On Shell-less Shellfish

Auckland Star, Volume LXX, Issue 88, 15 April 1939, Page 3 (Supplement)

On Shell-less Shellfish

ByA. W. B. Powell

ONE thinks of a shellfish in terms of a soft-bodied, succulent morsel encased in a hard shell. In fact, we have no difficulty in defining a shellfish as a creature with no internal skeleton, but a hard outer coating of carbonate of lime in the form of a one-piece spirally coiled shell or a pair of shelly valves as in the familiar bivalve. Sooner or later, however, our definition of a shellfish becomes badly upset by the realisation that certain softbodied creatures also are shellfish. Some have vestigial internal shells that render the relationship feasible, but what of others that are entirely softbodied yet are classed as shellfish nevertheless ? Such are the nudibranchs or seasltigs —entirely without shells, yet by their anatomy clearly related to the normal shelled kinds. They are instances of shellfish that have discarded the shell as a protective device and .rely upon their coloration for either concealment or defence. One does not expect aggressive behaviour from a brightly-coloured, softbodied organism, but some of our most attractive sea-slugs are voracious carnivores nevertheless. In 1901 a new species of sea slug was trawled between Kaipara and New Plymouth, and was subsequently named Tritonia incerta by the late Swedish specialist, Dr. Bergh. This specimen lost all its colour in the preservative, and it looked a very unattractive object. No further specimens were found during many years until 1920, when I obtained three from a local trawler. Shortly afterwards, in December, 1920, a particularly low tide enabled me'to reach a kelp-covered reef on the west coast of Motutara, one that I had never before been able td reach. Imagine my delight on parting the masses of kelp streamers that masked a deep rock-pool to see crawling on the sides not one, but fifteen examples of a vivid vermilion slug with white tentacles and feathery gills arranged all along the side—none other than the almost forgotten incerta. Upon dissecting one of the specimens, I found huge jaws almost the full width of the body, and a dental ribbon studded wi th thousands of sharp teeth. Although outwardly harmless enough, this soft-bodied, delicate-looking creature was shown to have strongly carnivorous tendencies. But, why the conspicuous coloration—surely advertising its own. unarmoured body? However, there seems to be an unwritten law regarding the colours of marine animals—yellows and reds seem to convey to other predaceous creatures a warning signal of -unpalatable or even poisonous qualities. It is on account of this phenomenon that this sea slug can afford to roam shell-less and otherwise unprotected. Most of the sea slugs are protectively coloured; that is, they harmonise per-

fectly with their surroundings. One of the best examples is a small greenish slug, Elysia, which until a few years ago was unknown in New Zealand, simply because its disguise is so perfect. It lives on a green velvety seaweed known as Codium, and its presence was noticed only from the spawn coils, which are whitish and formed like the hairspring of a watch. Will the Elysia eventually complete its disguise by producing spawn coils also that will harmonise with the Codium T The gills in most sea slugs are arranged as a circle of filmy plumes towards the tail; others have these set in a series around the edge of the body, and. in some specialised kinds there are no special breathing organs, respiration probably functioning through the skin. Near the front end in most species are a pair of retractible club-shaped appendages which, act as organs of smell. Step by step the sea slugs lead us further and further away from the orthodox shellfish, the most fantastic being the aeroplane-like Glaucilla and

the ribbon-like Phylliroe. Both are special adaptations to a life on the high seas, where they drift or swim about on the surface, often far from land. In the illustration of the Glaucilla will be seen numerous tentacle-like processes—a most weird .-tdaptation, for these represent the liver, utilised in this special case as swimming organs as well. Glaucilla has been taken in New Zealand waters but once, a specimen washed ashore at Mairangi Bay, Auckland, having been sent to the Auckland Museum in October, 1934, by Mr. A. S. Martin. The Phylliroe, likewise, ig known in New Zealand only by a single specimen that was caught swimming off Mokohinau Islands in July, 1930, by Mr. Anderson of the Fisheries Department, Auckland. This queer "shellfish" is not only shell-less, but it has the body compressed almost to paper thinness, so that its entire anatomy can be seen without recourse to dissection. Another remarkable feature is that it is shaped like a fish and swims by undulating the body in a fish-like manner, yet, of course, there is not the slightest relationship. All this goes to show how marvellously adaptable shellfish are, for there are kinds to suit almost every mode of life, from tha shore to the ocean depths, the surface of the vast oceans, and even

the dry land itself as well as the fresh waters. The greatest difficulty in studying the sea stage is to preserve the specimen*. They are like delicate flowers, in that we are allowed but a short time to examine them, after which they wilt and die. No satisfactory method of preserving the colours of sea slugs is known, and even the preservation of their shapes is very difficult. If plunged straight into alcohol or any other preservative the specimens immediately become shapeless and almost unrecognisable! The only satisfactory method of preserving sea slugs in an expanded and natural condition w to slowly narcotise*-them by the addition of "Epsom salts" or menthol crystals to the sea water, and later transfer the specimens to the preservative, when they have been thoroughly anaesthetised. i A marine aquarium can be made most • fascinating and colourful by the addi- , tion of these slags. In fact, we have in i Auckland what amounts to t mammoth - aquarium, although it was never in--3 tended as such. This is the Orakei - Basin, once tidal, but now ( enclosed by , sluice gates. Here a certain sea slug, t RanSosaclesia glauca, has welcomed the change in its habitat as far more desir- - able than, nature ever intended it to be. e If the bathers at Orakei Basin do not f know this nuisance as Raniosaclesia >, glauca, they probably have an even a worse name for it.