BREEDING HABITS. Fishes that lay eggs show no parental care, as a rule, either for their eggs or young. The eggs are fastened to rocks or weeds or other objects, and the eggs and young are left to shift for themselves. In many marine species the eggs are extruded into the water, and during their first period of development float at the surface and are carried about by the currents. In all such cases the loss both of eggs and young must be very great, and to meet tbis loss such species usually produce enormous numbers of eggs. Thus a single large cod may produce in a single year 10,000,000 eggs. On the other hand, in some species there is considerable care be stowed upon the eggs and young by the parents —this duty usually falling to the male. The sticklebaeks (q.v.) are well-known instances. The male builds a nest of sticks, grass, etc., ce menting them together with a sticky excretion, and guards the nest and eggs during incubation. In some of the Silurida-, after the young are ready to leave the nest, the male may be seen leading the brood about, guarding it until the individuals are better able to shift for themselves. This in stinct is found in other families. The cave blindfish (Amblyopsis) retains the eggs during incubation in its capacious gill-cavity. The sea horse develops a brood-pouch along the ventral side of the body, in which the eggs and young are harbored. Many marine fishes, like the shad and the salmon, ascend the rivers each season to spawn. These migrations may be for great dis tances, and against the greatest difficulties, such as rapids and falls. Such migratory species are known as anad•omous fishes. The reverse proc ess takes place in the case of the eel, which goes to the ocean to spawn.
The spawning season of most species is dur ing the spring months. In the tropics, where the rainy and dry seasons alternate, these are deter mining factors in the time of spawning of certain species. Many species spawn during the colder
months, for example the Salmonida.. Many fishes show on the approach of the breeding season a noticeable sexual difference, the male being marked by more brilliant colors, or by the tem porary growth of tubercles on the bead and other portions of the body. Many species, how ever, do not exhibit this sexual dimorphism.
The eggs of fishes vary greatly in size and shape. The typical fish-egg is globular, more or less transparent, having a tough protective mem brane, within which the yolk-laden egg proper lies. The yolk is present in sufficient amount to maintain the embryo until it can swim about and feed for itself. The outer protective mem brane is very commonly sticky, to enable it to cling to stones, weeds, etc. Sometimes eggs stick together in clusters. In other eases the outer coat has tufts of fine filaments with which the egg fastens itself to weeds. The shark's eggs are inclosed in large, horny, purse-shaped cases within which the embryo is developed. The pe riod of incubation is very various. In certain pelagic eggs the embryo emerges from the eggs in 24 to 48 hours after deposit, while in other eases, as the trout, the period extends over three to six months. See Eau: Form. The food of fishes includes all sorts of vegetable and animal matter and forms. Some are omnivorous. Others are exceedingly choice about their food, living almost exclusively certain species of erustaeea, for instance. Some, like the carp, are vegetarians, and the smaller fishes are the principal food of the larger, preda ceous forms, like the trout and bluefish. Many species subsist entirely upon the minute organ isms they can strain out of the water.