Gnathia indoinsularis, Jörundsdóttir, 2004

Jörundsdóttir, J. Svavarsson K., 2004, A new gnathiid species (Crustacea, Isopoda, Cymothoida) from Rodrigues, Mauritius, Indian Ocean, Journal of Natural History 38, pp. 3103-3111 : 3104-3109

Gnathia indoinsularis n. sp.

( figures 1–6)

HOLOTYPE: male, 2.7 mm, Rodrigues, Mauritius, RDS Stn 14, Gravier, 19‡43.878’S, 63‡29.610’E, 19 September 2001, inner margin of reef crest, dead coral, intertidal, coll. N. L. Bruce, NHM 2004.213.

PARATYPES: four males, Rodrigues, Mauritius, RDS Stn 13, Gravier, 19‡43.825’S, 63‡29.544’E, 19 September 2001, outer lagoon, Acropora patch in v 0.5 m, coll. N. L. Bruce, NHM 2004.214–217; two males (dissected male, 2.5 mm), one female, Rodrigues, Mauritius, RDS Stn 14, Gravier, 19‡43.878’S, 63‡29.610’E, 19 September 2001, inner margin of reef crest, dead coral, coll. N. L. Bruce, NHM 2004.218–219 (one male, one female), IMNH 2004.02 .11.1 (one male); three males, one female, Rodrigues, Mauritius, RDS Stn 15, Gravier, 19‡43.878’S, 63‡29.610’E, 19 September 2001, inner margin of reef crest, Porolithion clumps, coll. N. L. Bruce, NHM 2004.220–223 .

Diagnosis. Frontal margin unevenly transverse, with shallow, narrow excavation. Inferior, bifid mediofrontal process; superior frontolateral process rounded. Mandibles stout, simple, with dentate blade. Lobi laterales absent on pereonite 6.

Description. Body length 2.2–2.7 mm. Body approximately 2.7 times as long as wide ( figure 1A, B). Cephalosome ( figure 2A–D) lateral margins evenly convex, about 1.5 times wider than long. Frontal margin unevenly transverse, with shallow, narrow excavation, width of excavation around 0.2 of anterior cephalon border; mediofrontal process inferior, bifid; superior frontolateral process rounded; wide dorsal sulcus on cephalon; paraocular ornamentation present; eyes large, lateral and sessile.

Pereonite 3 widest, slightly wider than cephalosome ( figure 1A, B). Pereonites 2 and 3 subequal in width; pereonite 6 longest; pereonite 7 almost as wide as pleonite 1. Pleonites 1 and 2 subequal in width ( figure 1B), posterior pleonites decrease in width towards posterior end; pair of setae posteriorly on all pleonites. Lobi laterales absent.

Mandible fairly straight ( figure 2E, D), stout and simple, approximately 0.8 times length of cephalon, with dentate blade, with unarmed carina, internal lobe absent, mandibular seta about halfway from distal end.

Antenna 1 ( figure 2F) short, slightly shorter than antenna 2; peduncle articles 1 and 2 with few plumose setae distally; flagellum with three articles, approximately 1.2 times as long as article 3 of peduncle, articles 2 and 3 with aesthetascs distally.

Antenna 2 ( figure 2F) peduncle articles 3 and 4 with several long, plumose setae distally; flagellum with seven articles.

Maxilliped ( figure 3A) with five articles; article 1 1.9 times as wide as article 2; articles 1 and 2 partly fused; external margin of article 1 fringed with fine, small setae; external margins of articles 2–5 with stout plumose setae, three on article 2, seven on article 3, five on article 4, six on external margin and distally on article 5; two slender, distal setae on article 5.

Pylopod ( figure 3B, C) with three articles, internal margin fringed with fine setae; lateral margin of article 1 with about 21 plumose setae, two simple setae present on distal end, one simple seta near internal margin; article 2 fringed with

fine setae, two simple setae distally; article 3 minute.

Pereopods 2–6 ( figures 4A–C, 5A, B) stout, similar in shape, pereopod 6 largest. Small, thin cuticular extensions on most articles. Propodus of pereopods 2–6 with robust seta medioventrally and ventrodistally, ventrodistal seta slightly larger than medioventral one; slender short seta and plumose seta dorsodistally, slender seta dorsally. Cluster of setae distally on ventral margin of carpus, carpus of pereopods 2 and 6 distally with pectinate setae in cluster of simple setae; two pectinate setae dorsodistally on merus of pereopod 6.

Pleopod 1 ( figure 6A) protopod 1.7 times wider than long, medially with two coupling hooks; endopod of similar length of exopod; 2.1 times longer than wide, distally with seven plumose setae; exopod 2.1 times longer than wide, distally with eight plumose setae.

Pleopod 2 ( figure 6B) with protopod around 1.8 times wider than long, medially with two coupling hooks; endopod of similar length to exopod; 1.9 times longer than wide, distally with eight plumose setae; exopod 2.1 times longer than wide, distally with nine plumose setae. Appendix masculina absent from male paratype (2.5 mm) at station RDS 14.

Pleopods 3–5 similar to pleopods 1 and 2.

Pleotelson ( figure 6C) slightly shorter than wide; pair of small setae and pair of slender setae dorsally, near lateral margins; two setae distally.

Uropodal ( figure 6D) endopod approximately as long as exopod, approximately 2.1 times as long as wide, fringed with about six plumose setae on distomedial margin and on apex, longest plumose setae about 1.9 times endopod length, one long simple seta laterodistally, two long setae on lateral margin; four plumose setae on dorsal surface. Exopod approximately 4.1 times as long as wide, fringed with four plumose setae mediodistally and distally, six simple setae laterally and laterodistally.

Etymology. The name combines Indian Ocean with the Latin insula (island).

Remarks. The genus Gnathia is large, with many similar species. G. indoinsularis sp. nov. is most similar to G. calsi Müller, 1993b , G. cooki Müller, 1989 and G. lignophila Müller, 1993a in having a bifid, inferior mediofrontal process and a rounded, superior frontolateral process. G. indoinsularis differs, however, from these species in the presence of the shallow, narrow excavation on the frontal margin. It differs further from Gnathia firingae ( Müller, 1991) , the only species recorded from the Mascarene Islands, in having the bifid mediofrontal process considerably posterior to the superior frontolateral process.

Nine species of Gnathia have been described from the Indian Ocean and a further 11 species of the genus await description ( Kensley, 2001). Gnathia , with 31 species, is the most species-rich genus of the family in the Indian Ocean ( Kensley, 2001; Svavarsson, 2002; Svavarsson and Gísladóttir, 2002). Only two species have been reported from the Mascarene Islands ( Mauritius, Réunion and Rodrigues), Caecognathia sp. nov. from Mauritius ( Kensley, 2001) and Gnathia firingae ( Müller, 1991) from Réunion. G. indoinsularis sp. nov. was the only gnathiid species recorded from Rodrigues, and these scarce reports may indicate undersampling in the region in general. The sampling at Rodrigues was, however, fairly extensive (N. L. Bruce, personal communication), at least intertidally and shallow subtidally. This may indicate that gnathiids may be scarce on isolated oceanic islands. There are limited data on the diversity of coral fish species on Rodrigues. Gnathiid larvae are parasites of coral fish ( Grutter, 1999) and fish diversity may influence the diversity of gnathiids.