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Tuatara: Volume 9, Issue 2, November 1961

A Dangerous Sea-Urchin

page 84

A Dangerous Sea-Urchin

It is not generally known that a venomous and potentially lethal species of sea-urchin lives in the northern waters of New Zealand. Fortunately it seems to prefer the off-shore zone, and has not so far been reported from depths less than forty fathoms. The species in question is Araeosoma thetidis, one of the subtropical elements in the Aupourian fauna of the region from Bay of Plenty to North Cape. It is very easily recognised. When fully grown it is a spherical urchin, up to eight inches in diameter, of a deep red-purple hue. It is occasionally found in the contents of trawls, and when brought to the surface it rapidly collapses into a pancake-like disc, through loss of body-fluids through the natural openings of the shell. The latter is quite flexible, as in Palaeozoic sea-urchins, and the family to which it belongs is believed to have a very ancient lineage. The venomous organs are small glands carried on the tips of the shorter spines. Contact with the poison-glands results in stings which are very painful and can cause a general paralysis or even death. No cases of injury have been reported in New Zealand, so far as known, but the same species occurs on the north-east coast of Australia, where it has an ill repute. A related form in the Japanese Inland Sea is a known man-killer, as it becomes entangled in fishermen's nets at night, so that accidental contact can occur. In Indonesia another species comes up to the tidal zone to feed on sewage. The New Zealand species, like most of its kind, probably feeds on leaves of land-plants washed out to sea. This strange habit is shared by most of the deep-sea echinoids, whose gut-contents usually consist largely of fragments of phanerogams. The herbivorous habit is found in widely diverse and unrelated abyssal sea-urchins and seems to be a characteristic feature of their ecology on the floor of the deep-sea, even far from land. One genus almost invariably has the gut crammed with fine shavings of wood, and it can only be assumed that its various species seek out sunken logs and gnaw them. Another peculiar feature of Araeosoma is that some of the spines of the lower surface carry at the extremity a calcareous expansion shaped very like a horse's hoof; indeed, the spines are in fact known as hooves in the technical literature, though their function is unknown. One such hoof was recently found in a deep-water sample from Cook Strait, so some relative of the Bay of Plenty species evidently ranges our more southern waters. Needless to say, the greatest caution should always be exercised in dealing with any sea-urchin which exhibits the features here described, and they must never be handled.

H. B. F.