A new Stygocyathura (Isopoda, Cymothoida, Anthuridae) from the subterranean waters of Socotra Island (Indian Ocean)

Abstract

A new species of the Tethyan genus Stygocyathura (Isopoda, Cymothoida), Stygocyathura taitii n.sp., collected in a well of the Socotra Island, is described here. The new species, on the basis of the similarity of the structure of the appendix masculina, appears related to S. numeae, a thalassostygobiotic species from New Caledonia.

http://zoobank.org/urn:lsid:pub:E3865EEB-A3F6-4AA9-819F-0C7D4CAED246

Keywords:

• Stygobionts
• Anthuridae
• Stygocyathura taitii n. sp.
• Socotra Archipelago
• Tethys

Introduction

Several species of crustaceans from subterranean fresh water were collected during two study expeditions to the Socotra Island. This type of environment is located inside the plateau of Cretaceous and Tertiary limestone (Beydoun and Bichan 1970) overlying the old igneous and metamorphic center of the island.

During these investigations a new species of stygobiotic isopod crustacean belonging to the family Anthuridae was discovered. Anthuridae are organisms living in marine and brackish water. As it happens also in other groups of marine isopods, many species have colonized different subterranean aquatic environments, probably because of distinct paleoclimatic and/or geological events. The species of the genus Stygocyathura Botosaneanu & Stock, 1982 are the result of progressive steps in the invasion of groundwater ecosystems. For example, Stygocyathura rapanuia (Botosaneanu, 1987) from Rapa Nui (Easter Island), and S. numeae (Wägele, 1982), from New Caledonia, are thalassostygobionts living in the coastal interstitial environment. Other species, as S. munae (Botosaneanu, 2003) from Sulawesi and S. salpicinalis (Botosaneanu & Stock, 1982) from Haiti, live in brackish water. Stygocyathura milloti (Chappuis, Delamare & Paulian, 1956) from Reunion Island and S. broodbakkeri (Wagner, 1990) from Dominican Repubblic occour in littoral freshwater springs, whereas S. cuborientalis (Botosaneanu & Stock, 1982) from Cuba, S. parapotamica (Botosaneanu & Stock, 1982) from Jamaica, S. fijiensis (Wägele, Coleman & Hosse, 1987) from Fiji, S. papuae (Wägele, Coleman & Hosse, 1987) from Papua New Guinea, and S. wadincola (Botosaneanu & Stock, 1997) from Oman, are rhithro/potamo-stygobionts.

Other species are freshwater karsto-stygobionts living in caves or in phreatic environment. Most of them are found in the area of the Caribbean Sea: S. sbordonii (Argano, 1971) and S. mexidos (Botosaneanu, 2008) from Mexico, S. univam (Botosaneanu, 1983) from Venezuela, S. orghidani (Negoescu Vladescu, 1983) and S. specus (Bowman, 1965) from Cuba, S. motasi (Botosaneanu & Stock, 1982) from Haiti, S. hummelincki (Botosaneanu & Stock, 1982) from Aruba, and S. curassavica (Stork, 1940) from Curaçao. Some other species with similar ecology are present in the western Pacific: S. chapmani (Andreev, 1982a) from Sarawak, S. beroni (Andreev, 1982b) from New Guinea, and S. filipinica (Botosaneanu & Sket, 1999) from Philippines.

Stygocyathura is an extremely archaic taxon, as evidenced by its cosmopolitan distribution and its remarkable adaptive radiation in the subterranean aquatic realm, despite the lack of current dispersal ability. The genus Stygocyathura has had a troubled history but it is currently accepted by all scholars who have dealt with it (Boyko et al. 2008). Wägele (1982) proposed that the stygobiotic species ascribed to the marine-brackish water genus Cyathura Norman & Stebbing, 1886, mainly characterized by a similar general morphology of the distal end of masculine appendix, could have had a common ancestor. Botosaneanu and Stock (1982) introduced the taxon Stygocyathura as a subgenus of Cyathura to comprise the species of stygobiotic cyathurans. However, in their reconstrucion of the phylogenetic relationships among the various species, they did not take into consideration the importance of the masculine appendix. Botosaneanu (1987) casted doubts “upon the possibility to use this organ successfully for phylogenetics reconstructions”. This is probably the reason why he underestimated the extraordinary resemblance of masculine appendix between S. rapanuia, from the marine interstitial of Rapa Nui (still considered belonging to the subgenus Cyathura) and S. sbordonii from a Mexican cave. Subsequently, Botosaneanu and Stock (1997), when describing S. wadincola from the hyporheic environment of the underflow in an Oman wadi, re-evaluated the relevance of the morphology of the appendix masculina, along with other characters, following the considerations by Wägele (1985). Stygocyathura was elevated to genus rank by Poore (2001).

Actually, there are still many obscure points that still need to be clarified and many morphological characters proposed by various authors for the diagnosis of the genus should be confirmed. In fact, some species have no information on the morphology of males while all the species with known males do not shaw a similar general structure of the appendix masculina, same number of retinacula on the first pleopods, same shape of uropods, or similar richness of aesthetascs of the male first antenna.

Abbreviation

MZUF = Museo di Storia Naturale dell’Università, Sezione di Zoologia “La Specola”, Florence.

Taxonomy

Order Isopoda Latreille, 1817

Suborder Cymothoida Wägele, 1989

Family Anthuridae Leach, 1814

Genus Stygocyathura Botosaneanu & Stock, 1982

Type species: Cyathura curassavica Stork, 1940

Stygocyathura taitii n. sp.

(Figures 1, 2)

Material examined

Yemen, Socotra Archipelago, Socotra Island: Holotype ♂, Shelelehen, 12°38′05.5″N 54°11′41.4″E, with traps in well, 14.01.2003, Stefano Taiti leg. (MZUF 5015). Paratypes 2 ♂♂ (in 5 micropreparations), 9 ♀♀ (1 ♀ in 3 micropreparations), same data as holotype (MZUF 5016).

Etymology

The new species is named after our colleague and friend Stefano Taiti, who collected the specimens, for his dedication to the study of subterranean isopods.

Description

Maximum body length: ♂ 8.2 mm, ♀ 9 mm. Blind and unpigmented. Pleon shorter than pereonite VII, pleonites 1–5 medially fused, pleonite 6 with a median dorsal longitudinal groove, fused with telson (Figure 1a). Telson tapered distally, with six apical setae, two of which distinctly longer (Figure 1b). Flagellum of the antenna with a tuft of about 20 simple setae (Figure 1c). Male antennule with 19–21 flagellar aesthetascs (Figure 1d); only three aestetascs and three long setae on flagellum of female (Figure 1e). Second segment of mandibular palp three times longer than wide, third segment longer and thinner than first with a row of eight short and stout setae (distal one pectinate) and two thin and long subdistal setae (Figure 1f). Maxillule and maxilliped as in Figure 1g and 1h, respectively. Propodus of gnathopod with a row of simple setae on palmar margin and a tuft of about 10 simple setae on internal distal corner (Figure 2a). First pleopod more than two times as long as wide, subovoidal with three retinacula on protopod, exopod with about 14 plumose setae on outer margin (Figure 2b). Appendix masculina of male second pleopod strong, surpassing branch of pleopod, and with a subapical hook-like lobe directed laterally and backwards (Figure 2c,d). Uropod with narrow exopod, not folding over telson, reaching in length first third of distal article of endopod (Figure 2e).

Figure 1 Stygocyathura taitii n. sp. Paratype male (a–d and f–h), female (e): a, habitus; b, telson; c, antenna; d, male antennule flagellum; e, female antennule flagellum; f, mandible; g, first maxilla; h, maxilliped without endopodite.

Figure 2 Stygocyathura taitii n. sp. Paratype male: a, gnatopod; b, first pleopod; c, second pleopod, exopodite omitted; d, apex of appendix masculina of second pleopod; e, uropod; f, third pleopod.

Remarks

As already mentioned in the introduction Wägele (1982) identified a group of species of the genus Cyathura characterized by an appendix masculina with a lateral lobe of the bifurcate apex which extends and folds in various ways. The author assumed that it may be an homologous structure that could indicate a common ancestor after comparison among species. This group of species were subsequently included in the genus Stygocyathura: S. curassavica, S. specus, S. numeae, S. milloti, S. sbordonii, and S. chapmani. Two more species subsequently discovered, S. rapanuia and S. wadincola, were included in this group. Wägele (1985) supported the same positions. We agree with the proposal by Wägele and believe that the new species here described should also be included in this group. The distal end of the appendix masculina of S. taitii n. sp. closely resembles that of the tiny interstitial species S. numeae from New Caledonia. The latter species differs from S. taitii n. sp. in the general dimensions of the body (the length/width ratio is 11.53 in S. numeae and 13.25 in the new species), the length/width ratio of the propodus of the pereopod 1 (1.15 for S. numeae and 1.73 for the new species) and the setae disposition on its palmar margin, and the shape of the exopodite of uropods.

With the new species described here, a total of 23 species of Stygocyathura are known. Stygocyathura taitii n. sp. from Socotra is geographically close to S. wadincola from Oman, and it appears to be part of the complex theory of the Tethyan relicts. Socotra is a hotspot of biodiversity and, among the hundreds of endemics that populate the Island, there are some that share a similar natural history with the new species. In a study of a new freshwater subterranean amphipod genus of the family Hadziidae from Socotra, Indowekelia Holsinger & Ruffo, 2002, the authors found a close affinity with the genus Wekelia Shoemaker, 1942 from Cuba (Holsinger and Ruffo 2002). The two authors stated that other groups of marine crustaceans invaded the freshwater environments bordering the ancient coasts of the Tethys which in the early Cretaceous formed a continuous sea between Laurasia and Gondwana. Ancestors of recent mainland species are believed to have colonised subterranean freshwater during marine regressions in the late Cretaceous. Certainly, this is a valid hypothesis, based on some factual data. However, the genus Stygocyathura presents some aspects that seem even more intriguing, like that of the interstitial marine species S. rapanuia from the volcanic island of Rapa Nui. The apical part of the appendix masculina of this species is extremely similar to that of S. sbordonii from a Mexican cave, a possible affinity between the two species. The coasts inhabited by S. rapanuia are among the most isolated in the world (2,000 km from Chile) and the island was formed less than 1 million years ago. It might be difficult to explain how S. rapanuia (or its ancestor) reached so recently this remote island. Phoresy or rafting from Central or South American coasts may be summoned, and/or the existence of temporary bridges of intermediate emerging volcanoes.

A similar situation can be found by comparing the Socotran S. taitii n. sp. with S. numeae from New Caledonia. As in the previous case, these two species are very similar to each other, one living in groundwater and the other in marine interstitial, yet separated by a long distance, but this case may be easily set in a Tethyan relict explanatory framework.

Disclosure statement

No potential conflict of interest was reported by the authors.