The eggs of the cod, whiting, haddock, fluke, plaice, etc., are relatively small, varying from 1⁄6 of an inch in the case of a halibut, to 1⁄25 of an inch in a flounder. The eggs are discharged into the water by the female. This process takes place gradually, and generally occupies many weeks. A few of the eggs come to maturity at a time, and are extruded. They are fertilized in the water by the spermatazoa of the male, which are discharged into the water at the same time as the eggs. The fish, both male and female, are closely crowded together on the spawning grounds, so that the fertilization of the eggs is fairly complete. With few exceptions, the eggs of most species are buoyant and float to the surface, where they drift in the warm surface water until, happily, they hatch. Unhappily, however, a very large proportion of them never reach maturity, for, either as eggs, embryos or larvae, or post larval young fishes, they soon fall a prey to marauding fish. It is estimated that, of the thirty-two thousand eggs laid annually by each female herring, not more than two reach maturity.
The spawning grounds of the herring are not definitely known. Research is being carried out at present with a view to solving this question. Haddock are to be caught in various likely parts of the sea, marked with the place of capture, and their interiors examined for herring spawn.
Certain demersal fish, notably shark, dog fish and skate, deposit a few large, demersal eggs—about a dozen in the year—in a carefully selected spot. The incubation period of these eggs is unusually long, being from six months to over a year, according to the species and the temperature of the water.
Parental care is exhibited by very few fishes in this part of the world, although many foreign fish build nests and care for their young, often carrying them in their mouths. Certain kinds of dog fish and angel fish keep their young inside their oviducts until they are completely formed. The only notable example of a fish common to British waters that exercises parental care is the stickleback. Spawn is deposited by a number of different females in a nest constructed of stones and weed, and is guarded by a male until all the eggs are hatched.
The eggs of the crustacea, for example the lobster, are found attached in large numbers to the swimmerets—feathery processes that are situated underneath the tail. When in this condition, the lobster is known as “berried,” and, if captured, should be returned to the sea. The eggs are sticky and are laid while the lobster lies on her back, and so become attached to the hairs of these feathery processes. Berried crabs, prawns and shrimps may also be observed on the seashore in the spring and early summer.
The mollusca, e.g. mussels, periwinkles, oysters, deposit their eggs in the sea-water. The eggs float to the surface, hatch out, and drift about with the other constituents of the plankton. The fully developed larvae fall to the sea bottom and become attached to seaweed and stones.
The period of incubation of fish eggs varies according to the species of fish, and for the same species is prolonged by a low temperature. Plaice eggs, fertilized in January, hatched in eighteen days; others, fertilized in April, were hatched in nine days.
All fish, on emerging from the egg, enter upon a larval stage in which they resemble each other very closely (see [Fig. 1]). (Thus, the larvae of plaice are quite symmetrical, like those of the cod or other round fish.) The newly hatched larvae drift helpless in the water for two or three weeks, during which time they subsist upon the contents of the yolk sac, which they carry attached to their ventral surface. When this is exhausted, they feed upon the microscopic plankton which abound in the surrounding water.
The characteristic forms of the different species of flat fish are gradually assumed by the young fish during the period of their larval development. The appearance of a newly-hatched young plaice exhibits little change during the first week or so, other than that due to the gradual disappearance of the yolk sac. The young fish grows very slowly, and, twenty-one days after hatching, is only 3/8 of an inch in length. For thirty days the development of the young fish is entirely symmetrical. During the succeeding fifteen days, the shape and appearance of the fish become profoundly modified. The left eye gradually moves upwards and forwards, until it attains its final position above and in front of the right eye. At the same time, the fish gradually acquires a new swimming position, finally swimming on what is really its left side. This left side becomes colourless. With these changes in form and habit, there proceeds a transformation in the diet of the fish. At twenty-one days it feeds upon the young stages of various crustacea. Gradually it acquires a taste for copepoda and the larvae of mollusca and crustacea. After its metamorphosis is complete, it feeds upon various worms, small shrimps and small, bottom-living crustacea. The adult plaice feeds upon mollusca of the cockle and mussel families.