MARINE NATURE NOTES.

Otago Daily Times, Issue 21266, 21 February 1931, Page 3

MARINE NATURE NOTES.

Written for the Otago Daily Times By David H. Graham, F.Z.S. Biologist to the Marine Fisheries Invesigation and Biological Station, Port Chalmers. HERMIT CRABS. These interesting creatures must be familiar to all seaside naturalists and those who visit the seashore. If not, readers will be well advised to take advantage of a low tide on some rocky beach or pool when they will, no doubt, see shells moving about in all directions, much mope quickly than shellfish move. These are not shell-fish, but hermit crabs, which have taken possession of these empty shells and are using them as their place of residence. Hermit crabs differ considerably from the true crabs, rather resembling lobsters or crayfish in the length of the body and feelers. They have only three well-developed pairs of legs, of which the last two are considerably reduced. The usual fins of the tail become transformed into a kind of holding or sucking apparatus, by the aid of which the animal is able to take a firm hold of the interior of its house, and thus able to carry it about with little or no difficulty, To be scientific, these hermit crabs are marine decapod crustaceans, belonging to the order Anomura, and the group Puguridae. The word “ decapod ” implies that it is an animal with 10 feet. “ Crustaceans ”is the term applied to those animals which are covered with a soft shell or crust, and consist of crabs, lobsters, crayfish, prawns, and many others. The word “hermit” was given to this creature by Aristotle, because it lived by itself; retiring from society and living in solitude. In other countries it is frequently called _ “ soldier crab,” probably because a soldier dwells in a tent or sentry box, and lives alone, or possibly because of the fighting tendencies of the crab. These crabs are the oddest and most interesting of crustaceans. A number of them are kept in the tanks at this station for scientific observation, and for visitors to observe. They are active animals, despite their awkward-looking house, and are always on the lookout for food, and ever ready to kill and eat any creature, dead or alive, and woe betide any animal becoming weak (even one of its own kind) with a herfhit crab about. Baby hermit crabs, as soon as they can walk or crawl, look for an empty shell of a small size to suit their body and thus make their first home. Small hermit crabs, not a quarter of an inch in length, have been collected by the writer, each one with a minute shell for its house. It will be seen, therefore, that they soon find their own homes and fend for themselves —quite differently from civilised human beings who want eveything provided for them.

Visitors frequently ask one or two questions about these hermit crabs. Why does this species of crab inhabit empty shells? When did this tendency of hermit crabs to occupy empty shells become a habit? There may be two reasons: First, the crab may have been originally created as it is now, designed with the express purpose of inhabiting empty shells, to which its structure was arranged—an explanation I very much doubt. Or, secondly, the hermit crab was furnished with a hard crust as in all other crustaceans, and, to escape his cnepiies, which found him a tasty dish, made a practice of hiding in empty shells, and thus the practice has become an organised habit in the species. There is a considerable number of species throughout the world, 10 different kinds alone being recorded around the Otago coast,* and each and every one will be found either living in shells or hollow tubes of coral.

The fore part of the hermit crab’s body is furnished with claws and armour as in other crustaceans, but the hind parts are soft and useless for walkings crawling, or swimming, and as they arc thus unprotected, nothing remains for these helpless creatures but to seek shelter, and then the empty shells or shellfish are utilised. The front legs of these crabs are strong and massive in comparison to the rest of the animal, for it is upon* these legs that the whole of the work of locomotion, lighting, defence, and securing of food must devolve. The extra work that is thrown on the front claws has not only made them strong but exceedingly sharp and hardened, and they are capably used by the owners for lighting and for hauling their homes about the rocky shores. These front legs well repay a closer examination. They are peculiarly adapted in form to suit the house in which the crabs'live. They are of very unequal size—one is remarkably short and slender and the other larger and of unequal size. When the animal retires inside his house, the larger claw- is the last to be drawn up, and when the slender claw is safely inside the larger claw is drawn in as far as possible, when it forms a door to the house—a door almost the same shape as the opening in the shell. This claw, being much more developed than its mate, makes a door which defies the most daring intruder. Thus nature provides for her lowly creatures in no small measure of ingenuity, for, had both claws been of equal Size, it would have been impossible for the crab to withdraw inside thei shell.

Not only do these hermit crabs have to change their old coat each year as ■ all other crustaceans do, but as they change their old coat they naturally grow, and become too large for their shell house. They then have to look around and find other shells larger than those they previously inhabited. Here comes the most dangerous time in a hermit - crab’s life. In order to find a new home, he has to trundle his house along till he finds an empty shell, and at the same time keep a sharp look out that there is no preying, hungry animal about looking for food, as these soft tailed crabs are a favourite dish with many of our sea creatures. Nothing is more ludicrous than to see these crabs in search of new homes. Every shell they come to is carefully examined with their front claws to see that no intruder is within ready to grasp the hermit crab’s soft tail. When the hermit has made sure there is no occupant within, he looks carefully around to see there are no fish or other preying animals about. Hauling his present home up as close as possible to what be considers may be his new home, be has a flnai good look around for any danger. When everything seems safe, in a twinkling ho has crawled out of his old home and backed his soft tail into the new one, and if it is suitable, he stops there, but if he is not too sure he will retreat into his old homo, and abothcr attempt will be made to see if he can use the new home. Sometimes he will try a number of , empty shells, and returning again to the first one tried, will possibly find it suited his soft tail better than any of the others. It is a case similar to that of persons trying on new clothes. The hind parts of the hermit crab being soft make it an easy prey for other animals. Hence lie does not survive long out of his home. It is a curious sight to watch two of these crabs that have been deprived of their houses and placed in a bowl of water. Each era!) is ever on the alert, and realising their soft, undefended tails, they always keep head on to each other. When an empty shell was dropped into the bowl on one occasion, both crabs made a forward move towards the shell; they met at the door, and struck with their front elaws, and each had a firm hold of the other. A tug of war now began. First one backed bis opponent round (he “erstwhile ring,” then the other took the offensive, and more tug-

ging and pushing occurred. So determined were these crabs that the fight continued for two hours, neither releasing hold. To hasten the end of the fight I touched the tail of the larger crab. This must have made him relax his hold (though it was not visible to me)' for the smaller crab either saw my movement, or felt the larger crab relax, for in an instant it threw the larger one over on to his back, and scuttled to the shell, quickly pushing a claw inside fo satisfy his cautious mind that all was safe. Then with surprising agility he slipped his tail into the opening and retreated inside the shell. By this time the defeated crab was up to attack the owner of the shell and attempted to claw the outside of his claws, but without any effect. The defeated crab would wait outside behind the shell for the owner to protrude, but he was far too cute for him. The homeless crab then tried other tactics. He would roll the shell over and over, tip it upside down and wait for the possessor to come out and right himself, but the hermit was too wary a creature for such tricks and remained firmly established inside. Hermit crabs are very timid or cautious, for the least movement or a shadow passing over will cause them to retreat, drawing in the smaller claw, leaving the larger to bar the entrance. They are also very quarrelsome, and two hermit crabs cannot meet without showing “ fight ” to each other. They extend their claws, and try to touch each other, like two boxers “shaping” up to each other, “sparring” and endeavouring to get a grip.

On one occasion I observed two hermit crabs fighting for the supremacy of another’s house. One caught hold of the other by the back of the head and after killing it at once proceeded to eat it literally “ out of house and home,” and afterwards lived in the house, affording the striking example of the old but true saying— The short, the simple plan, That they shall take who have the power,

And they shall keep who can. These pugnacious creatures, however, have their own particular friends in the sea wor.ld. They are very partial to having anemones (sea flowers) on their borrowed shell houses. The old time naturalists asserted that the crab drags the anemone from its habitat and places it upon its shell house. I have spent a good deal of time watching these hermit crabs in the observation tanks of this station, but never once have I seen this happen. But I have often seen one trundling its shell-house till it was hard up against an anemone which had fixed itself to the floor of a tank. The crab would then wait for some time, after which it would begin tapping at the edge of the base of the sea anemone. If the sea anemone was willing, it would be seen to lift ' its foot or base slowly and crawl on to the back of the hermit’s house, taking about an hour in the process. During this interesting operation, the crab kept the house perfectly still, but frequently came out to watch the proceedings, and when the anemone was gafely on top of his house, the hermit would go for a walk round the tank as though showing off his ornamentations. The favourite anemone for this curious act is a very pretty species called Tealia Thompson!, with violet tipped tentacles. It is curious that most of these hermit crabs have the habit of associating with other animals, decorating their houses as it were. Some have anemones, other sponges, tunicates, shells, and hydroids. This is probably of mutual benefit to the animals. The animals mentioned are mostly distasteful to fish, especially the sea, anemone, with its batteries of stinging cells, and its presence on the shell is a protection to.the crab. The anemone and other animals profit, too, by the association, as the crab moves about from place to place searching for food, giving them more opportunities to gather food than if they were stationary. Then again the anemone will be seen to bend over in such a way as to catch up pieces of fish that fall from the hermit crab’s mouth. Then, again, I have seen the anemone catch a fish, and after eating its fill, drop the balance on the flooi*, when the hermit crab would finish the remainder. This benefit companionship is scientifically called “ commensalism ” —in other words eating or sharing the same table. Sometimes, worms live in association with the hermit crab, and, when the crab is feeding, the worm can be seen to come out of its, hiding place and share the meal. These hermit crabs with their heavy shells and other growths frequently cause a good deal of trouble in a deli-cately-stocked tank, for, as soon as darkness falls, they begin to crawl about, dragging their houses with them, killing, destroying, and eating anything that they can get hold of; even their own brothers and sisters are not spared. As they crawl about, their large, lumbering house breaks and mangles everything of a delicate nature, and truly it may bo said, when they have sway in the tanks:--Ruin itself stands still for lack of worth, And desolation keeps unbroken sabbath. As Diogenes held in contempt the common laws and aims in life, so the hermit crab may well be called the Diogenes of the aquarium tanks, as he obeys no laws, spares no one, neither kith nor kin escaping his pugnacious nature.