Perhaps the simplest form of association between two animals is found where one utilizes the other as a means of transport. The little Gulf-weed Crab, previously mentioned, is very often found clinging to the carapace or skin of large marine turtles. It is not a parasite, since it can hardly derive any food from the Turtle itself; neither is it a commensal, for there is no evidence that it shares in the Turtle's meals. It probably takes to a Turtle, when it can find one, as giving it a wider range of operations than is afforded by its usual drift-log or tuft of sargasso-weed. A somewhat similar case is afforded by some of the Barnacles that are found on the skin of Whales. The species of Conchoderma, for instance, are often found on certain Whales, but they may also occur on inanimate floating objects. Other Whale-infesting Cirripedes, however, are specially adapted to their habitat, and never occur elsewhere. For example, Coronula ([Plate XXVIII].) is a genus of sessile Barnacles in which the shell is elaborately folded, forming a series of chambers into which prolongations of the Whale's epidermis grow, securely fixing the shell. Tubicinella is even more effectively protected against dislodgment, for its shell is sunk in the thickness of the Whale's skin, with only the opening exposed. Other genera of sessile Barnacles (Chelonobia, etc.) are found adhering to the shell of Turtles. The increased food-supply made available by the host's movements through the water is probably the chief advantage that the Barnacles gain in such cases. This is indicated by the fact that certain small stalked Barnacles (Dichelaspis, etc.), found on large Crabs and Lobsters in tropical seas, generally cluster on the mouth parts of their hosts, near the entrances to, or even within, the gill chambers, profiting no doubt by the respiratory currents and the food particles they carry.
GROUP OF BARNACLES, Coronula diadema, ON THE SKIN OF A WHALE. JAPAN. (REDUCED)
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A great variety of Crustacea find shelter and defence in association with Sponges, Corals, and other more or less sedentary animals. Sponges are not eaten by many marine animals, the needle-like spicules which often form their skeleton no doubt helping to render them distasteful, and many small Crustacea, Amphipods, Isopods, Prawns, etc., profit by their immunity from attack, and take up their abode in the internal channels and cavities of the Sponge. The beautiful siliceous Sponge known as "Venus's Flower-basket" (Euplectella) very often contains imprisoned within it specimens of a delicate little Prawn (Spongicola venusta) or of an Isopod (Æga spongiophila). As these Crustacea share with the Sponge the food particles drawn in by the currents of water passing through the pores in its walls, they are in the strict sense commensals.
The Corals and various other animal organisms commonly known as "Zoophytes," forming together with the Jellyfishes the group Cœlentera, are very effectively protected against the attacks of most predatory animals by the possession of "stinging cells," and this protection is shared by many other animals which shelter among them. Thus, the branching Coral stocks which grow in great luxuriance on tropical coasts support a rich and varied assemblage of animals, some of which may actually prey upon the Coral polypes, but all of which profit by the fact that few enemies venture to pursue them in their retreats. Innumerable prawn-like animals of the Alpheidæ and other families, and many kinds of Crabs, are found among living Corals. The Crabs of the family Trapeziidæ are especially characteristic of such habitats, and their thin, flat bodies seem to be adapted to slip into slits and crannies of the Coral blocks. The most highly specialized of all Coral Crabs, however, are the species of the family Hapalocarcinidæ, which modify in various ways the growth of the corals on which they live. In some of the more delicately branched kinds of Coral there may sometimes be found hollow bulbous growths, each of which contains imprisoned within it a little Crab—Hapalocarcinus marsupialis ([Fig. 66]). It seems that the female Crab (the habits of the male are not definitely known) settles down among the branches of the Coral, and that the irritation of its presence causes the branches to grow up and surround it, coalescing with each other to form a kind of cage, and ultimately leaving only one or two small openings. Through these openings water can enter to enable the Crab to breathe, and no doubt food particles find their way in, but it is not possible for the Crab to leave its prison. The production of these abnormal growths of the Coral is closely analogous to the formation of "galls" on plants as a result of the irritation set up by the presence of insect larvæ or other parasites, and it is not inappropriate, therefore, to speak of them as "Coral galls."
