It has been mentioned above that the Common Shrimp is protected, not only by its habit of lying half buried in sand, but also by its close resemblance in colour to the sand among which it lives. There are many others among the shore Crustacea which show what seems to be a "protective resemblance" in colour and form to their surroundings. It is necessary to be cautious in interpreting these resemblances as necessarily protective, since the fish and other enemies which prey on these Crustacea see them with eyes very different from ours, and probably, in many cases, are guided to their prey by the sense of smell rather than by sight. The "masking" habit of the Spider Crabs, already described, strongly suggests, however, that concealment from sight is an important protection to some shore Crustacea, and helps to make it probable that the same end is reached in other cases by modifications of form and colour.
There can be no doubt, at all events, that many Crustacea are very inconspicuous to human eyes when they remain motionless in their natural surroundings. Thus, for example, the Caprellidæ, or "Skeleton Shrimps" (see [Fig. 22], p. 54), are hard to detect without very close search, as they cling to the feathery branches of the hydroid zoophytes among which they are usually found. They are strangely modified Amphipods, in which the body is slender and thread-like, and generally of a semi-transparent, whitish or yellowish colour, like the zoophytes on which they live. They clamber about among the branches with a movement like that of a "looper" caterpillar, and often remain clinging by means of the hooked claws of the hinder pairs of legs, with the fore part of the body gently waving about.
Fig. 40—A, a Piece of a Tropical Sea-weed (Halimeda); B, a Crab (Huenia proteus) which lives among the Fronds of Halimeda, and closely resembles them in Form and Colour. Reduced. (After Borradaile.)
The little Crabs of the family Leucosiidæ (Oxystomata), of which the British representatives are several species of the genus Ebalia, are often extremely like pebbles of the gravel among which they live. In many tropical species the carapace is pitted and eroded, so as to resemble a worn fragment of coral shingle. One of the most striking cases among the Crabs, however, is that of Huenia proteus ([Fig. 40]), one of the Spider Crabs (Oxyrhyncha), which is found in the Indian and Pacific Oceans. In this little Crab the carapace is flat, and is extraordinarily variable in form. In most of the males it is triangular in outline, but in most of the females and in some males it is broadened by leaf-like expansions of the side edges. Borradaile has pointed out that these broad individuals are usually found among the sea-weed Halimeda, and that they closely resemble the fronds of this weed in form and in their greenish colour.
A number of Crustacea are known to possess a chameleon-like power of changing their colour. The mechanism by which this change is effected is similar to that found in other animals, such as fish and frogs, which have the same power. The pigment which gives its colour to the animal is lodged in microscopic star-shaped bodies known as chromatophores, lying for the most part just below the skin. Each chromatophore consists of a central body from which a number of branching filaments radiate. The pigment may contract into the centre of the chromatophore, forming a minute and hardly visible speck, or it may spread out into the branching filaments, forming a distinct spot of colour. Each chromatophore may in some cases contain several colours of pigment, and these may expand or contract independently of each other, so that a whole series of changes may be produced by a single chromatophore. In the larger Crabs and Lobsters the visible colour of the animals depends on pigment in the shelly exoskeleton, which is thick enough to hide the chromatophores in the living tissues underneath, and no very rapid or considerable changes are apparent; but in the smaller forms, in which the exoskeleton is thin and translucent enough to allow the underlying colours to appear through it, the changes in the chromatophores may produce striking effects. Thus, Fritz Müller describes a species of Fiddler Crab of the genus Gelasimus, in which the hinder part of the carapace was brilliantly white, but five minutes after the Crab was captured it had changed to a dull grey. Many other cases of colour change have been described, but most remarkable and the most fully studied is that of the Prawn, Hippolyte varians, which is very common on our own coasts, and has recently been the subject of a very elaborate series of researches by Professors Keeble and Gamble. The specimens of this Prawn show "a bewildering variety of colour and of colour-pattern"; they may be uniformly coloured in various shades of brown, green, or red, or they may be "blotched," "barred," or "lined," with colour. These different varieties are generally found among sea-weeds, which they resemble in colour and pattern, the "lined" forms, for instance, frequenting finely branched and feathery weed. Like many other protectively coloured animals, they are of sedentary habits, clinging to the weed, and seldom moving by day. If a specimen be removed from its habitat and placed in an aquarium with different kinds of sea-weed, it will take refuge among that which it most closely resembles. It appears that this resemblance in colour-pattern is acquired during the growth of the Prawn, and that a young specimen kept among finely branched sea-weed will acquire the "lined" pattern, while others, living among coarser weed, become "barred," "blotched," or "monochrome." Even in the adult Prawns the colour (though not the pattern) becomes changed in a day or two if they are placed among weed of a different colour—from green to brown, or the like. Within certain limits still more rapid changes of colour take place. If kept in the dark, or if placed on a white background (for example, in a porcelain dish) in the light, the Prawn quickly becomes nearly colourless, by contraction of the chromatophores, a transparent bluish tint alone remaining, due to a substance which diffuses from the chromatophores into the fluids of the body. In natural conditions this phase is assumed at night; and the interesting observation has been made that Prawns kept in the dark continue for three or four days to show a periodic expansion and contraction of the chromatophores, corresponding to the alternation of day and night. It seems that the rhythm of light and darkness has become impressed on the chromatophore system of the animal, and the movement of the pigments is regulated by something analogous to memory.
Fig. 41—The Common Porcelain Crab (Porcellana longicornis), slightly enlarged, and One of the Third Maxillipeds detached and further enlarged to show the Fringe of Long Hairs