Type species. Gondwanorchestia tristanensis sp. nov., original designation.
Included species. Gondwanorchestia includes 2 species: G. scutigerula ( Dana, 1852) and G. tristanensis sp. nov.
Category. Mascupod.
Ecological type. Beach-hoppers, field-hoppers.
Etymology. Gondwana reflecting the origin of this genus coupled with the stem orchestia.
Diagnostic description ( male). Head. Eye medium (1/5–1/3 head length). Antenna 1 long, reaching to at least midpoint of peduncular article 5 of antenna 2. Antenna 2 peduncular articles slender or slightly incrassate (expanded) in male; article 3 without plate or process ventrally. Labrum epistome without many large robust setae over surface; median groove with robust setae. Mandible left lacinia mobilis 5-cuspidate, fifth article rudimentary. Maxilliped palp article 2 with distomedial lobe; article 4 apparently fused with article 3.
Pereon. Gnathopod 1 sexually dimorphic; subchelate; posterior margin of carpus and propodus each with lobe covered in palmate setae; propodus ‘subtriangular’ with well-developed posterodistal lobe, palm transverse. Gnathopod 2 subchelate; posterior margin of merus, carpus and propodus each without lobe covered in palmate setae; propodus palm oblique; dactylus not modified distally, blunt or acute, shorter than posterior margin of propodus. Oostegites setae with simple smooth tips. Pereopods 3–7 bicuspidactylate. Pereopod 4 subequal or slightly shorter than pereopod 3; dactylus thickened proximally with notch midway along posterior margin. Pereopod 5 short, less than 2/3 length of pereopod 6; dactylus long, slender. Pereopods 6–7 without row of short setae along posterior margin of dactyli. Pereopod 6 not sexually dimorphic; basis expanded; shorter than pereopod 7. Pereopod 7 sexually dimorphic; basis broadly expanded, plate-like; merus and carpus very weakly expanded.
Pleon. Pleonites 1–3 without dorsal spines. Pleopods 1–3 all well-developed. Epimera 1–3 slits absent; a row of slender setae along ventral margin of each epimeron. Uropod 1 male exopod not sexually dimorphic; peduncle distolateral robust seta absent; endopod with marginal robust setae in 2 rows; exopod with marginal robust setae in two rows. Uropod 2 rami without apical spear-shaped setae; exopod with marginal robust setae in 1 row; endopod with marginal robust setae in 2 rows. Uropod 3 ramus linear, weakly narrowing distally; shorter than peduncle. Telson as broad as long, tapering distally, entire, without groove, with marginal and apical robust setae, with 7–9 robust setae per lobe.
Remarks. Gondwanorchestia is most similar to Orchestia, a genus that also occurs in southern South America (undescribed species, Myers & Lowry, in prep.). The main difference between these two genera is in the form of sexual dimorphism in male pereopod 7 and a prominent row of ventral setae on epimera 1–3 in Gondwanorchestia. In Gondwanorchestia, the basis is expanded, plate-like, and the merus and carpus are only slightly expanded. In Orchestia, the basis is unexpanded, while the merus and carpus are variously expanded and the ventral epimeral setae are absent.
Distribution. Tristan da Cunha ( Stephensen 1949); Tierra del Fuego ( Dana 1852), South Atlantic Ocean.