormoids, with shorter stem internodes (and, consequently, more approximated cladia) provided with 1–2 superior nematothecae characteristically arranged in a row, and its hydrothecae are slightly shorter and narrower. Additional differences to other congeners are summarized in Appendix 1.

Distribution: New Caledonia (present study), French Polynesia (Vervoort & Vasseur, 1977; as H. buskii).

Halopteris polymorpha – Millard & Bouillon, 1973 (pro parte): 83, fig. 10F, G, H, J. – Bouillon et al., 1995: 49. – Schuchert, 1997 (pro parte): 66, 72, fig. 22A-D [non Halopteris polymorpha (Billard, 1913)].

non Halopteris polymorpha – Millard & Bouillon, 1973 (pro parte): 83 (= H. platygonotheca Schuchert, 1997).

Halopteris buskii – Rees & Vervoort, 1987 (pro parte): 119, fig. 25A-B [non Halopteris buskii (Bale, 1884)].

Holotype material: MACT2700; Seychelles, Mahé I., coll. J. Bouillon (MRAC Expedition); 1966; numerous cormoids, 1.5–7 cm high, with both female and male gonothecae [material studied by Millard & Bouillon (1973), as H. polymorpha (Billard, 1913); not studied here due to ongoing renovation of MRAC; however, 4 microslides (MHNG-INVE-37494, H12/32–35) prepared from the holotype, were examined for the purpose of the present study; according to Schuchert (1997: 55), cormoids of H. platygonotheca Schuchert, 1997 co-occur in the original sample].

Paratype: MHNG-INVE-98634; Republic of Maldives, Faafu Atoll, 3.06497° 72.9212°, 35 m, coll. D. Maggioni and S. Montano; 14.04.2016; colony composed of 6 sterile stems, 1.7–3.9 cm high; 16S sequence MF773747.

Diagnosis: Halopteris with tall cormoids, reportedly reaching 7 cm high; stems homomerously-segmented, each internode moderately-long, carrying a hydrotheca, a lateral apophysis, and up to 7 nematothecae: 1 mesial, a pair of laterals, a scale-shaped axillar one, as well as 2–3 superior ones arranged in two parallel rows; cladia alternate, heteromerously-segmented; ahydrothecate internodes with 2 laterally-displaced nematothecae (distally, only 1 of these subsists); hydrothecate internodes with a hydrotheca and its 4 associated nematothecae: 1 mesial, a pair of laterals, and a small, scale-shaped axillar one. Colonies monoecious. Female gonotheca borne on stems, large, ovoid, laterally flattened, with 2 basal nematothecae, and a distal, transverse aperture closed by glass-watch-shaped operculum. Male gonothecae borne on both stems and cladia, small, fusiform, with narrow distal aperture, and one basal nematotheca.

Etymology: This species honors the late N.A.H. Millard (1914–1997) for her outstanding contribution to the hydrozoan research.

Description: Colonies composed of a varied number of tall stems (reportedly up to 7 cm in height) arising from tortuous, creeping, branching, anastomosing hydrorhiza, devoid of nematothecae. Stems erect, simple, monosiphonic (Figs 1F, 2G), composed of a basal, ahydrothecate part of varied length, and a much longer, distal part bearing hydrothecae and hydrocladia; basal part arising directly from hydrorhiza without constriction above origin, usually not segmented by transverse nodes, and carrying a varied number of nematothecae in two parallel rows; distalmost node deeply-cut and oblique; remainder of stem homomerously-segmented into regular internodes by means of oblique nodes; each internode moderately long, with a hydrotheca in its basal half, a short lateral apophysis supporting a cladium, and up to 8 nematothecae: 1 mesial, a pair of laterals, a small, scale-shaped axillar one, as well as generally 2, occasionally 3, or exceptionally 4, superior nematothecae arranged in two parallel rows (Fig. 7A, B); proximal most internode carries 2 opposite apophyses supporting a pair of cladia, and usually bears 3–4 superior nematothecae. Cladia borne on corresponding stem apophyses, alternate, composed of a proximal, short, quadrangular segment, followed by a succession of athecate and thecate internodes resulting from a heteromerous segmentation (Fig. 7A); ahydrothecate internodes with straight node proximally and oblique node distally; the reverse in hydrothecate internodes; the latter, up to 5 per cladium in the material in hand, comprising a centrally-placed hydrotheca, and its 4 associated nematothecae: 1 mesial, a pair of laterals, as well as a minute, scale-shaped axillar one (Fig. 7C, D); ahydrothecate internodes shorter than their hydrothecate counterparts, with generally 2 laterally-displaced nematothecae, either opposite or subopposite, more often found in proximal most internodes, whereas only one of these is retained by the distalmost internodes. Hydrothecae cup-shaped, moderately-deep, fused for about 1/3rd their adaxial length; abaxial wall straight for most of its length, slightly everted below aperture; the latter perfectly circular in apical view, slightly flaring and showing a sinuated rim in lateral view, though not producing an abaxial cusp (Fig. 7D). All nematothecae, except the axillar ones, bithalamic and movable; mesial ones triangular in frontal view, with deeply-scooped rim on adaxial side (Fig. 7E^{5, 6}); laterals borne on rather short apophyses, conical, with thickened walls (Fig. 7E⁷), not surpassing the hydrothecal rim (Fig. 7D); cauline (Fig. 7E^{1, 2}) and cladial (Fig. 7E^{3, 4}) nematothecae characteristically turned posteriad (Fig. 7A, C), long, conical, with tall basal chambers and comparatively shallow apical chambers, with adaxially-scooped rims; axillar nematothecae associated to the hydrothecae of both caulus (Fig. 7E⁸) and cladia (Fig. 7E⁹), monothalamic. Stems monoecious. Female gonothecae borne below the stem hydrothecae through short lateral apophyses and a single-segmented, quadrangular pedicel; large, ovoid, laterally-flattened, with two nematothecae on base, and a distal, transverse, conspicuously thickened aperture closed by a glass-watch-shaped operculum. Male gonothecae borne on both stems and cladia, through short, lateral apophyses and a single-segmented, quadrangular pedicel; comparatively smaller than female, fusiform, with distal, narrow, circular aperture, and a basal nematotheca. Color in life: brownish (Fig. 1F). Cnidome (Fig. 7F) composed of 3 types of microbasic mastigophores: large, elongated-ovoid [(19.9–21.3) × (7.3–8.0) μm, in nematophores, as well as scattered in the coenosarc]; small, banana-shaped [(5.8–6.5) × (2.1–2.3) μm, in tentacles]; small, ovoid capsules [(5.1–5.8) × (2.9–3.1) μm, scattered in the coenosarc].

Dimensions: See Table 5.

Remarks: Upon comparison of the newly-collected Maldivian specimens with the slide material MHNGINVE-37494 (H12/32–35) prepared from the holotype designated herein [sample MACT2700 studied by Millard & Bouillon (1973), as H. polymorpha (Billard, 1913)], it appears that both are conspecific.

The description of the gonothecae, often distorted in the slide material available (H12/32: ♂; H12/33: ♂ & ♀; H12/35: ♂), was taken after Millard & Bouillon (1973: 84, fig. 10H & J) and Schuchert (1997: 22, fig. 22D, as H. polymorpha).

The tallest cormoid examined here (3.9 cm high) bears 54 cauline hydrothecate internodes. Not only the proximal most internode gives rise to a pair of cladia, but this situation is also repeated in several subsequent, consecutive, more distal internodes. In one cormoid, a secondary stem arises from one of the paired, basalmost hydrocladia.

The first cladial ahydrothecate internodes do not differ much in length compared to their subsequent counterparts; they generally bear 2 nematothecae, though exceptionally 3 were noted. The remaining internodes equally bear 2 nematothecae (although, exceptionally, two pairs could be found) when they are found in the proximal parts of the cladia, while only one nematotheca occurs in those internodes confined to the distalmost parts of the cladia. Rees & Vervoort's (1987) record from Zanzibar (Stn. 112) assigned to H. buskii likely belongs to the present species. Indeed, the occurrence of pairs of suprahydrothecal nematothecae, and of a single axillar nematotheca on the cauline internodes, are distinctive. However, the authors mention only one nematotheca per ahydrothecate cladial internode; as stated above, this situation is, quite often, met with in the distalmost internodes of the material MHNG-INVE-37494. It should also be stressed that only one nematotheca of a couple is visible when the cladia are seen laterally, especially in material mounted between slide and coverslip. Strangely, Rees & Vervoort do not mention a sinuated hydrothecal rim, though it should be underlined that this peculiarity is only noticeable towards the adaxial thecal wall, where the presence of lateral nematothecae could make it less obvious upon a routine examination.

Halopteris millardae comes close to a few congeners with homomerously-segmented cauli and a heteromerous division of their cladia, and whose both cauline and cladial hydrothecae are provided with an axillar nematotheca, namely H. nuttingi (Billard, 1911) and H. polymorpha. Halopteris nuttingi has proportionally shorter cauline internodes [compare fig. 21B in Schuchert (1997) with Fig. 7A herein], provided with up to three pairs of superior nematothecae in two parallel rows, and the upper chamber of its lateral nematothecae is globular, with the rim scooped on both ad- and abaxial walls [Billard (1913), as H. buski (sic!); Schuchert (1997, fig. 21C, H), as H. polymorpha]. Halopteris polymorpha has comparatively longer stem and cladial ahydrothecate internodes (Fig. 3A, B), its hydrothecae are shallower (Fig. 3D, F) and are provided with an even rim. Additional differences to other congeners are summarized in Appendix 1.

Distribution: Seychelles [Millard & Bouillon (1973), as H. polymorpha (Billard, 1913)], Maldives (present study), Zanzibar [Rees & Vervoort (1987), as H. buskii (Bale, 1884)].

Halopteris buskii. – Migotto, 1996: 48, fig. 9F-H [non Halopteris buskii (Bale, 1884)].

Halopteris polymorpha. – Schuchert, 1997 (pro parte): 72, fig. 22F-H [non Halopteris polymorpha (Billard, 1913)].

Holotype material: MHNG-INVE-37495; Brazil, São Sebastião Channel, 6–8 m, coll. A.E. Migotto; 06.10.1987; two slides, H12/36 & 37, each containing a ca. 1.6 cm high cormoid provided with a female gonotheca.

Diagnosis: Halopteris with medium-sized plumes, reaching 3 cm high; stems simple, monosiphonic, homomerously-segmented; internodes rather short, with a lateral apophysis, a hydrotheca, and its up to 7 associated nematothecae (1 mesial, a pair of laterals, a pair of axillar, and generally 2 superiors, the latter either opposite or subopposite). Hydrocladia alternate, heteromerously-segmented; ahydrothecate internodes very short, with 1 nematotheca; hydrothecate internodes comparatively longer, with one hydrotheca and up to 5 nematothecae (1 mesial, a pair of laterals, and commonly 1, rarely 2, axillar). Female gonothecae borne on stems; large, piriform, with 2–3 long, basal nematothecae, aperture distal, wide, circular, perpendicular to long axis of theca, closed by glass-watch-shaped operculum.

Etymology: Named after the country of occurrence.

Description: Colonies composed of reportedly up to 3 cm high cormoids arising from creeping, branching hydrorhiza. Stems erect, simple, monosiphonic (Fig. 2H), composed of a basal, ahydrothecate portion, and a much longer, distal part bearing both hydrothecae and hydrocladia. The former of varied length, irregularly divided into a number of segments by means of transverse nodes, bearing nematothecae arranged in two longitudinal rows; last node deeply-cut and oblique. Remainder of caulus homomerously-segmented into rather short internodes by means of oblique nodes (Fig. 8A); each internode with a hydrotheca in its proximal half, a cladial apophysis lateral to it (two opposite in proximal most internode), and up to 7 nematothecae, of which 5 are associated to the hydrotheca (1 mesial, a pair of laterals, and a pair of axillar) (Fig. 8B), and 2 (slightly displaced laterally and, thus, forming an opposite or a subopposite pair) occur distally on the internode (occasionally, only one of these is present; however, in the basalmost internodes bearing pairs of cladia, 2–3 of these occur). Hydrocladia alternate, borne on corresponding cauline apophyses; composed of a short, athecate, quadrangular segment, followed by an alternation of ahydrothecate and hydrothecate internodes resulting from a heteromerous segmentation (Fig. 8C); ahydrothecate internodes with proximal node transverse and distal node oblique; the reverse in hydrothecate internodes; first ahydrothecate internode comparatively longer than subsequent ones, and carrying a single nematotheca; ordinary ahydrothecate internodes very short and provided with single nematotheca; hydrothecate internodes, reportedly up to 7 per hydrocladium, accommodating a hydrotheca and its up to 5 associated nematothecae (1 mesial, a pair of laterals, and commonly one – rarely a pair – of axillar nematothecae) (Fig. 8D). Hydrothecae cup-shaped, rather deep, adnate for about half their height, walls slightly divergent, abaxial one imperceptibly sigmoid (concave proximally, convex below aperture), free part of adaxial one straight; aperture wide, circular, rim even (Fig. 8C). Hydranths with conical hypostome and 16–17 filiform tentacles. All nematothecae, including the axillar ones, bithalamic and movable; mesial ones short, with lower and upper chambers of nearly same depth (Fig. 8E⁴), and rim of upper chamber lowered on adaxial side; lateral nematothecae borne on well-developed apophyses, with tall basal chamber and shallow upper chamber with sigmoid rim on adaxial side (Fig. 8E⁵); whole nematotheca barely reaching hydrothecal rim (Fig. 8C); cauline (Fig. 8E¹) and cladial (Fig. 8E^{2, 3}) nematothecae similar in shape to the laterals, but with lowered rim on the adaxial side of upper chamber; cauline axillar nematothecae displaced laterally and facing outwards in opposite directions (Fig. 8E⁷); cladial axillar nematothecae commonly occurring singly (Fig. 8E⁸), reportedly in pairs in rare instances. Gonothecae (only female known) borne on both cauli and cladia, given off laterally from below the hydrothecal bases; large, piriform, with 2–3 long, basal nematothecae, and a broad, circular aperture with thickened rim, perpendicular to long axis of the theca (Fig. 8F), and closed by glass-watch-shaped operculum. In life, coenosarc yellowish, hydranths white. Cnidome: at least pseudostenoteles [(18.0–21.5) × (7.5–9.0) μm] and microbasic mastigophores [(6.0–7.0) × ca. 2.0 μm] reported to date.

Dimensions: See Table 5.

Remarks: The description given above is based on the holotype, although some additional data, such as the cnidome composition, are taken from Migotto (1996, as H. buskii).

Through the presence of pairs of cauline axillar nematothecae and the heteromerous segmentation of its cladia, H. brasiliensis comes close to a number of congeners, viz. H. australis sp. nov., H. liechtensternii, H. sibogae, and H. vervoorti. Halopteris australis is immediately distinguished through it distinctly sinuated hydrothecal margin. Halopteris liechtensternii, when fertile, has comparatively longer and more tubular female gonothecae. Halopteris sibogae has distinctive, exceedingly long nematothecae (Fig. 2C). Halopteris vervoorti forms comparatively shorter stems (compare Fig. 2C-E and 2H), their cauli and cladia are thinner (compare Fig. 5A, B, M and 8A), and their cauline hydrothecae are placed in the lower halves of the corresponding internodes, leaving enough place for 1–2 superior nematothecae, arranged in one row, to be confined to their distal halves (Fig. 5C). Additional differences to other congeners are summarized in Appendix 1.

Distribution: Only known from Brazil (Migotto, 1996).