Schindleria nana, a new extremely progenetic gobiid fish species (Teleostei: Gobiiformes: Gobiidae) from Lizard Island, Great Barrier Reef, Australia

Abstract. Here, we describe a new species of Schindleria, Schindleria nana, from Lizard Island, Great Barrier Reef, Australia. The new species belongs to the long dorsal-fin type (LDF) of Schindleria and is the first very small (‘dwarf’) LDF species (< 13 mm TL) to be described. It is characterized by an elongate and narrow body; a dorsal fin longer than the anal fin (predorsal-fin length 63.3% of SL: preanal-fin length 72.1% of SL); a long, relatively narrow head (head width 46.2% of head length) with a straight profile; small and round eyes (24.9% of head length); a large postorbital distance (52% of head length); a narrow, slender pectoral radial plate (width at origin 46.4%, maximum width 57.0% of pectoral radial plate length); 16 dorsal-fin rays; 11-12 anal-fin rays; first anal-fin ray ventral to the sixth dorsal-fin ray; six procurrent rays gradually increasing in length, last ray elongated, twice the length of the penultimate ray; premaxilla with tiny, conical, densely set teeth; dentary with zero teeth in the holotype and with two teeth on the left dentary and five teeth on the right dentary in the adult paratype; females with few (approx. 4-7) but very large eggs (3.4-3.9% of SL); urogenital papilla inconspicuous, de facto just an urogenital opening; swim bladder not pigmented; black eyes; no other external pigmentation on the body.


Introduction
During the last years, a series of studies on the ecology, faunistic, morphology, and taxonomy of Schindleria (infantfishes) have increased our current knowledge on this enigmatic gobiid genus (e.g.Ahnelt & Sauberer 2018, Robitzch et al. 2021a, b, Ahnelt et al. 2022).These tiny (10-23 mm TL) and extremely rapidly reproducing fishes (with up to nine generations per year; Kon et al. 2007) inhabit the entire tropical and sub-tropical Indo-Pacific, from Central and South America to East Africa and the Red Sea (Fig. 1).At its discovery, the first two described species, Schindleria praematura (Schindler, 1930) and S. pietschmanni (Schindler, 1931), were morphologically distinguished by a very conspicuous character in their dorsal and anal fins, where S. praematura featured a notably longer dorsal fin compared to its anal fin, while S. pietschmanni's dorsal and anal fins were approximately equal in length (Schindler 1930(Schindler , 1931(Schindler , 1932)).Thus, subsequent studies started grouping new species of Schindleria in two species groups based on this character: the long dorsal fin type (LDF) as in S. praematura and the short dorsal fin type (SDF) as in S. pietschmanni (Ahnelt 2019).Virtually all reports of Schindleria across their Indo-Pacific distribution range resemble morphologically one of these two types (summarized in Ahnelt & Sauberer 2020).Nevertheless, the evolutionary history of this morphological trait is not known, nor whether these two fin types might have evolved convergently in allopatry multiple times.
More than 70 years after the first descriptions of the two Hawai'ian species of Schindleria, a third species, S. brevipinguis, was described from the Great Barrier Reef (Australia) close to Lizard Island.This species was morphologically clearly distinguishable from the two other species by a deeper body, toothless jaws, larger eyes, lower number of dorsal and anal fin rays, a non-pigmented swim bladder, and especially its much smaller size at maturity (< 10 mm TL) (Watson & Walker 2004).A further intriguing and distinctive character in the description of S. brevipinguis was the few but extremely large eggs of females.Among all described Schindleria species, females of the larger species S. nigropunctata, S. pietschmanni, S. praematura, and S. squirei have relatively numerous (in total 50-60) eggs of a size from 1.3-1.8% in SL, while the two smaller sized (< 11 mm SL) species, S. brevipinguis and S. qizma, have only a few (approx.10) and distinctly large (2.5-5% of SL) eggs (Ahnelt et al. 2023).
Herein, we describe a new species of Schindleria of the LDF species group, two adult females and one juvenile, from Lizard Island (Queensland, Australia).This species is characterized by very small size, few and very large eggs, oral jaws with no or few teeth on the lower jaw, an inconspicuous urogenital papilla, de facto just a urogenital opening, and an unpigmented swim bladder.

Material and Methods
The three specimens of S. nana sp.nov.are deposited in the Australian Museum Sydney (AMS), Sydney, Australia, and are registered as AMS.I.23115-004 (holotype) and as AMS.I.23115-040 (paratypes).The sample was collected by J.M. Leis with a plankton tow at 6-10 m depth off of Lizard Island (Queensland, Australia), in the lagoon 'Lizard Island Reef', 14°41' S, 145°27' E (Fig. 2).The specimens were deposited at the Australian Museum, Sydney.
Based on three genetic studies in which Schindleria clustered within Gobiidae (Thacker 2009, Agorreta et al. 2013, Tornabene et al. 2018), we consider Schindleriidae a junior synonym of Gobiidae.We separate the species of Schindleria into two species groups, a long dorsal-fin type group (LDF) and a short dorsal-fin type group (SDF), as defined by Ahnelt (2019).The 12 formally described species of these two species groups are listed in Table 1.
Pictures of the specimens were taken with a Nikon DSRi-2 camera mounted on a Nikon SMZ25 stereo microscope using NIS-Elements Microscope Imaging Software (BR V.  Paratypes: Two specimens, same data as for holotype: female, 8.5 mm SL; juvenile, 6.8 mm SL; AMS.I.23115-040 (Fig. 4).

Diagnosis:
The new species S. nana stands out from its congeners because it is the first small-sized species (< 10 mm SL) in the LDF species group and the first LDF Schindleria with only a few (4-7) and very large eggs   anal-fin 5.9-6.6% of SL; head length 14.4-15.6% of SL; head depth 7.8-8.1% of SL; eye diameter 3.3-3.6% of SL and 23.1-26.1% of the head length; pectoral radial plate length 5.6-5.8% of SL; maximum width of the pectoral radial plate 3.2-3.3% of SL and 56.9-57.1% of pectoral radial plate length; depth of the hypural late 66.7% of the urostyle length; 16 dorsal-fin rays; 13 anal-fin rays, first anal-fin ray positioned below the 5-6 th dorsal-fin ray; six procurrent rays; swim-bladder not pigmented; continuous row of small, conical teeth on premaxillary but zero teeth on the dentary of the holotype or five teeth (right) plus two isolated teeth (left) in the dentary of the paratype.See detailed comparisons below.
Comparisons: Schindleria nana, a member of the LDF species group, can be distinguished from all members of the SDF group by a combination of the following characters: i) length of dorsal fin (distinctly longer than the anal fin vs. dorsal and anal fin of about the same length), ii) longer preanal-fin length (72.2% vs. 60.1-71.8% of SL), iii) smaller eyes (24.6% vs. 27.4-35.6% of head length), iv) longer postorbital length (52% vs. 43.8-59% of head length), and v) a relatively narrow hypural plate (66.7% vs. 79.6-89.1% of urostyle length).
Schindleria nana can be distinguished from the other species of the LDF species group by its i) small adult body size (8.8 mm SL vs. > 15 mm SL), ii) the unique type of dentition (0-5 teeth on the dentary vs. at least 8-9 teeth on the dentary), and iii) the females' few (approx.10 vs. 50-200) and large (3.4-3.9% of SL vs. 1-1.8% of SL) eggs (Table 4).
Schindleria nana can be further distinguished from S. macrodentata by i) the size, position, and number of teeth (many, tiny, conical, and densely set on the premaxillary and few or no on the dentary vs. few, large, and widely spaced on both jaws), ii) fewer dorsal-fin rays (16 vs. 19), iii) more anal-fin rays (13 vs. 10), iv) a longer head (15% vs. 11.1% of SL), v) a longer tail (30.4% vs. 22.1% of SL), vi) a distinctly wider pectoral radial plate (57% vs. 32.6% of pectoral radial plate length), vii) the shape of the last and longest procurrent ray (simple vs. with an additional spiny process at the base), and viii) females with few large eggs (total approx.10) vs. with numerous small eggs (possibly up to 150-160).
Schindleria nana can be further distinguished from S. multidentata by i) the number and position of the teeth on the lower jaw/dentary (zero or up to five vs. approx.30 divided by a diastema in two series, approx.21 + 8), ii) fewer dorsal fin rays (16 vs. 19), iii) the position of the first anal-fin ray (below the dorsalfin ray 5 or 6 vs. 10), iv) a deeper body (body depth at the pectoral-fin base 6.3% vs. 4.1% of SL and at the origin of the anal-fin 6.3% vs. 5.6% of SL), and v) a swim bladder unpigmented vs. dorsally pigmented.
Schindleria nana can be further distinguished from S. nigropunctata by i) more anal-fin rays (13 vs. 12), ii) the number and position of the teeth on the lower jaw/dentary (zero or up to five vs. approx.8-9 teeth), iii) a longer predorsal-fin length (63.3% vs. 61.1% of SL), iv) a deeper body (body depth at the pectoralfin base 6.3% vs. 4.8% of SL and at the origin of the  Description: Morphometric (Table 2) and meristic (Table 3) information is given separately for the holotype and the paratypes.Body slender, elongate, and somewhat compressed; body depth increases slightly from head to anus; head long and elongate in the lateral view with a straight head profile; jaws reach a vertical, posteriorly through the origin of the orbit; lower jaw projecting the upper jaw, mouth superior; premaxilla with approx.20 tiny, conical, and densely set teeth along the entire ventral margin; dentary with 0-5 tiny teeth on the anterior third: zero teeth on both dentaries (Fig. 5 4).
Colouration in life: Unknown.

Colouration preserved:
The body is uniformly yellowish except for the ventral side of the abdomen and the head, which are translucent.The eyes are rubiginose to blackish (Figs. 3, 4).The fins are translucent.

Etymology:
The specific name 'nana' (from the Latin 'nanus' -dwarf) refers to the small size of this species.

Discussion
The relative position of the dorsal fin to the anal fin is a reliable character to group species of Schindleria and has resulted in two groupings: the SDF species group and the LDF species group (Ahnelt 2019).Although adult specimens can generally unambiguously be assigned to one of these two groups, the assignment is less evident in juvenile LDF species because juveniles have fewer dorsal fin rays than adults (see Fig. 177D in Watson 2000, Fig. 1 in Ozawa & Matsui 1979).This ontogenetic difference in fin ray number is characteristic of teleost fishes because the dorsal fin rays appear sequentially in a bidirectional pattern during larval development (e.g.Mabee et al. 2002, Ott et al. 2012, Figs. 2, 6 in Koch et al. 2022).This difference is also true among gobiid fishes (e.g.Daoulas et al. 1993, Figs. 2, 3 in Strydom & Neira 2006, González-Navarro et al. 2021), where late larvae have more fin rays developed than earlier larval stages of the same species.Schindleria species are extremely progenetic (Johnson & Brothers 1993) and do not develop beyond a larval stage (Kon & Yoshino 2002, Kon et al. 2007).Generally, the first anal fin ray is positioned below the dorsal fin ray eight to eleven in LDF species such as S. macrodentata, S. multidentata, S. nigropunctata, S. praematura, or S. squirei (Ahnelt & Sauberer 2018, Ahnelt 2019, 2020, Robitzch et al. 2024) and below the dorsal fin ray one to four in SDF species such as S. brevipinguis, S. parva, S. pietschmanni, or S. qizma (Schindler 1932, Watson & Walker 2004, Abu El-Regal et al. 2021).In the new species, S. nana, the Table 4.The species of Schindleria grouped in typical 'large' species (> 15 mm SL), and very small ('dwarf') species (< 11 mm SL).
Additionally, the number of ovaries (from S. edentata, no females are known), the size of the eggs and the origin of the type localities (for details, see Fig. 1) are given.The five 'dwarf' species are shaded in grey.No data concerning the ovaries and the size and number of the eggs are available from the twelfth species, S. elongata (Red Sea); therefore, this species is not considered in this only known small-sized (< 11 mm SL) LDF species, the first anal fin ray is positioned below the dorsal fin ray six.This is not as distinctly far anterior as in the larger (> 15 mm SL) adult LDF species and could be related to an even faster progenic development in S. nana sp.n., where specimens reach maturity at an earlier larval stage than larger LDF species.
Another character which allows the differentiation of Schindleria species is the dentition of the oral jaws, which is also hypothesized to be related to size (Ahnelt et al. 2022).In many gobiid fishes, teeth first appear in a larval stage of 4-9 mm TL (e.g.Kondo et al. 2012, Zanella et al. 2017, Hwang et al. 2018).Watson (2000) found teeth in Schindleria sp.larvae at a size of 4.2-4.5 mm SL, and Watson & Walker (2004) in S. pietschmanni larvae of 4.5-5 mm SL, with teeth first forming on the premaxilla and subsequently on the dentary.The large Schindleria species S. macrodentata, S. multidentata, S. praematura, S. nigropunctata, S. pietschmanni, and S. squirei have teeth on both oral jaws (Schindler 1930, Fricke & Abu El-Regal 2017, Ahnelt & Sauberer 2018, Ahnelt 2020, Robitzch et al. 2024).The smaller species (< 11 mm SL) S. brevipinguis, S. edentata, and S. qizma are characterized by the lack of teeth on the oral jaws (Watson & Walker 2004, Ahnelt et al. 2022, 2023) or by teeth only on the premaxilla and no teeth on the dentary as is the case for S. parva (Abu El-Regal et al. 2021).Ahnelt et al. (2022) discussed the possibility that the toothlessness of S. brevipinguis, S. edentata, and S. qizma and the lack of teeth on the dentary of S. parva could be the result of a more truncated development of these small species (< 11 mm SL) compared to the larger Schindleria species (> 15 mm SL).The few teeth on the lower jaw and their asymmetric position, or the overall lack of teeth in S. nana sp.n., a species of only 10 mm SL, supports this hypothesis.Seemingly, S. nana sp.n. reaches maturity during and prior to the full development of dentition, which is hence completed for the premaxilla but not for the dentary.This pattern also aligns with the observation by Watson & Walker (2004), where the formation of teeth (in S. pietschmanni) starts earlier in development on the premaxilla than on the dentary.This chronological appearance of the first teeth on the oral jaws is also known from other gobioids such as Bathygobius soparator Valenciennes, 1837 (Peters 1983) and Tridentiger obscurus Temminck & Schlegel, 1845 (Hwang et al. 2018), but it is reverse in some percomorphs such as Porichthys notatus Girard, 1854 (Vaz & Hilton 2022) or Sander lucioperca Linnaeus, 1758 (Löffler et al. 2008).
Another interesting morphological character of Schindleria is the relation between the depth of the hypural plate and the length of the urostyle.The urostyle is extremely elongated (Johnson & Brothers 1993) in Schindleria and plays a crucial role in swimming behaviour (Robitzch et al. 2022).Hence, we compared the relation of the depth of the hypural plate and the length of the urostyle of ten of the twelve described species and concluded that the hypural plate is distinctly less deep in the large species compared to the small species, regardless of whether they belonged to the LDF or the SDF species group (Table 1).Except for S. nana sp.n., the other three small species have less elongated but deeper bodies than the large species and deeper caudal peduncles (e.g.Ahnelt 2020, Ahnelt et al. 2022, 2023) (Fig. 8).This is also characteristic for S. edentata, the fifth small Schindleria species described, based on a single male.These characters are linked to the swimming abilities of fishes (Langerhans & Reznick 2010).Occurrence in different habitats often results in adaptive differences in fish species, particularly affecting locomotion (e.g.Walker 1997, McGuigan et al. 2003).Detailed habitat descriptions are missing for S. brevipinguis, S. edentata, S. nana sp.n., S. parva, and S. qizma, but seemingly all five small-sized species occur in calm waters of sheltered coastal lagoons (S. edentata, S. nana sp.n., S. parva) (Abu El-Regal et al. 2021, Ahnelt et al. 2022, present study) or in especially sheltered sites of a coral reef (S. brevipinguis, S. qizma) (Watson & Walker 2004, Ahnelt et al. 2023).
Yet an enigma, is the spawning strategy and the mode of reproduction in Schindleria.Although some authors assume a demersal egg deposition (Watson & Leis 1974, Whittle 2003, Thacker & Grier 2005), offshore records of up to 360 km distance to the next shore (Ahnelt & Sauberer 2020) may indicate that at least some species could be pelagic spawners.High diversity in the shape of urogenital papillae (Kon et al. 2007, Ahnelt et al. 2023) and in the size and number of eggs (Ahnelt & Sauberer 2018, Ahnelt et al. 2023) in Schindleria can be further indicative of differences in spawning strategies.Kon et al. (2007) described ten different types of male urogenital papillae.Among females, three morphotypes of urogenital papillae are known, i) two long, flat, and flexible bifurcated projections flanking the urogenital opening (Ahnelt et al. 2023), ii) a roundish and bulbous papilla with two short horn-like projections (e.g.Bruun 1940, Sardou 1974, Ahnelt 2019), and iii) a plain urogenital opening (Ahnelt & Sauberer 2018, Ahnelt 2020, this study).
The most common urogenital papilla morphotype among female Schindleria across the Indo-Pacific is a bulbous-like opening with two horn-like projections (Bruun 1940, Sardou 1974, Ahnelt & Sauberer 2020).Furthermore, in the ovaries of some large species (i.e. S. pietschmanni and S. praematura), more than one size class of eggs have been found in the same specimen (Jones & Kumaran 1964, Whittle 2003, Thacker & Grier 2005), indicating that these species are capable of reproducing multiple times.On the other hand, in small-sized species with few but large eggs (S. brevipinguis, S. nana sp.n., and S. qizma), only one size class of eggs has been found (Ahnelt et al. 2023, this study), which may hint towards a shorter lifespan in these species, with only a single spawning event, and/or at least the production of only a single, very small egg clutch.
Like in most teleost fishes, the ovaries of female gobioid fishes are paired and of about equal size (Thacker & Grier 2005).However, a shift from paired ovaries to eggs in a single, unpaired ovary, as in some very small-sized Schindleria species, is not likely linked to their small size.Similar to the larger Schindleria species, the very small S. parva has paired ovaries and relatively small eggs (1.0-1.8% of SL).
In contrast, the eggs of S. nana are about twice the size (3.4-3.9% of SL) of those found in S. parva and are arranged in a single, unpaired ovary (Table 4).Therefore, we conclude that the distinctly increased egg size likely induced the shift from paired-ovary eggs to a single-ovary egg row in Schindleria.
The high diversity in the shape of urogenital papillae and the size and number of eggs suggest reproductive isolation (Langerhans et al. 2016).Based on two genetic studies, Kon et al. (2007Kon et al. ( , 2011) ) identified 25 genetic species-level lineages from the Ryukyu and Ogasawara Islands (Japan) and Palau (Western Pacific).Many of these lineages were restricted to single islands, and because of the short lifespan and extremely short generation times of Schindleria (Kon & Yoshino 2002), high levels of endemism were proposed (Kon et al. 2011).
Diversity in egg size and the shape of urogenital papillae could result in reproductive isolation and favour a high level of endemism.Interestingly, nine of twelve to date known and formally described Schindleria species occur sympatrically at least with one other species: S. praematura and S. pietschmanni at Hawai'i, S. elongata, S. nigropunctata and S. edentata in the northern Red Sea, S. parva and S. qizma from the Central Red Sea near Jedda and S. brevipinguis and S. nana sp.n. from Lizard Island -Carter Reef from Queensland, Australia (Fig. 1); and the speciespairs are comprised of an LDF and SDF species.
Considering the vast distribution range of Schindleria, there may be hundreds of undescribed species within this unique genus.

Fig. 2 .
Fig. 2. The sampling sites of Schindleria nana (red star) and Schindleria brevipinguis (red square), the two smallest Schindleria species of the Pacific Ocean.The sampling location, Lizard Island (black square), is off the north-eastern Australian coast.Available at https:// apps.gov.au/reef-monitoring/reefs.
(3.1-3.6% of SL) (Figs.3, 4A).It differs from its congeners in the combination of the following characters: body elongated, slender, and not pigmented in preserved specimens; tail (postabdominal region) distinctly shorter than abdomen; origin of the dorsal fin distinctly anterior to origin of the anal fin (LDF type); predorsal-fin length 63.1-63.5% of SL; preanal-fin length 71.2-73.0% of SL; body depth at the origin of the

*
Both specimens: likely some eggs were already released.teethon the left and the right dentary, respectively, in the paratype(Figs.5, 6); except for the posterior-most tooth on the right dentary, all other teeth (left and right) of the paratype are very small and seemingly not fully developed; premaxilla long, thin with indistinctive postmaxillary process; maxilla long and thin, slightly widening distally; postabdominal region distinctly shorter than abdominal region, ends in an elongated, relatively long caudal peduncle (Figs.3, 4); urostyle elongated and flexed, its upturned tip attached to the antero-dorsal rim of the large, triangular hypural plate (depth of the hypural plate in urostyle length is 66.7%); externally, the preserved body without pigmentation except for the eyes; the two females have few (4-7) but very large eggs (3.4-3.9% of SL) in the abdominal cavity, arranged in a single row (Fig.7, Table

Fig. 5 .
Fig. 5. A, B -Dentition of Schindleria nana, paratype (AMS.I.23115-040), 8.5 mm SL; C -right lower jaw of Schindleria sp.(S. pietschmanni or S. praematura), Hawai'i (CAS uncatalogued) in dorsomedial view; the teeth were lost during the staining process, only the bases for the teeth are visible.A -dorsal view of the head, B -ventral view of the head.Teeth on the right dentary are encircled; two isolated teeth on the left dentary are marked with arrows.Note continuous row of small, conical teeth along the entire ventral margin of the premaxilla.ang -anguloarticular, bt -bases for the teeth, den -dentary, mck -Meckel's cartilage, pmx -premaxilla.Scale bar: 0.2 mm.

Fig. 7 .Fig. 8 .
Fig. 7. A, B -ventral view of the abdomen of A) the holotype of Schindleria nana, AMS.I.23115-004, 9.0 mm SL, four eggs (photo Oliver Macek and Harald Ahnelt) and of B) the holotype of Schindleria brevipinguis, AMS.I.23552-006, 8.4 mm SL, 10 eggs (photo Anthony Miskiewicz, Australian Museum) to show the single rows of few and large eggs.a -anterior, asterisks -squeezed and/or distorted eggs, white arrowheads point to anus, black arrowheads point to urogenital opening.Scale bar: 0.5 mm.

Table 1 .
Ratio of the depth of the hypural plate (HYP) to the length of the urostyle (UL) of adult Schindleria species of the 'long dorsal fin type' (n = 7) and of Schindleria species of the 'short dorsal fin type' (n = 3); latter are shaded in grey.Values for two species, S. elongata and S. parva, were not available.

Table .
* S. qizma: the ovaries are paired anteriorly but end up as a single ovary.