Vallorchestia, Lowry, 2012

Lowry, J. K., 2012, Talitrid amphipods from ocean beaches along the New South Wales coast of Australia (Amphipoda, Talitridae), Zootaxa 3575, pp. 1-26 : 19-20

Vallorchestia gen. nov.

Type species. Orchestia dispar Dana, 1852 .

Included species. Vallorchestia includes 1 species: V. dispar ( Dana, 1852) .

Diagnostic description (based on male). Eyes medium (1/5 to 1/3 head length). Antenna 1 short, rarely longer than article 4 of antenna 2 peduncle. Antenna 2 peduncular articles slender or slightly incrassate; article 3

without plate or process ventrally. Labrum without robust setae. Mandible left lacinia mobilis 4-dentate. Maxilliped palp article 2 distomedial lobe well developed; palp 4 reduced, button-shaped. Gnathopod 1 subchelate; posterior margin of merus, carpus and propodus each with lobe covered in palmate setae. Gnathopod 2 subchelate; merus and carpus free; dactylus attenuated distally. Pereopods 3–7 cuspidactylate. Pereopod 4 dactylus thickened proximally with a notch midway along posterior margin. Pereopod 6 sexually dimorphic (T-setae on merus and carpus). Pereopod 7 sexually dimorphic (merus and carpus swollen). Pleonite 1–3 without dorsal spines. Pleopods all well developed. Epimera without vertical slits just above ventral margins. Uropod 1 rami without apical spear-shaped setae; outer ramus slender; outer ramus without marginal robust setae. Uropod 3 well developed; ramus subequal or shorter than peduncle. Telson as long as broad, apically incised, with 3–6 robust setae per lobe.

Remarks. Dana’s (1852) original illustrations indicate a male in which the seventh pereopod has developed secondary sexual characteristics but the second antenna is still slender. His material came from the ‘shores of Illawarra’. We have never collected a species like this from the modern day Illawarra area, but we have collected one from farther north at Valla Beach and Nambucca.

Vallorchestia belongs among talitrid taxa with cuspidactylate dactyli, with a 4-dentate left lacinia mobilis, with male antenna 2 slender, with a subchelate male second gnathopod, with pleopods all well developed and without marginal robust setae on the outer ramus of uropod 1. These include the Indo-Pacific genera Chroestia Marsden & Fenwick, 1984 , Floresorchestia Bousfield, 1984 and Pseudorchestoidea Bousfield, 1982 .

All of these genera except Pseudorchestoidea have attenuated dactyli on male gnathopod 2. Pseudorchestoidea differs from Vallorchestia in having a parachelate male gnathopod 1 without a palmate lobe on the merus; in having a pinched pereopod 4 dactylus; and in not having sexually dimorphic pereopods 6 and 7. Chroestia differs from Vallorchestia in the same characters.

Vallorchestia shares with Floresorchestia the unusual character of palmate lobes on the merus, carpus and propodus of male gnathopod 1, but lacks the vertical epimeral slits found in Floresorchestia . Like the preceding taxa Floresorchestia also differs from Vallorchestia in having a pinched pereopod 4 dactylus; and in not having sexually dimorphic pereopods 6 and 7.

Among these taxa Vallorchestia is the only one with a sexually dimorphic pereopod 7, a character it shares with Platorchestia (group 1) species. Vallorchestia differs from this taxon in the non-sexually dimorphic antenna 2, the development of palmate lobes on the merus, carpus and propodus of male gnathopod 2 and the development of T-setae on the merus and carpus of male pereopod 7.

Vallorchestia is differentiated from all other talitrid amphipods by the unique setae on the merus and carpus of the male, known here as T-setae. T-setae are long, slender setae in which the distal tip is expanded to form a Tshaped structure. Although they occur only in adult males it is difficult to postulate their functionality.

Morphological changes to males of Vallorchestia during the three final molts are as follows: the antepenultimate stage has no change in pereopods 6 or 7 nor to antenna 2. In the penultimate molt antenna 2 becomes slightly incrassate and pereopod 6 develops T-setae, but there is no change to pereopod 7. In the final molt pereopod 6 develops many T-setae on the merus and carpus and in pereopod 7 the merus and carpus expand and the carpus develops medial robust setae.