CHAPTER IX.

_CAVE-FISHES, SAND-EELS AND THEIR ALLIES, AND FLAT-FISHES._

BY W. P. PYCRAFT, A.L.S., F.Z.S.

The subterranean fresh-water caves of Cuba furnish the most interesting and most remarkable members of the family in certain small fishes known as CAVE-FISHES. Living in complete darkness, the eyes have degenerated so as to be no longer useful as organs of sight; indeed, in many species they are entirely wanting. By way of compensation delicate organs of touch have been developed, taking the form, in different species, of barbels, hair-like processes, or tubercles. These blind fishes are closely allied to certain marine forms found in the tropical Atlantic and Indian Oceans, and it is curious to note that amongst these about seven very rare species are found at great depths in the southern oceans, so great that light fails to reach them, and they too are blind.

The SAND-EELS, or LAUNCES, are extremely common on the sandy shores of Europe and North America, living in vast shoals, and displaying a wonderful unison in their movements, rising and falling as with one accord. They burrow in the sand with amazing rapidity, forcing their way by means of a horny projection on the lower jaw, and remaining buried at ebb-tide some 5 or 6 inches under the sand, when they are captured by fishermen, armed with rakes, for bait. When swimming in shoals, their presence is often betrayed by schools of porpoises, which feed greedily upon them, preventing their return to the bottom by getting under the shoal, whilst others swim round it. Mackerel also make large raids upon the ranks of such shoals.

FLAT-FISHES may be reckoned among the most important of food-fishes, and besides are of quite peculiar interest, on account of the remarkable modifications of structure which they have undergone. They differ from all other vertebrates in that, save for the first few weeks of existence, they spend the whole of their lives with one side of the body uppermost--the right or left, according to the species. Whether resting or swimming, this position holds good. The newly hatched fish, however, maintains the normal poise of the body, the back being uppermost. Of the many changes which the organs of the body undergo during this strange transformation from a "round" to a "flat" fish, one of the most interesting is that which concerns the eyes. These, in the very young fish, lie one on either side of the head; but as the fish grows older it begins to lie on its side on the ground, and ultimately, when it is two or three months old, loses the power of sustaining itself in an upright position altogether. The most remarkable feature in this very strange mode of development is, that as the fish comes to lie more and more on its side, so the eye which is undermost begins to move round to the other side, till eventually the two eyes lie side by side on the upper-surface. Strangely enough, in some species the eye moves _round_ the head, passing over its edge, and so to its place beside the stationary eye, whilst in others it acquires its ultimate position by moving _through_ the head, sinking in on one side and appearing again on the other. The coloration of these fishes is also peculiar, in that the two sides are quite differently coloured, the upper side resembling in tone that of the sea-bottom, whilst the under side is pure white. In the young fish, before the habit of lying on one side has been acquired, both sides are coloured alike. The difference in coloration between the two sides of the adult fish appears to be due to the effect of light, since in flat-fishes kept in a tank with a mirror at the bottom the under-surface was found in many cases to be very largely coloured. But the colour of the upper-surface is by no means constant. On the contrary, the flat-fish appears to possess the power, in common with all other fishes, of changing its colour so as to harmonise with the tone of its surroundings. Thus on a light sandy ground the exposed surface of the body becomes pale, while on a dark muddy bottom it is almost black. By this power of changing the colour of the exposed portion of the body so as to harmonise with the surroundings, the fish is enabled to become in a large measure invisible, and in proportion to the effectiveness of the change to escape its enemies. Some soles are quite invisible.

The most important of the flat-fishes are the PLAICE, FLOUNDER, DAB, HALIBUT, SOLE, TURBOT, and BRILL.

The PLAICE is one of the commonest of the British flat-fishes, and may be distinguished by the numerous orange or red spots which are scattered over the upper surface of the body. From 15 to 18 inches in length, specimens are occasionally captured which have attained a length of 28 inches. Plaice feed on shell-fish, such as mussels or scallops, which they crush by means of strong, blunt teeth in the throat; but worms are also eaten.

The eggs of the plaice are amongst the largest fish-eggs known.

The FLOUNDER is a smaller fish than the plaice, and lacks the red spots. It differs from the other flat-fishes in the preference it shows for the mouths of rivers, ascending these, indeed, so far as to enter fresh-water.

The HALIBUT is the largest of the flat-fishes, specimens of as much as 20 feet in length being on record, while examples of from 6 to 7 feet long are not uncommon in Grimsby market. This fish has a wide distribution, occurring on both sides of the North Atlantic and North Pacific, being most abundant in deep water. The halibut which are brought into the markets of Grimsby and Hull are chiefly caught off the coasts of Iceland and the Faröes with long lines.

The SOLE is a shallow-water fish, feeding chiefly upon worms, crabs, and shrimps. These it apparently hunts by smell, gliding over the sea-bottom, and tapping with the lower side of its head, which is provided with sensitive organs of touch in the shape of filaments. By day it conceals itself by burrowing in the sand, coming out after dark to feed.

The TURBOT resembles the brill, from which it may be distinguished by the greater breadth of the body in proportion to the length, the absence of scales, and the presence of large bony tubercles scattered over the surface. In Great Britain the turbot is most abundant in the English Channel.

The BRILL closely resembles the turbot not only in general form, but in the numerical superiority of the females, and in the habits of the young, which, like the turbot, are surface-feeders and possess an air-bladder.

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