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

Bathymetric Range

Bathymetric Range

The single specimen from Collection 83 was taken during the day in a beam trawl which struck mud bottom at a depth of about 550 fathoms. This had been intended as a bathypelagic haul, and it only unintentionally collected on the bottom. Associated with this specimen of Sergestes potens were one specimen each of the typically bathypelagic natants Sergestes arcticus, Pasiphaea sp. and Acanthephyra cf. quadrispinosa, as well as a specimen of the characteristically benthic genus Sclerocrangon. Similarly the single specimen from Collection 100 was taken during the day in a beam trawl working on the bottom in about 380 fathoms. Although this was primarily a bottom haul, both Sergestes arcticus and Pasiphaea sp. were associated as above. As the three bathypelagic forms, but not S. potens, have been taken on numerous occasions by mid-water nets in Cook Strait, it is reasonable to suppose that the latter, like the crangonid, is a benthic species, at least during the day.

However, all Sergestidae, with the single exception of the shallow-water genus Sicyonella, have been regarded, from the evidence of their statoliths, as nektonic (Burkenroad, 1937b). In particular all members of the genus Sergestes examined, page 22have "autogenous statoliths" (i.e., self-secreted cuticular pellets) rather than exogenous concretions (containing material introduced from outside), such as are found in Sicyonella and other benthic penaeids. I have examined the statoliths of specimens of S. potens from both 1941 and the 1942 collections, as well as one from the Collection 83 specimen, and in every case they consist of transparent oval cuticular pellets, slightly pointed at one end, with no trace of extraneous matter included. I do not consider that the possession of autogenous statoliths is necessarily indisputable evidence against S. potens being at least a partially benthic species as apparently is the case in the Japanese S. (Sergia) lucens as recorded by Nakazawa (1915). This paper is in Japanese, but I have seen, in the British Museum (Natural History), an English abstract, provided by the author himself, and referred to by Gordon (1935: 313). Nakazawa states that S. lucens (recorded as S. prehensilis) lives on a bottom of mud and fine sand during the day in depths down to about 100 fathoms and rises at night to mid-water in winter or to near the surface in summer for "swarming", where, on dark nights in May and June, their luminescence makes a wonderful sight. They are fished in great quantities, about two miles off shore in Suruga Bay, with floatless purse seins lowered to between 50 and 100 fathoms on dark nights, or in conditions of high turbidity after flooding, during the daytime. S. lucens is of great economic importance in Japan, and although the fishery started accidentally in 1894, the total annual catch in 1915 was about 10 million lbs.

Now if S. potens had the same habits as S. lucens and rose to mid-water nocturn-ally, shoals could be swept by tidal currents over shallower water and on sinking with the approach of daylight would be taken in large numbers by groper and ling. These fish are voracious bottom-feeders, being recorded down to at least 150 fathoms. That this commonly and regularly happens in Cook Strait is borne out by the fact that the stomach contents of these two fish seasonally contain bathypelagic fish, squid and shrimp, which normally are taken well below the depth at which the fish were caught. It should be pointed out that the Pawley brothers lifted their lines in the morning and from the fresh appearance of these luminous shrimps they were probably taken only a short time before the fish themselves were actually caught.