Syllophopsis (Hymenoptera: Formicidae) is a genus of small, inconspicuous ants, with 21 described species (Bolton 2016). Most Syllophopsis species are known only from the Afrotropical bioregion. One Syllophopsis species, however, has achieved broad distribution through multiple bioregions: S. sechellensis (Emery). Here, we examined the worldwide distribution of S. sechellensis, and evaluated where it may be native and where it has recently spread through human commerce.

Santschi (1915) described Syllophopsis as a subgenus of Monomorium. Some subsequent authors followed this designation, whereas others considered Syllophopsis to be a distinct genus. Most recently, Ward et al. (2014) revised the genus Monomorium and confirmed Syllophopsis as a distinct genus.

Syllophopsis workers are small, monomorphic, yellow to light brown in color, and have 12-segmented antennae with 3-segmented terminal clubs (Bolton 1987). Syllophopsis species resemble small Solenopsis thief ants. However, the 2 genera can be distinguished easily because Solenopsis workers have 2-segmented terminal clubs.

Of the 21 recognized species of Syllophopsis, 17 species are known only from the greater Afrotropical region: Africa, Madagascar, the Arabian Peninsula, and western Indian Ocean islands: S. adiastolon (Heterick), S. aureorugosa (Heterick), S. cryptobia (Santschi), S. dentata (Sharaf), S. elgonensis Santschi, S. ferodens (Heterick), S. fisheri (Heterick), S. gongromos (Heterick), S. hildebrandti (Forel), S. infusca (Heterick). S. jonesi (Arnold), S. kondratieffi (Sharaf & Aldawood), S. malamixta (Bolton), S. modesta (Santschi), S. saudiensis Aldawood, S. sersalata (Bolton), and S. thrascolepta (Bolton). In addition, 3 Syllophopsis species are known solely from outside the Afrotropical bioregion: S. subcoeca (Emery) (from the Neotropics), S. australica (Forel) (from Australasia, Indomalaya, and Oceania), and S. vitiensis (Mann) (from Fiji). Only S. sechellensis has been widely reported both from islands of the western Indian Ocean islands and from other parts of the world.

Emery (1894a) described Monomorium fossulatum sechellense (= S. sechellensis) from the Seychelles before Emery (1895) described the nominal subspecies of M. fossulatum from Burma. When Wilson & Taylor (1967) synonymized the 2 subspecies, they designated M. fossulatum as the senior synonym. However, based on the publication dates of the 2 names, Bolton (1995) designated M. sechellense as the senior synonym.

Like most Syllophopsis species, S. sechellensis has tiny eyes. One character, however, that distinguishes S. sechellensis from other members of the genus is that its entire mesopleuron is matte and reticulate punctate, whereas in all other described Syllophopsis species the mesopleuron is glossy and smooth.

Materials and Methods

Using published and unpublished records, we documented the worldwide range of S. sechellensis. We obtained unpublished site records from museum specimens in the collection of the Museum of Comparative Zoology, Cambridge, Massachusetts (MCZ). In addition, we used online databases with collection information on specimens by AntWeb ( www.antweb.org). We excluded records where identifications were recorded as uncertain, e.g., Onoyama (1976) reported “M. fossulatum (?)” from Okinawa, Japan.

We obtained geographic coordinates for collection sites from published references, specimen labels, maps, or geography websites (e.g., earth.google.com,  www.tageo.com, and  www.fallingrain.com). If a site record listed a geographic region rather than a “point locale,” and we had no other record for this region, we used the coordinates of the largest town within the region or, in the case of small islands and natural areas, the center of the region. We did not map records of S. sechellensis on boats, found in newly imported goods, or intercepted in transit by quarantine inspectors.

Published records usually included collection dates. In some cases, records did not include the collection dates for specimens, but we were able to determine or approximate the date based on information from museum specimens, on the collector's travel dates, or limit the date by the collector's date of death. We categorized each site record as belonging to 1 of 7 terrestrial bioregions (following Olson et al. 2001): Afrotropics, Palearctic, Indomalaya, Australasia, Oceania, Nearctic, and Neotropics.

Fig. 1.

Worldwide distribution records of Syllophopsis sechellensis.

Results

We compiled published and unpublished S. sechellensis specimen records from >160 sites (Fig. 1). We documented the earliest known S. sechellensis records for 43 geographic areas (countries, island groups, and West Indian islands), including many islands for which we found no previously published records, in the Indian Ocean (Comoro Islands, Îles Éparses, Mascarene Islands), Pacific (Palau), Atlantic (Cape Verde), and Caribbean (Guadeloupe, Grenada, Martinique, Mona, Puerto Rico, St. Croix, St. Lucia, St. Martin, St. Thomas, St. Vincent, and Trinidad) (Tables 1 and 2).

Records ranged in latitude from Blackall Range near Maleny, Queensland, Australia (26.8°S; Nakamura 2007) in the south to Tokyo, Japan (35.7°N; Schmidt 2010) in the north. The Tokyo record, however, is much further north than the next highest latitude record, on Amamioshima, Japan (28.3°N; Fukumoto et al. 2016), and therefore deserves confirmation. The geographic areas with the most sites records were Fiji (27), Hawaii (20), Madagascar (12), the Seychelles (12), and the Mascarene Islands (11).

J. K. W. collected S. sechellensis at the following 21 sites on 16 islands, including 3 on Pacific islands, 1 in Southeast Asia, 1 on an Atlantic island, and 16 on West Indian islands, and M. S. confirmed identifications (arranged from south to north, collection date and geographic coordinates in parentheses):

Tonga, 'Eua, eastern slope forest (23 Aug 1995, 21.375°S, 174.915°W)

American Samoa, Tutuila, Mapusaga, yard (16 Jun 1995; 14.324°S, 170.744°W)

American Samoa, Tutuila, Mapusaga, forest (18 Jul 1995; 14.322°S, 170.744°W)

Singapore, Katong, Amber Road, woods (28 Jul 2014; 01.299°N, 103.900°E)

Trinidad, La Seiva, chapel, parking area (24 Sep 2003; 10.684°N, 61.411°W)

Grenada, Woodlands, sugarcane (28 Jun 2006; 12.025°N, 61.741°W)

Barbados, Flower Forest, garden (29 Nov 2003; 13.202°N, 59.572°W)

St. Vincent, Wallilabou Falls, forest patch (3 Jul 2006; 13.246°N, 61.259°W)

St. Vincent, Sans Souci, grass by beach (6 Jul 2006; 13.296°N, 61.126°W)

St. Lucia, Grande Cul de Sac, north side of bay (8 Jul 2006; 13.987°N, 61.013°W)

Martinique, 1 km southwest of Simon, bananas (12 Jul 2011; 14.584°N, 60.876°W)

Martinique, Grosse Gouttière, secondary forest (18 May 2008; 14.670°N, 61.049°W)

Guadeloupe, Chateaubrun, roadside (7 Jun 2011, 16.250°N, 61.357°W)

Cape Verde, São Nicolau, Ribeira Brava, garden (22 Jul 2003; 16.617°N, 24.294°W)

St. Croix, St. George, botanical garden (1 Nov 2005; 17.716°N, 64.831°W)

St. Croix, Morning Star, roadside (3 Nov 2005; 17.762°N, 64.764°W)

St. Martin, Saint Jean, road to Mt. Fortune, roadside (24 May 2006; 18.049°N, 63.087°W)

St. Martin, Colombier, forest (23 May 2006; 18.073°N, 63.063°W)

Mona Island, Sardinera Beach, beachfront (2 Jun 2006; 18.089°N, 67.938°W)

St. Thomas, Nadir, weeds under large trees (7 Nov 2005; 18.321°N, 64.878°W)

Puerto Rico, San Juan, Park Luis Muñoz Marín (15 Oct 2005; 18.408°N, 66.073°W)

Table 1.

Earliest known records for Syllophopsis sechellensis in Old World bioregions.

Table 2.

Earliest known records for Syllophopsis sechellensis on Caribbean islands, Neotropics.

M. S. also identified numerous specimens of S. subcoeca that J. K. W. collected in the West Indies (e.g., from Barbados, Grenada, Guadeloupe, St. Croix, St. Lucia), as well as some from Singapore. Although all previously published records of S. subcoeca come from the New World, Torres & Snelling (1997) considered it to be an Old World exotic.

Discussion

Syllophopsis sechellensis has widespread records from the Old World tropics and subtropics, particularly from islands (Table 1; Fig. 1). Recently, Wetterer et al. (2016) reported the first record of S. sechellensis in the New World, on the island of Barbados. Here, we report additional Neotropical records, from 11 other islands of the West Indies.

Syllophopsis sechellensis is now known from 6 of the 7 terrestrial bioregions where ants occur: Afrotropics, Indomalaya, Australasia, Oceania, Palearctic, and Neotropics. For such a wide-ranging species, S. sechellensis has remarkably few site records. Despite sampling at many sites on Pacific, Atlantic, and West Indian islands, J. K. W. found S. sechellensis at no more than 2 sites per island (e.g., 1 of 123 Barbados sites and 1 of 106 Grenada sites; see Results). Whereas S. sechellensis may simply be relatively rare throughout most of its range, it seems more likely that this species is overlooked at many sites where it occurs. We expect that more thorough surveys will find that the actual distribution of this species is even broader than currently documented.

Although S. sechellensis is certainly native to the Old World, the extent of its original native range (i.e., before being spread by human commerce) is unclear. Wilson & Taylor (1967) wrote “taken at face value, the limited evidence seems to suggest that fossulatum is native to tropical Asia and includes in its variation a weakly sculptured form (sechellense) that has been distributed to a limited extent by commerce.” Dlussky (1994) similarly listed M. fossulatum (= S. sechellensis) as an Asian species. Sarnat & Economo (2012) listed S. sechellensis as a Pacific native. However, the geographic ranges of S. sechellensis and other Syllophopsis species suggest that S. sechellensis may well be native to Madagascar and neighboring islands in the western Indian Ocean. Heterick (2006) noted that in Madagascar, S. sechellensis “is found in dry tropical forest and gallery forest habitats.” Occurrence in intact native forests is usually indicative of a native species. We found no compelling evidence that S. sechellensis is exotic to the western Indian Ocean or to Southeast Asia. It is possible that S. sechellensis is native to both these areas. There is, however, an enormous distributional gap between these areas: neither S. sechellensis nor any other Syllophopsis species has been reported from India, Sri Lanka, or Bangladesh.

Wilson & Taylor (1967) listed S. sechellensis (as M. fossulatum) in Polynesia as a tramp species “certainly distributed by recent human commerce.” Wetterer (2002) and Wetterer & Vargo (2003) listed S. sechellensis as an exotic species in Tonga and Samoa. Jourdan (2006) considered S. sechellensis to be an invasive species in New Caledonia. Ward (2007) and Ward & Beggs (2007) listed S. sechellensis as an invasive species in Fiji. It is striking that the greatest numbers of S. sechellensis site records are from Fiji and Hawaii. It is possible that these apparently exotic populations of S. sechellensis reach high densities due to escape from natural enemies.

Syllophopsis sechellensis is almost certainly exotic to the isolated Atlantic islands of St. Helena and Cape Verde, and to the New World. Syllophopsis sechellensis was first found in the New World in 2003, but is now known from 12 West Indian islands, suggesting that the spread of this species through the West Indies may be fairly recent. Alternatively, earlier specimens of S. sechellensis from the West Indies may have been misidentified as S. subcoeca, a species that Emery (1894b) described from St. Thomas in the US Virgin Islands, and that was previously the only Syllophopsis species known from the New World.

Research on the genetic diversity of S. sechellensis at sites around the world is needed to resolve questions concerning the native range and invasion history of this species. Genetic studies should also evaluate Old World specimens identified as S. subcoeca to determine whether this species, previously reported only from the Neotropics, may actually be native to the Old World, as are all other known members of the genus.