Rhabdosoma White, 1847

Zeidler, Wolfgang, 2016, A review of the families and genera of the superfamily PLATYSCELOIDEA Bowman & Gruner, 1973 (Crustacea: Amphipoda: Hyperiidea), together with keys to the families, genera and species, Zootaxa 4192 (1), pp. 1-136 : 76-79

Genus Rhabdosoma White, 1847

( Figs 35–36 )

Xyphicéphale Guérin-Méneville in Eydoux & Souleyet, 1842: 271.

Rhabdosoma White, 1847: 130 .— Dana 1852: 316.— Dana 1853: 1009.— Bate 1862: 344.— Claus 1871: 155.— Claus 1879: 43 (key), 49–51.— Streets 1879: 286.— Gerstaecker 1886: 487.— Claus 1887: 68 (key), 73–74.— Stebbing 1888: 1606.— Stebbing 1895: 367.— Spandl 1927: 207.— Barnard 1940: 541 (key).— Yoo 1971: 63 (key).— Bowman & Gruner 1973: 49 (key), 52–53.— Zeidler 1978: 30 (key), 36.— Vinogradov et al. 1982: 404 (key), 431–432.— Nair 1995: 6 (key), 24–25.— Shih & Chen 1995: 190 (key), 206.— Vinogradov 1999: 1195 (key), 1197.

Macrocephalus Bate, 1858: 361 -362.

Rhabdonectes Bovallius, 1887: 39 .

Xiphocephalus Bovallius, 1890: 116 .— Pirlot 1929: 168.

Pseudanurus Garbowski, 1895: 199 .

Type species. Oxycephalus armatus Milne-Edwards, 1840 by monotypy. A probable syntype is in the ANSP, CA4200 (Guérin-Méneville Coll., no. 458) but the supposed type, a male (27.5 mm), in the MNHN (Am 4807) is lost (see Zeidler 1997a). It was collected by the expedition of Quoy and Gaimard (1833) from between Ambon and Tasmania .

Type species of synonyms. See Remarks for Xyphicéphale and Xiphocephalus .

The type species of Macrocephalus is M. longirostris Bate, 1858 by monotypy. Type material could not be located at the NHM or MNHN and is considered lost. The brief description by Bate (1858) is suggestive of Rhabdosoma , and later ( Bate 1862) he recognised the synonymy. No specific type locality is provided, just the South Atlantic, E. Belcher, surface.

Rhabdonectes was proposed by Bovallius (1887) as a replacement name for Rhabdosoma because he believed that it was already preoccupied. Thus, there is no type material.

Pseudanurus was instituted by Garbowski (1895) for R. brevicaudatum Stebbing, 1888 because he believed that this species differed sufficiently to warrant a separate genus. Thus, the type material is the same as for Stebbing’s species. The unique holotype female of R. brevicaudatum is in the NHM (89.5.15.326), the whole animal on one microscope slide. The type locality is the north-east Atlantic, off Guinea [10°55’N 17°46’W], Challenger stn. 352, surface, 13 April 1876.

Diagonsis. Body shape very elongate and narrow. Head oval. Rostrum usually very elongate in both sexes. Eyes occupying most of head surface, except for neck and rostrum; grouped in one field on each side of head. Antennae 1 of males with 2-articulate peduncle; flagellum with large, crescent-shaped callynophore, with relatively large antero-distal lobe, with aesthetascs arranged in two-field brush medially; with one small article inserted below antero-dorsal corner. Antennae 1 of females with 1–2-articulate peduncle; callynophore narrowly rectangular; sometimes with one additional, small, terminal article. Antennae 2 absent in females. Antennae 2 of males 5-articulate; strongly zig-zagged, with most articles folded back on each other; extending anteriorly under head and posteriorly between the gnathopoda to pereonite 1; basal article elongate, sub-equal in length to following article; terminal article very short, not folded, pointing posteriorly. Mandibular palp 3-articulate in males, with extremely long first article. Mandibular incisor relatively broad, with several teeth, with small distal lobe medially; in male orientated more or less parallel to palp. Maxillae 1 & 2 absent. Maxilliped with inner lobes completely fused; media l margin of outer lobes with membranous fringe. Coxae all fully fused with pereonites. Gnathopods 1 & 2 complexly chelate; carpal process knife-shaped, armed with microscopic teeth or setae; propodus with postero-distal corner produced, overlapping dactylus. Pereopods 3 & 4 sub-equal in length to pereopods 5 & 6. Pereopod 5 relatively slender; basis relatively narrow, only about twice as wide as merus; articles 3–7 inserted terminally to basis. Pereopod 6 with basis slightly wider than for P5, otherwise similar. Pereopod 7 reduced in size with large basis; with only 0–3 terminal articles. Uropoda with endopoda often reduced, especially for U2 & U3. Uropod 1; endopod articulated with peduncle. Uropods 2 & 3; endopod fused with peduncle. Telson articulated with double urosomite. Oostegites on pereonites 2–6. Gills on pereonites 2–6, or 5 & 6 in female; 4–6, or 5 & 6 in male; all without folds.

Species. Rhabdosoma armatum ( Milne-Edwards, 1840) ; R. whitei Bate, 1862 ; R. brevicaudatum Stebbing, 1888 and R. minor Fage, 1954 .

Sexual dimorphism. Sexually mature males of R. brevicaudatum and R. minor have not been recorded, and these species may be parthenogenic. In the other two, currently recognised species, females have a relatively longer rostrum; the telson is longer relative to uropod 3; the gnathopoda have a slightly longer carpal process, and gills occur on pereonites 2–6. In males, gills occur on pereonites 4–6, or 5 and 6.

Remarks. As suggested by Stebbing (1895), Guérin-Méneville (1842) merely gave an opinion on the taxonomic status of Oxycephalus armatum Milne-Edwards, 1840 (= R. armatum ), and fore-shadowed a suitable name, Xyphicéphale, but did not institute a new genus. Thus, Bovallius (1890) is wrong in accepting this, and changing it to Xiphocephalus . In rejecting this name we are left with Rhabdosoma as the earliest name for the genus and, except for Pirlot (1929), this has been accepted since Stebbing (1895).

Rhabdosoma is one of the most bizarre genera of Hyperiidea, easily characterised by the extremely slender, elongate body, and very long, needle-shaped rostrum. It seems to bear little resemblance to any other genus of Oxycephalidae , and it is the only genus in which the telson is not fused with the double urosomite, although juveniles may have the telson fused (e.g. R. brevicaudatum ). It resembles C ranocephalus in the reduction of the number of articles of the first antennae of females. The second antennae of males resemble those of Leptocotis and Glossocephalus , in that the juncture of articles 3/4 extend forward of the juncture of articles 1/2. In the absence of maxillae it resembles Oxycephalus . The maxilliped is like that of Oxycephalus and Cranocephalus . The character of coxae fused with pereonites is shared with Oxycephalus , Calamorhynchus and Cranocephalus .

Fage (1960) provides the most comprehensive biogeographical information for this genus. Most species seem to be epipelagic in habit, preferring tropical to sub-tropical waters.

The only recorded association with gelatinous plankton is for juveniles of R. whitei and Rhabdosoma sp. with the ctenophore, Beroe sp. ( Harbison et al. 1978).

Four species are currently recognised as valid ( Fage 1960, Vinogradov et al. 1982). Two large species, R. armatum and R. whitei , are distinguished by the morphology of the gnathopoda, and uropoda, and the two small species, R. brevicaudatum and R. minor , by the relative length of the telson. The two small species (<30 mm) could be mistaken for juvenile R. armatum , in which the urosome is similar, and the telson is also reduced, but the presence of ovigerous females demonstrates the validity of these species. They appear to be parthenogenic, as mature males have not been found. Fage (1960) studied 643 specimens of R. brevicaudatum and 1921 specimens of R. minor without finding any males. It is sometimes difficult to distinguish between these two species as the length of the telson seems to vary, and Vinogradov et al. (1982) suspect that they may even be synonymous. However, in all the specimens that were examined the first antennae of R. brevicaudatum consist of the basal article, callynophore, and one small terminal article while in R. minor the first antennae are like those of female R. whitei , consisting of just the basal article, and callynophore, although, the callynophore seems to have an incomplete article proximally, thus approaching the condition found in female R. armatum ( Fig. 36 ).

Figures

FIGURE 35. Rhabdosoma armatum (Milne-Edwards, 1840), male (approx. 110 mm), Indian Ocean, off South Africa, SAMA C 5808. A, habitus. Scale bars = 2.0 mm (A), 0.2 mm (A 1), 0.5 mm (remainder).

FIGURE 36. Rhabdosoma species, all from Indian Ocean, off South Africa. Md & Mxp from male R. armatum (Milne- Edwards, 1840), approx. 110 mm, SAMA C 5808; A 2 from male R. whitei Bate, 1862, approx. 60 mm, SAMA C 5811. A – D, A 1 from females, R. armatum, approx. 34 mm, SAMA C 5809; R. whitei, approx. 62 mm, SAMA C 5810; R. minor Fage, 1954, approx. 17 mm, SAMA C 5819 and R. brevicaudatum Stebbing, 1888, 8.9 mm, SAMA C 5820, respectively.