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New Zealand Coelenterates Ctenophores from Cook Strait
Order Cydippida
Order Cydippida
Pleurobrachia pileus (O. F. Muller).
This species has been recorded from New Zealand waters by Moser (1909) and Ralph (1949) and the present study only places on record that the New
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Zealand specimens of P. pileus are efficient fish catchers. On several occasions larval fish observed when the net was taken on board were missing when the haul was later examined. Once P. pileus was seen to come in contact with a larval fish as large as itself. The fish seemed to die immediately and in approximately an hour was within the stomodeum. The economic importance of this habit has been emphasized elsewhere. (Bigelow, 1924.)
Pleurobrachia helicoides sp. now Text Fig. I .
Specific Description: Body of living specimen cylindrical, with long comb rows extending from the aboral pole to nearly the oral pole; meridional canals slightly greater in length than the comb rows broad paragastric canals, nearly as wide as the stomodeum; tentilla. tightly coiled into a barrel-shaped helix, orange-pink in colour.
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Text Fig. I.
Pleurobrachia helicoides n.sp. P, paragastric canal; TB, tentacle base; TS, tentacle sheath; TT, coiled helix of tentilla; S, stomodeum.
Specimens were taken on two occasions—July 22nd and 23rd 1935. They were very active swimmers and intolerant to any irritation. The specimen described below
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was 11 mm. long, approximately cylindrical, with blunt oral and aboral ends. The eight bands of swimming plates almost cover the eight meridional canals and extend from the aboral pole to nearly the mouth rim. A faint pink colour can be distinguished underneath the comb rows. The two long contractile tentacles are based in the usual position for Pleurobrachia—midway between the outer body wall and the funnel axis. The tentacles give rise at long intervals to lateral filaments tightly coiled into barrel-shaped helicles which are orange-pink in colour. To the naked eye the tentacle resembles the fishing thread of a young Physophora. The helicles were not seen to uncoil. The bottom of the tentacle sheath is filled with liquid of refractive index similar to the oil bubble in a fish egg. The apical sense organ when first examined in the living animal is prominent, projecting above the polar surface, but as the specimens become fatigued it sinks into a pit and the surrounding tissue between the swimming plates swells upward to form the walls of the pit. The funnel tube is nearly one-third the length of the body, and of constant diameter. The paragastric canals are greatly flattened and very wide, as wide as the stomodeum. No genital products are visible in the meridional canals.
Discussion
The very broad paragastric canals distinguish P. helicoides from all known species of Pleurobrachia. They can further be distinguished by body shape, pigmentation, etc. Firstly they are distinguished from P. pileus O. F. Muller 1776 by their cylindrical body shape, long comb rows and by their colour—pale pink underneath the comb rows and orange-pink tentilla; from P. bachei A. Agassiz 1865 and P. globosa Moser 1909 by the difference in position of the junction of the adradial canals with the meridional canals; from P. pigmentata and P. striata Moser 1909 by the absence of pigmented "paddle plates" and from P. crinata Moser 1909 and P. australis (Benham) 1907 (Syn. Euplokamis australis Benham 1907) by the absence of a funnel-shaped collar round the mouth. The general body shape, comb rows, and canal system of P. helicoides is similar to P. australis and both species have been taken from Cook Strait, but not at the same time. The weight of evidence is such that despite the general similarity in appearance and canal structure P. helicoides does not fall within the specific description of P. australis as given by Benham. The broad paragastric canals, tightly coiled tentilla, the presence of colour, and the lack of a funnel-shaped collar round the mouth distinguish P. helicoides from P. australis. Nevertheless it must be borne in mind that only a few specimens of each species have been examined and it would not be surprising to find in a genus where wide variation in form, colour, and even canal structure is permissible within a species that examination of a large number of specimens would yield sufficient evidence for the specific description of P. australis to be widened to include those forms at present distinguished as P, helicoides. P. helicoides may be in fact only a seasonal variant.