Microscina rostrata, Zeidler, 2012

Zeidler, Wolfgang, 2012, A review of the hyperiidean amphipod families Mimonectidae and Proscinidae (Crustacea: Amphipoda: Hyperiidea: Scinoidea) 3533, Zootaxa 3533, pp. 1-74 : 68-71

Microscina rostrata sp. nov.

( Figs. 36–37)

Material examined. Holotype. Female , approx 12.0 mm ( ZMUC CRU-20441); south-eastern tropical Atlantic, in vicinity of Gulf of Guinea (03°45’S 10°00’W), Dana stn. 3999 III, 600 mw, 2 March 1930. Mouthparts mounted on microscope slide, remainder in spirit.

Description of holotype. Female, approx 12.0 mm; probably mature judging by the well-developed oostegites, armed with long setae on distal margin. Cuticle very thin, translucent. Head with curved, elongate, triangular rostrum, over-lapping most of A1. Pereonites broad and deep anteriorly, becoming smaller posteriorly, with pereonite 7 the smallest. Pereonites 1 & 2 mostly fused, only slightly joined ventrally. Pleonites relatively narrow. Coxae relatively small. Antennae 1 as long as head (minus rostrum) and first four pereonites combined; with stout, lanceolate callynophore, triangular in cross-section, with serrated margins; with 2–3 short, terminal articles. Antennae 2 slender, about half length A1; consisting of four articles. Gnathopod 1; basis length 1.4x carpus; propodus length 0.8x carpus, both articles covered in well-spaced, fine setae; dactyl narrow, slightly curved, length about 0.2x propodus. Gnathopod 2 sub-equal in length to G1, with more slender articles, especially carpus and propodus; basis length almost twice carpus; propodus slightly longer than carpus, both articles with several fine setae on posterior margin; dactyl narrow, bent at right angles near base, length slightly more than 0.1x propodus. Pereopods 3 & 4 similar in structure, with P3 marginally longer; all articles relatively elongate, with posterior margin slightly serrated; basis length 1.6–1.7x merus; carpus and merus of similar length; propodus length 0.9x carpus (P3) or 0.8x carpus (P4); dactyl a small, curved nail, fully retractile. Pereopod 5 slightly shorter than P4; all articles relatively elongate, with anterior margin slightly serrated, also serrations mid-way along face; basis length 1.3x merus; carpus length 0.6x merus; propodus length 0.8x carpus; dactyl as for P3 & 4. Pereopod 6 with carpus and propodus missing but similar in structure to, but much longer than, P5, judging by the relatively longer basis and merus; basis length 1.2x merus. Pereopod 7 slightly shorter than P5 but of similar structure; basis length 1.6x merus; carpus length 0.8x merus; propodus slightly shorter than carpus; dactyl as for P5. Uropoda with relatively slender peduncles and rami; all with slightly serrated margins; all with inner ramus slightly longer than outer, both much longer than peduncle. Uropod 1; inner ramus length 1.2x outer, and 1.8x peduncle. Uropod 2; inner ramus only slightly longer than outer, and 1.4x peduncle. Uropod 3; inner ramus only marginally longer than outer, 1.4x peduncle. Telson rounded, not triangular, about as long as wide; length almost 0.3x peduncle of U3.

Colour not known for living specimen.

Etymology. The specific name emphasises the unusual character of the relatively long rostrum that distinguishes this species.

Remarks. Although there is only one, slightly imperfect specimen available, the characters are so unusual that I have no hesitation in describing it as new to science, and assigning it to a new genus and family. In having a head with a rostrum, it is unlike any other species of the Scinoidea , and in having pereonites 1–2 fused, it only resembles Acanthoscina and Spinoscina (Scinidae) . It also resembles some species of Scinidae in having serrated margins on the fist antennae, and on some pereopod articles, and on the uropoda. However, the characters of the uropoda, with an articulated inner ramus, and the maxilliped, with free inner lobes, alone preclude it from this family.

It is most similar to species of Mimoscina (Mimoscinidae) in having slender pereopods with retractile dactyls, but in that family the dactyls of pereopods 3 & 4 are not retractile, the head lacks a distinct rostrum, the pereonites are all free, the first antennae are of a different structure, and the mandibles have a relatively broader lacinia mobilis and the incisor is of a different structure.

In having pereopods with retractile dactyls, and a head with a distinct rostrum, this species also resembles some members of the Lanceoloidea , particularly Chuneolidae (see Zeidler 2009), species of which also possess mandibles lacking a palp. Initially I thought that this might be an unusual species of Chuneola , but species of this genus are all very robust, the pereonites are all separate, the coxae are relatively large, the first antennae are not lanceolate and have a three-articulate peduncle, the second antennae are reduced to a relatively large gland cone plus one tiny article, the mouthparts are relatively large, the mandibles have a relatively broad incisor lacking the three prominent teeth characteristic of Microscina rostrata , and the retractile dactyls of the pereopods are characteristically hooded.

Distribution. Known only from the type locality, from the unique holotype, as detailed above.