Mexorchestia Wildish & Lecroy, 2014, n. gen.

Mexorchestia n. gen.

“ Tethorchestia ”: LeCroy, 2011, pp. 757.

Type species. Mexorchestia carpenteri carpenteri n. sp. and subsp.

Component species. Mexorchestia carpenteri carpenteri n. sp. and subsp.; M. carpenteri raduloviciae n. sp. and subsp.

Etymology. Refers to the Gulf of Mexico, where the new taxon was originally found, and to the genus Orchestia, to which it is related.

Diagnosis. Eyes large, greater than one third head length; antenna 1, approximately one half length of antenna 2 peduncle, not extending beyond peduncle article 4; antenna 2 not sexually dimorphic, that of male slender, not incrassate, without ventral plate on peduncle article 3; upper lip without robust setae; mandible, left lacinia mobilis 4–5 dentate; maxilliped, palp 4-articulate, article 2 with well-developed medial lobe, article 4 reduced; gnathopod 1 of male subchelate, palm well-developed, transverse, longer than dactyl; carpus and propodus, posterior margin with rounded lobe covered with palmate setae; gnathopod 1 of female parachelate, palm poorly developed, shorter than dactyl; gnathopod 2 of male subchelate, basis stout, without tubercles on anterior margin, merus and carpus free, unfused, dactyl distally attenuate, extending two thirds length of propodus, without tooth on cutting edge; gnathopod 2 of female, oostegite spatulate, with 15–35 long, simple marginal setae, basis subovate, expanded medially, anterior margin evenly convex; peraeopods 3–7 cuspidactylate; peraeopods 5–7 without slender setae lining anterior margin of dactyl; peraeopod 7 weakly sexually dimorphic, merus and carpus of male not or very weakly incrassate, propodus of male with 2–4 tufts of long, stiff, slightly medial slender setae on anterodistal and distal margins; pleon segments 1–3 without dorsal spines; epimera 1–3 without vertical slits; pleopods 1–3 slightly reduced, peduncles slender; uropods 1–2, rami without apical spade-like robust setae; uropod 1 not sexually dimorphic, peduncle without well-developed dorsolateral robust seta distally, outer ramus without marginal robust setae; uropod 2, outer ramus subequal to inner in length; uropod 3 well-developed, ramus shorter than peduncle, at least twice as long as deep, cylindrical, not laterally compressed, tapering distally, tip subacute; telson apically notched, with 8–10 robust setae per lobe, distinctly shorter than uropod 3, not extending beyond distal end of peduncle.

Remarks. Mexorchestia belongs to the cuspidactylate group of non-substrate modifying (sensu MacIntyre 1963) beachfleas of Bousfield (1982, 1984). This group also includes the genera Australorchestia Serejo & Lowry, 2008; Chroestia Marsden & Fenwick, 1984; Floresorchestia Bousfield, 1984; Notorchestia Serejo & Lowry, 2008; Orchestia Leach, 1814; Paciforchestia Bousfield, 1982; Platorchestia Bousfield,1982; Tethorchestia Bousfield, 1984; Tongorchestia Lowry & Bopiah, 2013; Transorchestia Bousfield, 1982, Traskorchestia Bousfield, 1982 and Vallorchestia Lowry 2012. Mexorchestia can be distinguished from Australorchestia, Notorchestia, Orchestia, Paciforchestia, Tongorchestia, Transorchestia and Traskorchestia by the absence of marginal robust setae on the outer ramus of uropod 1 and from Chroestia, Tethorchestia and Vallorchestia by the lack of a large distolateral robust seta on the peduncle of uropod 1. The remaining genera (Floresorchestia and Platorchestia) are distinguished from Mexorchestia by the shape of the basis of the female second gnathopod. In the first two genera, the basis is inverted pyriform in shape, with the broadest expansion occurring proximally; in Mexorchestia, the basis is evenly expanded anteriorly, with the broadest expansion occurring medially.

Mexorchestia is close to the genus Tethorchestia and Bousfield (1984) originally considered Mexorchestia carpenteri n. sp. to be a member of that genus (Bousfield 1984, as Tethorchestia sp. B). Both genera have the distinctive, apparently unique, tufts of stiff, elongate, anterodistal and distal slender setae on the propodus of peraeopod 7 in the male, although the number of setal groups differs between the two (Mexorchestia has 2–4 groups; Tethorchestia has 5–6 groups). However, Mexorchestia differs from Tethorchestia in the subovate basis of gnathopod 2 in the female (inverted pyriform in Tethorchestia), the shape of the oostegites (strap-like in Tethorchestia; spatulate in Mexorchestia), the lack of a well-developed distolateral robust seta on the peduncle of uropod 1 (present in Tethorchestia) and the presence of a strong dorsal pigmentation pattern (pattern absent or obscure in Tethorchestia). Additional support for the new genus is provided by a CO1-based phylogeny of North Atlantic and Gulf of Mexico regional talitrids (Radulovici 2012).