Hyperiella dilatata Stebbing 1888

Hyperiella dilatata Stebbing

Hyperiella dilatata Stebbing, 1888:1403–1404, pl. 171.–Bovallius, 1889:247.–Walker, 1907:8.–Spandl, 1927:162–164, fig. 5a-h.–Barnard, 1930:413–414; 1932:274–275, fig. 161 [distribution]; 1937:4.–Stephenson, 1947:76.–Hurley, 1961:600; 1969:33, sheet 2, map 5[distribution].–Vinogradov, 1962:25.–Emison, 1968:passim [food of Adélie penguin].

DERIVATION OF NAME.–Refers to the inflated pereon.

TYPE-LOCALITY.–Indian sector of Antarctic, 63° 30′S, 88°57′E.

DIAGNOSIS.–Length 6–8 mm. Posteroventral corners of pleonal epimera sharper and more prominent than in H. antarctica. P5–7, anterodistal corners of s2–4 produced into conspicuous triangular processes. Up3 ♂, exopod 2/3 or more as wide as endopod.

RELATIONSHIPS.–The differences that previous workers have used to distinguish between H. antarctica and H. dilatata are not altogether convincing, but the material at my disposal has not allowed me to shed much light on the problem. My specimens of H. dilatata, collected from the stomachs of Adélie penguins by William B. Emison, are somewhat deformed and partly digested, but were sufficiently intact to permit illustration of most of the significant details. With regard to H. antarctica, I have examined only 3 males from the collections of the Copenhagen Museum, identified by Bovallius and believed to be syntypes. The original label for these specimens is lost, and a label in K. Stephensen’s handwriting reads (translated from the Danish by Torben Wolff), “Amongst the ‘Icebergs’ of the South Polar Sea. Wessell.” In his original description Bovallius (1887b) gave the locality as “Antarctic Seas, around Cape Horn.” In 1889 Bovallius cited the type-locality more precisely: “The American Antarctic region: Lat. 58°43′; Long. 76°W.” This locality lies in the western part of the Drake passage, south of the northern limit of drift ice, hence is compatible with the syntype label.

Accepting the 3 males as syntypes leaves unresolved the question of whether H. antarctica and H. dilatata are distinct species, since the condition of the specimens is only fair. A brief history of the taxonomy of the two species may help to show the nature of the problem.

The original description of each species was based on only one sex, the male for H. antarctica and the female for H. dilatata. Bovallius (1889) gave a key to the two species that distinguished them by 3 characters: (1) The pleonal epimera, with posterolateral corners rounded in H. antarctica and pointed in H. dilatata. (2) The anterodistal corners of s2–4 of P5–7, rectangular in H. antarctica and acutely produced in H. dilatata. (3) The endopod of Up3, “ovate” in H. antarctica, “narrowly elongate and sharp-pointed” in H. dilatata.

The usefulness of the first character became doubtful when Stewart (1913) noted pointed pleonal epimera on a ♂ H. antarctica. This finding was verified by Barnard (1930, 1932), although he stated that the points were not as well developed as in H. dilatata. In the specimens seen by me the points are somewhat stronger in H. dilatata (compare Figures 20m and 20n), but it is risky to rely on such a purely quantitative character when nothing is known about its variability. Moreover, the appearance of the points may be affected by the action of the preservative; inflation of the epimera, obscuring the nature of the margins, is frequently seen in preserved hyperiids.

That Bovallius’ second difference is also a matter of degree was noted by Barnard (1930, 1932), who found pointed anterodistal corners on s2–4 of P5–7 in H. antarctica less prominent than those on H. dilatata. In specimens seen by me the points are distinctly more prominent in H. dilatata, but I cannot rule out the possibility that the points have been abraded in the H. antarctica syntypes.

The third difference in Bovallius′ key concerned a sexually dimorphic character. The broader “ovate” endopod of Up3 of H. antarctica was that of a male Hyperiella, and the “narrowly elongate” endopod of H. dilatata was an attribute of a female Hyperiella (compare Figures 21>a and 21b of the ♂ and ♀ uropods of my specimens of H. dilatata). As Barnard (1930) observed, Spandl (1927) added to the problem by describing the uropods of an immature male of H. dilatata; uropods of young males are narrow, like those of females. But Barnard himself compounded the confusion by stating that the endopods of Up2–3 of the male H. antarctica are lanceolate and slightly narrower than those of the female H. antarctica. My drawing of the uropods of the ♂ H. dilatata (the first for this species) shows the endopod of Up3 ovate and only slightly narrower than that of the male H. antarctica. Figures of the uropods of the female H. antarctica have not yet been published, and I am unable to evaluate Barnard’s statement concerning them.

In summary, none of the characters said to be useful in distinguishing between H. antarctica and H. dilatata are of proven reliability. Both species (if they are not conspecific) are in need of detailed redescriptions, including studies of their variability.

DISTRIBUTION.–From the Antarctic Convergence to the Antarctic continent (Hurley, 1969).