Fig. 235.—Diagrams of the pelagic ova of a Cod or a Plaice (A) and of a Ling (Molva). G, Germinal disc; M, micropyle; O.G, oil globule; Y, yolk. (From Cunningham.)
The eggs of Elasmobranchs are deposited singly or in pairs at considerable intervals, and the period of egg-laying is prolonged over a considerable part of the year. In most other Fishes, as in Teleosts, the period of spawning is limited to a few months, usually in the spring and summer in temperate latitudes; and in the case of a single Fish it may last only a few days or weeks, but the number of eggs produced is often enormous. Thus, in a Ling 61 inches long and weighing 54 pounds the ovaries contained 28,361,000 eggs. A Turbot, 17 pounds in weight, had 9,161,000 eggs; and a Cod of 21½ pounds 6,652,000. The least prolific of the British food Fishes is the Herring, in which the number of ovarian eggs varied from 21,000 to 47,000 in four specimens examined.[[490]] The extraordinary fecundity of many Fishes seems to bear no relation to the relative abundance of the Fishes themselves, but rather it is to be associated with certain disadvantages attendant on the sexual relations of Fishes, involving a considerable waste of the sex-cells, while in many Fishes it no doubt helps to compensate for any subsequent mortality among the larvae, which may result from an uncertain and precarious food supply and from the attacks of enemies. Whenever internal fertilisation is the rule, or when, as in nest-building and marsupial Fishes, the propinquity of the sexes in the breeding season ensures the fertilisation of a larger proportion of the eggs and the protection of the young, the number of eggs produced is small.
The male sex-cells or spermatozoa are essentially similar to those of other Vertebrates, although in different Fishes they may vary in such details as length, and the shape and size of the head, which may be rod-like and wavy, elliptical or globular.
As a rule, in Fishes females are more numerous than males, and generally they are larger, but to both statements there are notable exceptions. The relations of the sexes in the breeding season are usually very promiscuous, especially in those Teleosts which discharge their sex-cells while swimming together in shoals. A female may, however, consort with several males (polyandry), or a male with several females (polygamy); or, as in some of the nest-building Fishes (e.g. Gastrosteus), there are not wanting examples of the pairing of one male with one female (monogamy).
Fishes often migrate at the commencement of the breeding-season to localities most suitable for the deposition of the eggs. Many marine species seek banks or shallower water near the shore, and some, like the Salmon and the Sturgeon, are anadromous, and ascend rivers for long distances to deposit their spawn.
In all Fishes except the Elasmobranchs and a few Teleosts the fertilisation of the eggs takes place in the water after their extrusion, the male depositing its seminal fluid over the eggs or in their neighbourhood. The waste of the sex-cells is often, no doubt, very considerable, especially when the eggs are adhesive and fixed, and the seminal fluid is liable to drift at the mercy of tides and currents. With pelagic ova the waste is perhaps not so great, inasmuch as the eggs as well as the spermatozoa would probably drift at the same rate and in the same direction. Liability to waste must also be greatly diminished in many Fishes by their habit of living in shoals, or of congregating together in the breeding season, in which they are sometimes aided by their power of emitting characteristic sounds, and in the case of nest-building Fishes by the still more intimate relations of the sexes. Even when the liability to waste is very great, compensation may be afforded by exceptional fecundity. The copulation of the sexes and the internal fertilisation of the eggs occur only in Elasmobranchs and some Teleosts. The copulatory organs of Elasmobranchs are the so-called "claspers" with which the males are provided. Some form of copulation is probably the rule in the viviparous Teleosts, where the eggs are fertilised in the oviducts, or even while they are still in the ovaries, and the young are born alive. As mentioned above, an intromittent organ is often formed by the prolongation of the genital or the urinogenital orifice into a papilla, or a longer or shorter tube.[[491]] Some Cyprinodontidae[[492]] (e.g. Anableps) have the anterior part of the anal fin modified in the male to form an intromittent organ along which the urinogenital canal runs (Fig. 374). In the females the genital aperture is covered by a special scale, which is free on one side and not on the other. "The male organ in some individuals is turned to the right, in others to the left, and in some females the opening beneath the special scale is to the right, in others to the left. Copulation thus takes place sideways, a left-sided male pairing with a right-sided female, and vice versa."[[493]] The anal fin also forms an intromittent organ in the "Half-beak" (Hemirhamphus). In a genus (Girardinus) of the same family the anal fin is modified to form an apparatus for holding the female during sexual congress.[[494]] The singular method of fertilisation practised by the males and females of Callichthys paleatus is referred to elsewhere.[[495]]
With the exception of the pelagic Antennarius, which builds its nest in the Sargasso weed in mid-ocean, nest-building and parental solicitude for the young are confined to freshwater Fishes and to marine forms with demersal ova. Pelagic ova must necessarily be beyond the scope of parental care. As a rule it is the male which acts as guardian nurse, the female troubling herself but little about the fate of her eggs or her offspring.
Fig. 236.—The Butter-Fish (Pholis gunnellus) coiling round a mass of eggs. (From Cunningham, after Holt.)