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Deep-Water Crustacea of the Genus Sergestes (Decapoda, Natantia) from Cook Strait, New Zealand

Sergestes (Sergestes) cf. S. seminudus Hansen, 1919

Sergestes (Sergestes) cf. S. seminudus Hansen, 1919

? 1919. Sergestes seminudus Hansen, Siboga Exped. XXXVIII: 18, Pl. I.

Material Examined

Victoria University, Zoology Department, Cook Strait Collections.

Collection VUZ 84 (Station CUK) as given above—1 ♀ 20 mm.

Extended Diagnosis

The single female specimen available has the cephalothorax separated from the abdomen and both 3rd maxillipeds and certain pereiopods missing. A short description of the main characters of this damaged specimen is given below.

A relatively large, attenuated, firm-bodied shrimp with long appendages and prominently ridged carapace.

Rostrum relatively small, compressed, very slightly raised above the anterodorsal mid-line of the carapace and hardly projecting beyond the anterolateral margin of the carapace. Anterior margin of the rostrum with an acute tip flanked dorsally and ventrally with short concave portions. Carapace relatively long and attenuated, greatest depth, at posterior margin, a little less than two-fifths the length. Supraorbital crest prominent, but supraorbital spine absent. Hepatic spine small but distinct. Cervical groove well defined dorsally and laterally, almost continuous anteriorly with a well defined lateral ridge running from anterior to the hepatic spine towards the base of the antenna. Suprabranchial groove and ridge prominent with a slightly less prominent horizontal ridge laterally across the branchial region.

Telson weakly grooved dorsally, distally setose with no dorsal spinules present.

Eyes not large, cornea considerably shorter but broader than distal segment of ocular peduncle. Ocular tubercle present, broader than high.

Antennular peduncle relatively narrow and attenuated, proximal segment a little longer than the third, which is itself longer than the second. Antennal scaphocerite, with outer margin distinctly convex, reaches a little beyond the proximal third of the third segment.

Mouth parts as for genus. As both 3rd maxillipeds are missing, it is impossible on this feature to be sure of the group to which this specimen belongs.

The 1st pereiopod reaches a little beyond the distal end of the antennular peduncle and is non-chelate. 2nd and 3rd pereiopods extend well beyond the antennular peduncle, and both bear minute chelae of the type apparently characteristic of the "S. corniculum" and the "S. sargassi" groups (see Burkenroad, 1937a: 320). Here the fixed finger is shorter than the free finger, both bearing distally a fan of setae, while the fixed finger and a portion of the hand bear a conspicuous longitudinal series of very long plumed setae exactly as shown by Hansen (1922, Pl. VII, figs. g–h). The ischia of 1st and 2nd pereiopods bear a spine disto-laterally. The 4th and 5th pereiopods are both of the compressed laminar type usual in the genus, the 4th reaching to the distal end of the antennal peduncle, while the 5th, with both margins of the two distal segments setose, reaches a little beyond the distal half of the merus of the 4th.

Setose portion of external margin of uropodal exopodite half as long again as non-setose portion. No tooth present on this outer margin.

Female with no teeth or lobes medially on coxa of 3rd pereiopod, but with a small spinule posterodistal to the opening of oviduct.

page 15

Branchial Formula

As given for S. arcticus above. The anterior arthrobranch of the 3rd pereiopod is nearly twice the length of the anterior branch of the 4th, which is a little longer than the posterior branch of the 4th, the latter being subequal to the posterior branch of the 3rd pereiopod.

Colour in Life

Organs of Pesta present. The coloration of this "half red" species was observed in the fresh state as follows. Red stellate chromatophores were scattered all over body and appendages, but with a greater concentration on the carapace. The posterior half of the dorsal surface of the carapace had a distinct purplish-blue cuticular pigment present, and another small patch of the same pigment occurred laterally on the carapace in the area of the anterior organ of Pesta. The stomach is clearly visible through the carapace as a large greenish-black anterodorsal mass.

Systematic Position

As the two distal segments of the 5th pereiopods are setose on both margins and organs of Pesta are present, the specimen belongs to either the "S. corniculum" or the "S. sargassi" groups of the subgenus Sergestes s.s. The absence of both 3rd maxillipeds in the only specimen available, does not allow it to be placed in one of these two groups, as the relative length of this appendage is apparently the only feature distinguishing one group from the other (see Burkenroad, 1937a: 320). However, owing to the great similarity in appearance between this specimen and the illustrations of S. corniculum, especially as regards the ridges, grooves and shape of the carapace, I believe it to be closely related to that species. Assuming it to belong to the "S. corniculum" group, the absence of a supraorbital spine distinguishes it from S. corniculum, S. disjunctus and apparently from the inadequately described S. nipponensis; the presence of a transverse groove between the hepatic spine and the lateral ridge anterior to it distinguishes it from S. rubroguttatus and S. coalitus; while the low ocular tubercle distinguishes it from S. erectus. This leaves only S. seminudus to be considered. This species described from Indonesian waters by Hansen (1919), is very similar indeed to the New Zealand specimen, differing only to a small extent in the relative lengths of the segments of the antennular peduncle. However, our female specimen cannot be definitely identified with S. seminudus (if. indeed, it belongs to the "S. corniculum" group) until mature males arc taken and the petasma compared to that of Hansen's species.

It should be noted here that Burkenroad (quoted by Gurney and Lebour, 1940: 38) states that there are three species in the Atlantic confused with S. corniculum "but he is not at present able to give a final opinion or names for the species which he has distinguished". In the same year Burkenroad (1940) described five new species belonging to the S. corniculum and the S. sargassi groups, by comparing them, without illustrations, to one another and to S. seminudus, S. henseni and S. "sargassi Ortmann" of Hansen. This does not allow us to know what he means by these species. Until his promised revision of the genus (1937a, 1940, 1945) is published, it is very difficult, if not impossible, to identify many species which he has dealt with in a preliminary manner.