Talorchestia gracilis ( Dana, 1852a )

Azman, B. A. R., 2017, The talitrid amphipod genus Talorchestia from the South China Sea to the Indonesian Archipelago (Crustacea, Senticaudata), Zootaxa 4319 (3), pp. 401-434 : 411-412

Talorchestia gracilis ( Dana, 1852a)

( Figs 9–12 )

Talitrus gracilis Dana, 1852a: 201 .

Orchestia (Talorchestia) gracilis .— Dana, 1853: 861, pl. 57, fig. 5.

Talorchestia gracilis .— Spence Bate, 1862: 15, pl. 2, fig. 5.— Stebbing, 1906: 551.—? Chilton, 1925: 535 (Tale Sap, north of Songkhla, Thailand).— Morino & Miyamoto, 1988: 97 (key).

Orchestia gammarellus .— Della Valle: 1893: 499, 511, 947 (in part).

Type material. Neotype, male, 12.2 mm, UKMMZ-1428 (SEM pin and 4 stubs), Pulau Layang-Layang , Swallow Reef, South China Sea (7°22.383'N 113°50.65'E), beach, supralittoral zone, B.A.R. Azman, 11 December 2010.

Additional material examined. 3 males, 1 female, 11.6 mm (SEM pin and SEM stub), 3 females, UKMMZ- 1427, same data as neotype .

Type locality. Pulau Layang-Layang , Swallow Reef, South China Sea (7°22.383'N 113°50.65'E).

Ecological type. Beach-hoppers (mainly coastal supralittoral/intertidal leaf-litter/wrack, non-substrate modifying talitrids).

Description. Based on neotype, male, 12.2 mm, UKMMZ-1428.

Head. Antenna 2 as long as or slightly longer than body (97%); peduncular article 5 longer than article 4; flagellum final article large, cone-shaped forming a virgula divina, with apical cluster of 'imbricated' setae. Labrum upper lip with apical setal patch; epistome without robust setae, without many pores. Mandible left lacinia mobilis with 5-cusps. Labium distolateral setal tuft absent; without inner plates. Maxilla 1 with small palp, 2-articulate.

Pereon. Gnathopod 1 carpus 1.8 × as long as propodus, 3.1 × as long as broad; propodus subrectangular, anterior margin with 5 or 6 groups of robust setae; dactylus subequal in length to palm. Gnathopod 2 sexually dimorphic; basis slender; merus and carpus separate; carpus triangular, reduced (enclosed by merus and propodus), posterior lobe absent, not projecting between merus and propodus; propodus subovate, twice as long as wide, palm reaching between about 62¾ along posterior margin, palm smooth, without protuberance near dactylar hinge, lined with robust setae, posteromedial surface of propodus with groove; dactylus longer than palm, curved, without anteroproximal bump, posterior margin smooth, with shallow posteroproximal sinus. Pereopods 2–4 coxae as wide as deep. Pereopod 3 dactylus without anterodistal patch. Pereopod 4 significantly shorter than pereopod 3; carpus similar in length to pereopod 3 carpus; dactylus without anterodistal denticular patch. Pereopod 5 propodus distinctly longer than carpus; dactylus without anterodistal setal patch. Pereopod 6 coxa posterior lobe medial view posteroventral corner rounded, posterior margin oblique to ventral margin. Pereopod 7 basis lateral sulcus present, slightly pronounced, posterodistal lobe absent; distal articles (merus and carpus) slender; merus posterior margin straight.

Pleon. Epimeron 1 robust setae along ventral margin unknown. Epimeron 2 subequal in length to epimeron 3, posterior margin smooth. Epimeron 3 posterior margin smooth, with minute setae, posteroventral corner with large subacute tooth. Uropod 1 peduncle with about 12 robust setae; inner ramus subequal in length to outer ramus, with 5–6 marginal robust setae, with 3 or more apical robust setae. Uropod 2 peduncle with about 12 robust setae; inner ramus subequal in length to outer ramus, with 7 medial and 5 lateral robust setae; outer ramus with 4 marginal robust setae in one row. Uropod 3 peduncle with 9 robust setae; ramus not fused to peduncle, 3 × as long as broad, linear (narrowing), with 6–7 marginal robust setae. Telson dorsal midline complete, with 13 robust setae per lobe.

Female (sexually dimorphic characters). Based on female, 11.6 mm, UKMMZ-1427. Antenna 2 more than half but not exceeding body length. Gnathopod 1 carpus 1.9 × as long as propodus, 3.4 × as long as broad; anterior margin with 5 groups of robust setae. Gnathopod 2 carpus well developed (not enclosed by merus and propodus), posterior lobe present, projecting between merus and propodus; not lined with robust setae, without cuticular patch at corner of palm; dactylus shorter than palm.

Remarks. Talorchestia gracilis has not been recorded since its original description ( Dana 1852a) 165 years ago. The original description is sparse and as the type species of the genus, its status is important. New material was collected in 2010 at the atoll Pulau Layang-Layang, Swallow Reef, about 300 km west of the original type locality in the Balabac Strait. We have designated a male from this collection as the neotype to stabilise the species and the genus.

Talorchestia gracilis belongs in the group without a distal protuberance on the palm of male gnathopod 2. It is the only species in the genus with antenna 2 longer than the body.

Distribution. South China Sea. Pulau Layang-Layang, Swallow Reef (this study); Balabac Strait ( Dana 1852a).

Figures

FIGURE 9. Talorchestia gracilis, male, neotype, 12.2 mm, UKMMZ- 1428, female, 11.6 mm, UKMMZ- 1427, Pulau Layang- Layang, Swallow Reef, South China Sea. Scale lines for male H, U 1, U 2, UR, EP 1 - 3 represent 0.5 mm, remainder represent 0.1 mm.

FIGURE 10. Talorchestia gracilis, male, neotype, 12.2 mm, Pulau Layang-Layang, Swallow Reef, South China Sea, UKMMZ- 1428. Scale lines represent 0.5 mm.

FIGURE 11. Talorchestia gracilis, male, neotype, 12.2 mm, UKMMZ- 1428; female, 11.6 mm, Pulau Layang-Layang, Swallow Reef, South China Sea, UKMMZ- 1427. Scale lines represent 0.5 mm.

FIGURE 12. Talorchestia gracilis, male, neotype, 12.2 mm, Pulau Layang-Layang, Swallow Reef, South China Sea, UKMMZ- 1428. Scale lines represent 0.5 mm.