Material Examined

Victoria University, Zoology Department, Cook Strait Collections

Specific Diagnosis

A large, fragile shrimp with small eyes and a soft and membraneous integument.

Rostrum relatively small and obtuse, hardly projecting beyond the anterolateral margin of the carapace and little raised above the dorsal surface of the latter. Supraorbital and hepatic spines absent. Cervical groove distinct and continuous over dorsum, weak anterolateral ridge from hepatic region towards base of antenna, suprabranchial ridge present.

Eyes relatively small, cornea barely wider than and about one-third as long as distal segment of ocular peduncle. No ocular tubercle. Segments of antennular peduncle not greatly attenuated, proximal longer than second, which is subequal to third. No distolateral spine on margin of scaphocerite. Third maxillipeds not greatly enlarged, subequal in length to 2nd or 3rd pereiopods.

Male with petasma as shown (Figs. 8, 9); lobus connectens very long, armed on lateral margin and distally on inner margin with crochets; processus ventralis shorter, with distinct distal nick; lobus armatus curved, shorter than processus ventralis, but reaching a little beyond lobus inermis, armed on inner margin with small crochets; lobus terminalis a simple armed projection, smaller than the unarmed lobus inermis, between the latter and the base of the lobus connectens.

Female with single blunt posteromedial projection on coxa of 3rd pereiopod.

Branchial Formula

The relative branchial size and arrangement of the New Zealand specimens agrees with the records for this species (as S. mollis) given by Smith (1884; 1886, Pl. XX, Fig. 5) and Hansen (1922) This differs from the more usual arrangement in the genus, as given for S. arcticus above, in having an arthrobranch and a branchial lamella above the 3rd pereiopod instead of two arthrobranchs.

Size at Maturity

Of the two largest male specimens, each with a carapace length of 21 mm, one is still slightly immature with the petasma as illustrated here, while the other shows the full development of crochets similar to that illustrated by Hansen (1922, Pl. IV, fig. 3 i-m).

Colour in Life

These specimens were uniform scarlet in life with some of the inner mouth parts tinged with purple. This pigment was mainly carried in the cuticle of the body and appendages, although small chromatophores were also present. No dermal photophores of any type present. Organs of Pesta absent.

Systematic Position

The membraneous integument and the lack of dermal photophores place these specimens in the "S. japonicus" group of the subgenus Sergia. The absence of an ocular tubercle and the fact that the cornea is barely wider than the ocular peduncle distinguishes it from S. tenuiremis, S. kröyeri and S. laminatus; the distinctive rostral profile distinguishes it from S. profundus and the branchial formula from S. inous.

The Cook Strait specimens agree in every way with the numerous descriptions and figures of the North Atlantic S. mollis Smith, which has been shown to be synonymous page 24with S. japonicus Bate taken by the Challenger Expedition off Japan (see Hansen, 1896, 1903; Burkenroad, 1940).

Geographic and Bathymetric Distribution

Sergestes japonicus is a well known deep-water species in the North Atlantic. Specimens were taken by the Michael Sars Expedition between about 125 and 1,050 fms. but those with a carapace length of 18 mm and over were never taken above 500 fms. (Sund, 1920). In 1938, Welsh and Chace were able to show from their "closing net" data that it is the only species of Sergestes in the North Atlantic which normally occurs below the level to which light penetrates (i.e., below the photic zone), being found usually below 500 fms. and regularly down to at least 1,000 fms.

Outside the North Atlantic, it has been recorded from only two localities in the Indopacific. The Challenger Expedition took it from deep water off Japan and the Philippines, these records being checked by Hansen (1903).

Thus it is now possible to record this small-eyed, deep-water species from Cook Strait, New Zealand, apparently the first record from the entire Southern Hemisphere.