Cheloscina antennula Shih & Hendrycks, 1996

Zeidler, Wolfgang, 2012, A review of the hyperiidean amphipod families Mimonectidae and Proscinidae (Crustacea: Amphipoda: Hyperiidea: Scinoidea) 3533, Zootaxa 3533, pp. 1-74 : 55-57

Cheloscina antennula Shih & Hendrycks, 1996

( Fig. 28)

Cheloscina antennula Shih & Hendrycks, 1996: 596–600 , figs. 4–5.

Type material. This species was described from one immature female specimen, 2.7 mm in length (head to end of inner ramus of U3), from the North Pacific, off the Alaskan Peninsula (53°20’N 155°16’W), 700–900 m, 13 April 1930. The unique holotype is in the USNM (264247); mouthparts and appendages from right side, including telson, mounted on microscope slide, remainder in alcohol .

Material examined. The unique holotype as detailed above.

Diagnosis. Following examination of female holotype, 2.7 mm, and according to Shih & Hendrycks (1996). Pereon not inflated. Head broadly rounded dorsally, without rostrum. Antennae 1 as long as head and first four pereonites combined (medially); of unusual structure with short callynophore, sub-equal in length to first peduncular article; terminal articles relatively long, together 2.6x length callynophore, the first about half length the second, and the second about half length the third, elongate terminal article; callynophore and first terminal article with enormous aesthetasc near distal corner. Antennae 2 reduced to small knob. Gnathopod 1; basis length twice carpus, slightly shorter than propodus; propodus narrowing distally, posterior margin slightly excavated for distal half and forming palm with dactyl, bearing two pairs of robust setae near the middle, both margins (especially posterior) prolonged distally to form process at base of dactyl; dactyl relatively strong, curved, length slightly more than 0.4x propodus. Gnathopod 2 similar in structure and length to G1. Pereopods 3 & 4 similar in structure, with P4 marginally shorter; basis length about twice merus; carpus length 1.6x merus; propodus length 1.2x carpus; dactyl relatively strong, curved, length about 0.3x propodus. Pereopod 5 slightly shorter than P4; basis length twice merus; carpus length 1.4x merus; propodus length 1.3x carpus; dactyl relatively strong, curved, length almost 0.3x propodus. Pereopod 6 similar in length and structure to P5. Pereopod 7 slightly shorter than P6, with marginally broader articles, especially carpus and merus; basis length 2.3x merus, similar in length to carpus and propodus; merus relatively short, with broad distal margin, about 0.8x length; carpus slightly broader than other articles; dactyl relatively strong, curved, length 0.3x propodus. Uropoda with slender, relatively long, lanceolate rami with serrated margins; all with inner ramus distinctly longer than outer and much longer than peduncle, especially U1 & 2. Uropod 1; inner ramus length 1.8x outer, and 3.6x peduncle. Uropod 2; inner ramus length 1.7x outer, and 3.4x peduncle. Uropod 3; inner ramus length 1.2x outer, and 2.8x peduncle; peduncle width 0.5x length. Telson with rounded apex, 1.4x as long as broad, about 0.6x length of peduncle of U3.

Colour not known for living specimen.

Remarks. The most distinguishing character of this species, and the genus, is the structure of the first antennae. Apart from the enormous aesthetascs, the callynophore is relatively short, whereas in all other species of the family (and even most of the species of the superfamily) it is by far the longest and largest article. In having gnathopods with excavations on the posterior margin of the propodus, this species also resembles Mimonectes sphaericus and M. neosphaericus , but the excavation is of a different structure, and except for gnathopod 2 of males of M. sphaericus , the carpus is relatively shorter. In addition, this species has uropoda with more slender and longer rami, and in M. sphaericus the dactyls are much weaker. The possibility that the characters of the first antennae and gnathopoda of C. antennula are juvenile characters of M. sphaericus was not supported by the examination of juvenile specimens of M. sphaericus .

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

Family MIMOSCINIDAE fam. nov.

Diagnosis. Sexually mature specimens not known. Largest recorded female, 9.0 mm (or 20 mm if Sphaeromimonectes scinoides Woltereck, 1906 is accepted as a species of Mimoscina ). Largest male recorded, 8.2 mm. Head rarely as long as first pereonite, with slight, rounded rostrum. Eyes absent. Pereon of females moderately inflated, mainly due to enlargement of pereonites 1–5. Pereon of males slender. Pereonites all separate. Coxae separate from pereonites. Antennae 1 as long as head and first four pereonites combined in females, almost as long as head and pereon in males; callynophore stout; three short terminal articles, the terminal one longer than the previous two combined; peduncle two-articulate. Antennae 2 reduced in females and immature males; sometimes as long, or longer, than A 1 in some male specimens. Mandibles with well-developed lacinia mobilis (left), slightly narrower than incisor; most inferior tooth of incisor more pointed and slightly offset from other teeth. Maxillae with broad palp, armed with few setae distally; outer lobe broad, armed with five strong setae, three distally and a smaller one either side; inner lobe with broad distal margin, armed with numerous fine setae. Maxillae 2 with relatively slender lobes of similar length, armed with 2–3 strong setae distally. Maxilliped with broad, rounded outer lobes, with strong setae along margin; inner lobes relatively narrow, separate, marginally exceeding half length outer lobes. Gnathopods simple, relatively slender. Pereopods 3 & 4 with normal dactyls. Pereopods 5–7 often very slender and much longer than P3 & 4, with very small, retractile dactyls. Urosomite 2–3 with partial suture ventrally. Uropoda slender, with articulated rami. Telson length 0.3–0.5x peduncle of U3; with rounded apex. Gills on pereonites 2–6. Oostegites on pereonites 2–5.

One genus: Mimoscina .

Remarks. This family is proposed here to accommodate the genus Mimoscina following a review of the family Proscinidae presented earlier. It is distinguished from all other families of the Scinoidea by the relatively slender pereopods 5–7, with their relatively small, retractile dactyls. Additional characters that distinguish the family are as follows; the head has a slight, rounded rostrum; the outer margin of the peduncle of uropod 1 is noticeably convex, and may be fringed with stout or long setae; the telson is rounded terminally, not triangular or pointed, and the most inferior tooth of the incisor of the mandible is sharper than the others, and is not separated by a large gap, as found in Mimonectes , but is slightly offset from the others.

Contrary to literature records, I have found that even very immature female specimens have a slightly inflated pereon, and in the largest specimen examined (about 9 mm), although still immature, the pereon is moderately inflated, mainly due to the enlargement of pereonites 1–5, as found in some species of M imonectes.