Deep-Sea Echinoderms of New Zealand
This deep-water group, though widely distributed, has not previously been recorded from New Zealand. The familial characters include the following features:
Abactinal surface roofed over by a membrane, supported by the elongated paxillae, enclosing a supradorsal brood-chamber, within which the eggs develop. Water is drawn in through numerous small pores on the actinal surface and expelled through a central opening on the abactinal surface, termed the osculum.
Pteraster Mueller and Troschel, 1842
Adambulacral armature comprising a transverse comb, united by a web. Tube-feet in two series.
Subgenus Apterodon Fisher, 1940
Marginal oral spines not united in a web.
Pteraster (Apterodon) bathami sp. nov. Plate 2, Figs. G, I, holotype.
Diagnosis: Body thick and cushion-like, flat below, rounded above, subpentagonal or pentagonal-star-shaped in outline, the arm-tips turned upwards. Supradorsal membrane reticulated and bristling with the exposed tips of the paxillar spines, which support it; the paxillae usually having five (sometimes four or six) paxillar spines, the central spine not projecting, and not evident. Prominent central osculum, surrounded by a web. Adambulacral spines 6, the innermost one third as long as the outermost and the intermediate ones of proportionately varying lengths, all united in a web which adheres to the spine of the actinolateral membrane. Tube-feet in two series. Oral plates carrying 8 furrow spines, the innermost largest, the spines decreasing regularly in size from within outwards. The inner three spines are flat and spatuliform with an expanded tip, the others more slender but also flat. Suboral spine robust, recurved, sharply tapering from a broad base to an acuminate hyaline tip, longer than the innermost furrow spine of the oral plate.
Material Examined: The holotype, also photographs of a second specimen taken at the same station as the holotype, from 250–300 fathoms, off east Otago, Dom. Mus. Station B.S.191, Drs. E. Batham and R. K. Dell.
Holotype: In the Dominion Museum, Ech. 517, R 42 mm, r 28 mm. The paratype, held at the Portobello Marine Biological Station is a little larger, R 49 mm, r 35 mm.
Colour and Behaviour: Dr. Elizabeth Batham was able to keep the paratype alive at Portobello for two days, and has kindly supplied the following information: The colour in life was deep cream, or pale warm yellow, Munsell YR-Y 8/6, a uniform colouring without patterning. The osculum opened to a maximum diameter of about six millimetres for an interval of approximately 5 seconds, once every 1–2 minutes. As it opened, the disc rose up as a whole, becoming depressed again as the osculum closed, i.e., the opening and closing of the osculum is not just a local sphincter action, but involves the body as a whole. Particles of azocarmine pipetted over the osculum were vigorously squirted away as it opened, confirming that it is exhalent. During the observations, which extended over about 30 minutes, the temperature of the sea-water rose from 8.7° C to 8.9° C, but there was no obvious change in the respiration rate. The tube-feet at the tips of the arm were about twice as long as those along its length (about 1 cm long at the arm-tip).
Remarks: Pteraster bathami appears to be rather closely related to P. myonotus Fisher, from the Philippines, 74–279 fathoms (Fisher, 1919, p. 458). It differs as follows: The paxillae of P. bathami lack the central spine of P. myonotus, so that the reticular areas have a smooth floor; the areas also tend to be pentagonal rather than hexagonal, since there are five paxillar spines usually, not 6 (plus 1 central) as in myonotus. The adambulacral combs regularly comprise 6 spines in P. bathami (not 5), and the oral spines are more flattened. Fisher examined 8 specimens of P. myonotus, apparently none exceeding R 28 mm, r 24 mm, whereas both New Zealand specimens are much larger, and have relatively longer arms. If the New Zealand specimens were merely older examples of Fisher's species, one might well expect the arms to be relatively longer, and the number of adambulacral spines to be increased to 6—but it is extremely improbable that the number of paxillar spines would decrease with age, or that the central paxillar spine would be retracted. The best course is then to regard the New Zealand form as representing a separate species, closely related to the Philippines one.
From Australia H. L. Clark (1916) has described Pteraster tetracanthus, a species which evidently falls in Apterodon since no membrane connects the oral spines. This species is stated to have four adambulacral spines, and only four page 15oral spines; the pentagonal outline and narrow adambulacral armature result in relatively wide oral intermediate areas. All these characters distinguish the species from the New Zealand form.