MARINE NATURE NOTES

Otago Daily Times, Issue 22712, 26 October 1935, Page 2

MARINE NATURE NOTES

Written for the Otago Daily Times. By David H. Graham, F.R.M.S., P.Z.S. COLLECTORS’ SPECIMENS. Two specimens of the well-known New Zealand shore crab have been sent to me from Timaru with an inquiry whether they are the European edible crab imported from London many years ago, and why one of the specimens should be soft, while the other has the usual hard back and legs. The questions raise interesting points of topical interest to most readers. Both specimens belong to the family Cancridse and known to science as Cancer novie-zealandian The reason why one crab is soft is that it had just discarded its last year’s coat and the new one had not hardened. In these circumstances it usually takes a few days to become like its companions. Crabs shed their coats in the same way as lobsters, shrimps and all other crustaceans. With regard to this soft crab, one of the most remarkable phenomena connected with the history of crustaceans (lobsters, crabs, etc.,) is the periodical change of shell or coat to which they are subject—a change that is obviously necessary when we consider the dense, unyielding nature of the stony armour in which their bodies are encased. That they must get rid of their old suit of mail when it becomes too small for their increasing size will be evident enough to all; but how this is accomplished is a problem, the solution of which presents so many difficulties, apparently insurmountable to the average reader. Nothing short of actual observation would enable one to believe that it is accomplished in the way it is, for these animals achieve a feat which might well be considered impossible. The mode adopted by Nature is as follows: —At certain seasons of the year, other than when a crab is about to change its coat, the legs and body are full of solid flesh; but when the time approaches for this change, the muscles become flabby and watery, and, in this state, are unfit for food. A soft skin is formed between the old solid shell and the body of the animal, and soon after the present shell cracks or opens horizontally between the hind pair of legs and the head part and allows the confined crab to escape, clothed in its new thin skin. To follow the course of this remarkable process of Nature is extremely interesting to the observer and will be witnessed, at least on the first time of its occurrence, with great surprise. Often and often have I had single crabs and single lobsters in a tank and left them at night as usual, and on returning next morning found what appeared at first sight to be two crabs or two lobsters in the tank where only one was left the night before. In fact, I have often - thought a stranger had somehow been introduced into the tank without my knowledge. But this was not the case. What had happened was that the crab had emerged from its old coat and -was there clothed in its new garb, and close beside it lay its former self, now an empty shell. In nearly every case the old shell is always found apparently quite entire, with all the accessories and minutest parts in their place;—bristles, antenme, eyes and breathing organs all exactly resembling the living and emancipated crab. When we consider the amazing complexity of structure presented by the individual portions of the cast-off skeleton, the joints of the various limbs, the many jaws, the stony teeth, the innumerable facets of the eyes, and reflect upon the difficulties attending tho_ reconstruction of organs so admirable and so delicate, we cannot help but say that few natural phenomena can be regarded as more wonderful than the process of a crab changing its coat. Before crustaceans change their coats they usually depart to some rocky part of the coastline, or under stones where they will be secure from prowling fish and other animals, or even other crabs, which delight in catching these poor unfortunate animals in this soft state, when they are so much easier to devour than they are while fully armoured. The new skin soon hardens and at the end of four days or so the crab is ready to go out into the open once again and rely on its armour for protection. It has been my good fortune on many occasions while shore collecting to have found a number of cither crabs or crayfish huddled in one sheltered nook, all having emerged from their old coats at the one time and lying one on top of the other, no doubt hoping against hope that no marauding animal would find them in their nakedness. In genera!, crabs are shy and fearful, yet they are bold and will advance fiercely with uplifted claws to attack any foe that comes near. But in their soft stage they arc ready to lie in some snug hole in safety. The famous naturalist Darwin tells of how the crabs which have just changed their jackets for new and soft ones lie in he ps inside a snug hole, while a sound crab which is not going to change his jacket or has done so some time ago will be seen paddling about in tront of the poor naked crabs doing duty as a sentinel, trying to keep all enemies off. The European crab is Cancer pagurus and is the best known of all the crabs in Britain, and the most important as an article of food. It is almost identical with our New Zealand species and only a specialist can distinguish the two. Young crabs are common between tide marks, but large ones are seldom found above the low-water mark. They are caught in crab pots, which are traps made of wicker or iron wire baited with fish or other refuse and sunk in suitable places. Very large crabs may weigh as much as 101 b or 121 b, and measure up to 10 inches across the broad carapace, while the largest cancer crab I have seen here measured five and a-half inches across its back. Up till a few weeks ago I had not seen crabs sold in New Zealand. While visiting Wellington last month, however, I saw several fish shops with crabs for sale, but in no instance did I see a specimen above three inches across its back. The New Zealand cancer crab is an extremely common species in and outside the Otago Harbour and up and down the coastline from lowwater mark to 40 fathoms. For some unknown reason very few cancer crabs are found with eggs, I suppose that during my fishery research we trailed thousands of them and only one was found with eggs. On another occasion one with eggs was taken from the stomach of a ling caught in Blueskin Bay. About 10 or 12 European crabs were sent out from London in 1907 and were kept at the marine station at Portobello, from which large numbers of young were liberated from 1908 to 1911. Again in 1913 about 40 more were landed in Port Chalmers for Portobello, where. I understand, most of them died, apparently in consequence of the mud in the ponds, as the gill cavities wore found to be choked with mud, which not only lowered their vitality but possibly prevented the oxidation of their blood. To give the others a fighting chance about 20 were liberated. According to statistics many million larvae were liberated, but up to the present time no crabs of these parents have been found. The reason of this is obscure. Should any reader find what ho considers a specimen of a European crab and forwards it on to me I shall be pleased to identify it. |