Stegocephalus Krùyer

Vader, Berge Wim, 2001, North Paci ® c species of the amphipod (Crustacea) family Stegocephalidae, with description of one new species and redescription of another, Journal of Natural History 35, pp. 985-1000 : 995-999

Stegocephalus Krùyer

Stegocephalus Krùyer, 1842: 150 .

Type species. Stegocephalu s in X atus Krùyer, 1842.

Species. Stegocephalus beringi n. sp., S. hancocki Hurley, 1956 , S. in X atus Krùyer, 1842.

Species found in the North Paci W c. Stegocephalus beringi and S. hancocki .

Remarks. The type species of the present genus has been the object of misunderstandings in the literature with respects to both identi®cations (see Berge and Vader, 1997b) and morphology ( Steele, 1967). The latter mainly concerns the number of articles on the palp of the ®rst maxilla (see Barnard and Karaman 1991: 682), which is uni-articulate in both juvenile and adult specimens.

Stegocephalus beringi n. sp. ( Figures 5 ±7) Phippsiella similis Oldevig, 1959: 23 (part).

HOLOTYPE. Swedish Museum of Natural History Nr. 7464: Female , 7 mm, Bering Island (Komandorskiy Islands), 117 m, 4 August 1879 (identi®ed as Phippsiella similis by Oldevig (1959 )).

Distribution. [281] the holotype is the only known specimen of this species.

Description. Rostrum very small. Antennae short. Antenna 1 longer than antenna 2; ¯agellum seven-articulate; accessory ¯agellum article 2 present. Antenna 2 peduncle (articles 3±5) longer than ¯agellum; article 3 short, about as long as broad; article 4 shorter than article 5.

Epistome curved (convex) and smooth; epistomal plate (medial keel) not produced.

Mouthparts not elongate or pointed.

Mandible incisor lateral; toothed; left lacinia mobilis present, powerful, laterally expanded; not conical.

Maxilla 1 palp uni-articulate; rectangular; apex not reaching above the apex of outer plate; outer plate distally rectangular; ST in pseudocrown; ST ®rst row with more than six setae (ST 1±5 expanded, ST 7); ST 1 ordinary (similar to ST 2±4); ST 6 absent; gap between ST 5 and ST 7; ST A present, located distally, part of ®rst row; ST B present, part of second row; ST C present; inner plate with a well developed shoulder; setae pappose.

Maxilla 2 gaping and geniculate; outer plate setae with distal hooks; no distal cleft; inner plate setae row A covering the entire margin, appressed to row B; row A setae pappopectinate; row B setae proximally pappose, distally with cusps; row C present; row D absent.

Maxilliped palp four-articulate; article 2 distally produced; distal inner margin weakly produced; dactylus distally simple (pointed); inner plate not exceeding base of palp article 3; nodular setae absent; medial setae-row present, not reduced, transverse, setae simple; distal setae-row present; inner setae-row present, row not reduced (more than two setae), setae conspicuously large and strong; outer plate outer setae-row present, marginal, setae attached normally, setae short, strongly curved upwards (hooks); inner setae-row absent; distal setae-group present, setae attached in a deep hollow, short, simple.

Labrum elongate; lobes symmetrical and not reduced.

Labium distally narrowing.

Coxal plates and basis on the pereopods smooth. Coxae 1±3 contiguous.

Pereopod 1 coxal plate deeper than basis; propodus subovate.

Pereopod 2 longer and thinner than pereopod 1; ischium elongate, ratio length:- breadth exceeding 1.5; ischium distal posterior margin with plumose setae; propodus subrectangular; palm absent.

Pereopod 4 coxa posteroventral lobe medium sized, not reaching the base of pereon segment 7; basis anterior margin without long setae; posterior margin with long setae; no plumose setae on distal anterior margin; plumose setae on distal posterior margin; ischium with plumose setae on posterior distal margin.

Pereopod 6 basis posteriorly expanded; expansion conspicuous, rounded posteriorly, with a row of long plumose setae.

Pereopod 7 basis anterior margin straight, posteriorly expanded with distal margin linear; no medial row of setae.

Oostegites on pereopods 2±5, gills on pereopods 2±7.

Pleonite 1±3 dorsally smooth.

Urosome: articulation between urosome segments 2 and 3 present. Uropod 1 peduncle longer than rami; outer ramus shorter than inner. Uropod 2 peduncle longer than rami; outer ramus shorter than inner. Uropod 3 peduncle longer than half the length of rami; outer ramus uni-articulate; shorter than inner.

Telson longer than broad; shorter than uropod 3 peduncle; cleft, apically rounded, no submarginal setae on apex of each lobe.

Males: unknown.

Remarks. The holotype of the present species was identi®ed as Phippsiella similis ( Sars, 1895) by Oldevig (1959), but he stated that`the determination [is] not quite certain’ ( Oldevig, 1959: 23).

The present species is easily separated from adult specimens of its two congeners by the unproduced and rounded epimeral plate 3 and the very small rostrum. However, immature specimens of S. in X atus, are rather similar to S. beringi , but can be separated by the distally rounded basis of pereopod 7 (versus linear in S. beringi ).