Atacamorchestia, Pérez-Schultheiss & Fernández & Ribeiro, 2024

Pérez-Schultheiss, Jorge, Fernández, Leonardo D. & Ribeiro, Felipe Bezerra, 2024, Two new genera of coastal Talitridae (Amphipoda: Senticaudata) from Chile, with the first record of Platorchestia Bousfield, 1982 in the southeastern Pacific coast, Zootaxa 5477 (2), pp. 195-218 : 197-198

Genus Atacamorchestia gen. nov.

http://zoobank.org/ urn:lsid:zoobank.org:act:09F893B7-C3CC-40BC-9D84-9A8B98A5F9E5

Type species: Atacamorchestia atacamensis sp. nov., by monotypy.

Etymology: The name results from the union of the word Atacama, the region where the species was collected, and the word of Greek origin orchestia, which means dancer.

Diagnosis: Head. Eye medium (1/5–1/3 head length). Antenna 1 short, not reaching midpoint of peduncular article 5 of antenna 2. Antenna 2 peduncular articles slightly incrassate; article 3 without plate or process ventrally. Labrum epistome without robust setae. Labrum median groove without robust setae. Mandible left lacinia mobilis 4-dentate. Maxilliped palp broad, article 2 with distomedial lobe; article 4 reduced, button-shaped.

Pereon. Gnathopod 1 sexually dimorphic; subchelate; posterior margin of merus, carpus and propodus each with lobe covered in palmate setae; carpus longer than propodus; 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 acute; dactylus attenuated distally, slightly longer than posterior margin of propodus. Oostegites setae with simple smooth tips. Pereopods 3–7 cuspidactylate. Pereopod 4 dactylus similar to that of pereopod 3, slightly thickened proximally along posterior margin. Pereopod 5 short, slightly longer than 2/3 length of pereopod 6; dactylus not inflated. Pereopod 6 not sexually dimorphic in shape, shorter than pereopod 7. Pereopods 6–7 without row of short setae along posterior margin of dactyli. Pereopod 7 not sexually modified in shape; propodus slender, length 7.0 × width. T-setae present in male antenna 2 and pereopods 3–7.

Pleon. Pleonites 1–3 without dorsal spines. Pleopods 1–3 all well-developed. Epimera 1–3 slits absent. Uropod 1 male exopod not sexually dimorphic, peduncle distolateral robust seta present; rami without apical spear-shaped setae; endopod with marginal robust setae in one row; exopod with marginal robust setae in one row. Uropod 2 rami without apical spear-shaped setae; exopod with marginal robust setae in one row; endopod with marginal robust setae in one row. Uropod 3 ramus shorter than peduncle. Telson longer than broad, tapering distally, apically incised, with complete dorsal groove, with at most 6 robust setae per lobe, only one marginal and the rest apical.

Remarks: The establishment of relationships between the members of Talitridae has been based almost exclusively on patterns of morphological similarity, so as there is no phylogenetic support, taxonomic discussions for the more than 50 described genera are based on combinations of characters, being still pending to test the monophyly in most of these taxa (e.g. Lowry & Myers 2019; Myers & Lowry 2020a).

The genus Atacamorchestia gen. nov. shares most of the characters that define the subfamily Platorchestiinae ; however, discrepancies were observed with respect to some of the diagnostic characters proposed by Lowry & Myers (2022). Two important features for the differentiation of the subfamily Platorchestiinae from Talitrinae are the absence of lobes with palmate setae on merus of gnathopod 1 and the absence of dorsal robust setae on exopodite of uropod 1, however, these features may be subject to ontogenetic variation. For example, recent observations indicate that in large male specimens of Vallorchestia Lowry, 2012 the presence lobes covered with palmate setae on merus of gnathopod 1 may be insinuated (L. Hughes, personal communication), whereas in Atacamorchestia gen. nov. are very well developed. Additionally, the appearance of robust setae on the exopodite of uropod 1 has been observed in most developed individuals of the new genus.

The presence of T shape setae in male individuals immediately relates Atacamorchestia gen. nov. to the genus Vallorchestia , described by Lowry (2012) from New South Wales, Australia. These special setae, longer and more flexible than normal in the distal half of the structure, have a characteristic wide apical expansion and are present only in these two genera among all known talitrids. However, T setae in Vallorchestia are restricted only to the merus and carpus of pereiopod 6, whereas in Atacamorchestia gen. nov. they are present on the last three peduncular articles of antenna 2, and on virtually all segments of pereiopods 3 to 7, although are more developed in merus and carpus.

In addition, there is other differences in fully developed males of Vallorchestia , whose sexual dimorphism in pereopod 7 involves merus and carpus, that are evidently incrassate; whereas in Atacamorchestia gen. nov. there is no indication of such modification in fully developed males, being these articles as slender as in smaller individuals. Another important difference is the presence of a well-developed lobe with palmate setae on the merus of gnathopod 1, a structure that varies little through male ontogeny, unlike Vallorchestia , where it is generally absent, and barely insinuated in terminal, fully developed individuals (Hughes, personal communication). Additionally, setation of telson in Atacamorchestia gen. nov. is reduced to only one dorsolateral seta per lobe; whereas in Vallorchestia there is 2–3 setae in this position.