Chorismus antarcticus (Pfeffer 1887)

"CHORISMUS ANTARCTICUS

Hippolyte antarcticus Pfeffer, Jahrb. Hamburg. Wiss. Anst. IV. (1887), p.51, pli. Figs. 22-27.

Description of ovigerous females. – Total length 87 to 101 mm. Length of carapace (including rostrum) about two-fifths of total length of body. Rostrum equal to or a little longer than the distance from the orbital notch to posterior margin of carapace in the middle line, curved upwards, moderately expanded below; upper margin with eight or nine teeth, of which the second is placed over, or a little in front of, or in one case a very little behind the orbital notch, and the last two are close to the apex of the rostrum; lower margin with six to nine teeth (usually seven.) No supra-orbital teeth, antennal tooth distinct from the lower orbital angle, antero-lateral corner of carapace with a minute (pterygostomial) tooth.

Third abdominal somite with dorsal surface strongly elevated or " humped," but without a definite tubercle as in Chorismus tuberculatus. Sixth abdominal somite without movable lateral spines.

Peduncle of antennules reaching to or nearly to two-thirds the length of rostrum, third segment one-half the length of second, spine of first segment ("stylocerite") narrow and acute, reaching to middle or quite to end of second segment; flagella subequal, or inner slightly longer than outer, extending beyond tip of rostrum by one-third to nearly half their length. Antennal scale broad, about equal in length to rostrum, outer edge distinctly and evenly convex, apex broadly rounded, outer spine short.

Mandible with small incisor process and palp of three segments, the first broader than, and as long as the second. Third maxilliped extending nearly to tip of antennal scale, terminal segment twice the length of preceding. Exopod well developed.

First leg extending beyond penultimate segment of third maxilliped, carpus slightly excavate distally, about equal in length to the hand. Second leg extending beyond tip of antennal scale, carpus of eleven, merus of three, and ischium of two segments. Remaining legs moderately stout, fifth leg extending forwards nearly to tip of antennal scale.

Telson not greatly narrowed distally, tip rounded, with seven spines. Inner plate of uropods subequal to the telson and very little shorter than the outer plate, which is broadly rounded at the tip.

The branchial system comprises five pleurobranchiæ on each side on the last five thoracic somites, an arthrobranchia on the third, and a podobranchia on the second maxilliped. There are epipods on the third maxillipeds and on the first two pairs of legs.

Young males (55-59 mm. in length) and females (42 mm.) Body a little more slender and the rostrum relatively longer, distinctly exceeding the length from orbit to back of carapace. Flagella of antennule little longer than rostrum. Outer edge of antennal scale straight. In two specimens (♂ and ♀) there are twelve segments in carpus of second leg.

The co-typical specimen of Hippolyte antarctica used for comparison is an ovigerous female, about 60 mm. in total length. It agrees perfectly with the 'Discovery' specimens except in the following points: — Of the seven teeth on the upper edge of the rostrum only one is close to the point, not two, as in our specimens; the antennal scale is slightly longer than the rostrum, and its outer edge (as in our specimens of similar size) is nearly straight; the third maxillipeds are a little shorter, and their terminal segment is hardly twice the length of the preceding; the carpus of the first legs is a little shorter than the hand.

Remarks. — This species agrees with the type of the genus Chorismus (C. tuherculatus, Sp. Bate.) in the characters given in my recently published synopsis of the Hippolytidæ (‘Ann. Mag. Nat. Hist.,’ xvii., 1906, p. 30), and further in having no supra-orbital spines, in the mandible-palp being composed of three segments,* the gills seven in number on each side, and the last three pairs of legs without epipods. It differs in having eleven or twelve segments in the carpus of the second legs while C. tuberculatus has only nine, in having an exopod on the third maxilliped, and in the fact that the first segment of the mandibular palp is not shorter than the second.

Occurrences. — January 22, 1902. 500 fathoms, 1 ♂.

W.Q., February 28, 1902. Less than 20 fathoms, 1 ♂, 1 ♀.

W.Q., January 10, 1903. 130 fathoms, 2 ♀.

W.Q., May 14, 1903. 127 fathoms, 1 ♂, 1 ♀.

W.Q., June 18, 1903. 130 fathoms, 2 ♀.

Fragments of this species were taken from the stomachs of seals on several occasions."

(Calman, 1907)