Glimpses of Ocean Life; Or, Rock-Pools and the Lessons they Teach

by John Harper

There is a certain species of crab, Pinnotheres pisum, or common Pea-Crab, frequently found in Mytilus edulis, the Oyster, and the Common Cockle. Indeed, one gentleman states, that on his examining, on two occasions, a large number of specimens of the Cardium edule, he found that nine out of every ten cockles contained a crab. Still, in no other instance than the one my own experience furnishes, have I ever heard of the Shore-Crab, or, indeed, of any other crustacean, becoming the guest of Patella.

The classical reader will not fail to remember Pliny's statement (somewhat analogous to that above narrated) of a small crab, Pinnotheres veterum, which is always found to inhabit the Pinna,—a large species of mussel. This latter animal being blind, but muscularly strong, and its juvenile companion quick-sighted, but weak of limb, the crab, it is said, always keeps a sharp look-out, and when any danger approaches, he gladly creeps into the gaping shell for protection. Some writers assert, that when the bivalve has occasion to eat, he sends forth his faithful henchman to procure food. If any foe approaches, Pinnotheres flies for protection with his utmost speed to the anxious bosom of his friend, who, being thus warned of danger, closes his valves, and escapes the threatened attack. When, on the contrary, the crab loads himself with booty, he makes a gentle noise at the opening of the shell, which is closed during his absence, and on admission, this curious pair fraternize, and feast on the fruits of the little one's foray.

For those of my readers who may prefer verse to prose, I here append a poetical version of this fable—equally pretty, but, let me add in a whisper, equally opposed to fact, at least in its principal details:—

'In clouded depths below, the Pinna hides,
And through the silent paths obscurely glides;
A stupid wretch, and void of thoughtful care,
He forms no bait, nor lays no tempting snare;
But the dull sluggard boasts a crab his friend,
Whose busy eyes the coming prey attend.
One room contains them, and the partners dwell
Beneath the convex of one sloping shell:
Deep in the watery vast the comrades rove,
And mutual interest binds their constant love;
That wiser friend the lucky juncture tells,
When in the circuit of his gaping shells
Fish wandering enters; then the bearded guide
Warns the dull mate, and pricks his tender side.
He knows the hint, nor at the treatment grieves,
But hugs the advantage, and the pain forgives:
His closing shell the Pinna sudden joins,
And 'twixt the pressing sides his prey confines.
Thus fed by mutual aid, the friendly pair
Divide their gains, and all their plunder share.'

There is one singular feature in the Crustacea which it may prove interesting to dwell a little upon. I allude to their power of living apparently without food, or at least without any other sustenance than is afforded by the animalculæ contained in the water in which they dwell. One accurate observer states that he kept a Cray-fish for a period of two years, during which time the only food the animal received was a few worms,—not more than fifty altogether. This statement I have often had ample means of verifying. Yet, on the other hand, strange to say, the crab is always on the hunt after tit-bits; and nothing seems to give him greater delight than a good morning meal, in the shape of a newly opened Mussel, Cockle, and above all—a Pholas. Let a youthful crustacean cast its shell, and rest assured, unless its companions have had their appetites appeased, they will endeavour to fall upon and devour the defenceless animal. This, to my chagrin and annoyance, I have known to occur repeatedly. When nothing else can be procured, not only the Lobster Crabs, but any Brachyurous Decapods who may be at hand, will set to work, and industriously pick off and eat the Acorn-Barnacles attached to any object within reach. These facts show that the asceticism of the crab is not voluntary, and that when opportunity occurs, he is as fond of a good dinner as are animals possessed of a higher degree of organization.

It will be gratifying if other observers are able to verify the circumstance which I shall allude to hereafter, and which would seem to show that the exuviation of crustacea is expedited by affording specimens an unlimited supply of food.

'The organs for pursuing, seizing, tearing, and comminuting the food of the Brachyurous Decapods,' says Professor Bell, 'are carried to a high degree of development; ... these appendages consist of six pairs, of which some are actual organs of mastication, as the mandibles or the true jaws, the foot jaws or pedipalps, generally serving to keep the food in contact with the former, whilst it is being broken up by them.

'The buccal orifice in the Brachyura occupies the interior face of the cephalic division of the body, and is bounded anteriorly by a crustaceous lamina of determinate form, which has been termed the upper lip, and posteriorly by another, termed the lower lip. The mandibles occupy the sides of the opening. After these, and external to them, are the first, and then the second pair of true jaws, followed by the three pairs of pedipalps or foot jaws, the last of which, when at rest, close the mouth, and include the whole of the preceding ones. In the Macroura the pedipalps are very different in their forms, and have the aspect of very simple feet.

'The means of comminuting the food are not restricted to the complicated machinery above referred to, for the stomach itself contains a very remarkable apparatus, consisting of several hard calcareous pieces, which may be termed gastric teeth. They are attached to horny or calcareous levers, fixed in the parietes of the stomach; they are moved by a complicated system of muscles, and are admirably adapted to complete the thorough breaking-down of the aliment, which had already been to a considerable extent affected by the buccal appendages. These gastric teeth may be readily seen and examined in the larger species of Decapoda, as in the large eatable crab and the lobster; and it will be readily perceived how perfectly the different pieces are made to act upon each other, and to grind the food interposed between them.'

Having been on a certain day at the sea-side collecting, I was amused to observe the movements of two ragged little urchins, who approached near to where I stood, bottle in hand, examining some beautiful zoophytes by aid of a pocket lens. One of them had a short iron rod, with which he very dexterously hooked out any unfortunate crab who happened to have taken up its quarters in some crevice or beneath a boulder. Having captured a specimen, it was handed over to his companion, who quickly tied it to a string which he held in his hand.