So far as is known at present, all sticklebacks construct a nest for the reception of the spawn, which is jealously guarded by the male until the young are hatched, which event takes place in from ten to eighteen days after oviposition. He also protects them for the first few days of their existence, and provides them with food, until they gradually stray from their home. The construction of the nest varies in the different species.

Sticklebacks are short-lived animals; they are said to reach an age of only three or four years; yet their short life, at least that of the males, is full of excitement. During the first year of their existence, before the breeding-season begins, they live in small companies in still pools or gently flowing brooks. But with the return of the warmer season each male selects a territory, which he fiercely defends against all comers, especially against intruders of his own species and sex, and to which he invites all females, until the nest is filled with ova. At this period he also assumes a bridal dress, painted with blue and red tints. The eggs are of comparatively large size, one female depositing only from 50 to 100; but, as the females deposit their spawn in nests of different males, the number of ova contained in one nest does not exceed one hundred.

Of the species known not one has so wide a geographical range, and has so well been studied, as the common British Three-Spined Stickleback (Gastrosteus aculeatus). It is found everywhere in northern and central Europe, northern Asia, and North America. The development of its scutes and spines varies exceedingly, and specimens may bo found without any lateral scutes and with short spines, others with only a few scutes and moderately sized spines, and again others which possess a complete row of scutes from the head to the caudal fin, and in which the fin-spines are twice as long and strong as in other varieties. On the whole, the smooth varieties are more numerous in southern than in northern localities. This species swarms in some years in prodigious numbers; in Pennant’s time amazing shoals appeared in the fens of Lincolnshire every seven or eight years. Their numbers may perhaps be conceived from the fact that a man employed in collecting them gained, for a considerable time, four shillings a day by selling them at the rate of a halfpenny a bushel. No instance of a similar increase of this fish has been observed in our time, and this possibly may be due to the diminished number of suitable breeding-places in consequence of the general introduction of artificial drainage. This species usually constructs its nest on the bottom, excavating a hollow in which a bed of grass, rootlets, or fibres is prepared; walls are then raised, aud the whole is roofed over with the like material. The nest is an inch and more in diameter, with a small aperture for an entrance.

The Ten-Spined Stickleback (Gastrosteus pungitius) is so called from the number of spines usually composing its first dorsal fin, which, however, may be sometimes reduced to eight or nine or increased to eleven. It is smaller than the three-spined species, rarely exceeding 2 inches in length. Its geographical range nearly coincides with that of the other species, but it is more locally distributed, and its range in northern Asia is not known. With regard to its habits, it differs from the common species only in the selection of the site for its nest, which is generally placed among weeds above the bottom of the water. Breeding males are readily recognized at a distance by the intensely black colour of the lower parts of their body.

Both these species are for their size extremely voracious, causing no small amount of injury if allowed in breeding-ponds in which valuable fish are preserved. During the whole time they are not engaged in their breeding operations they are in pursuit of food. A small stickleback kept in an aquarium devoured, in five hours’ time, seventy-four newly-hatched dace, which were about a quarter of an inch long. Two days after it swallowed sixty-two, and would probably have eaten as many every day, could they have been procured.

The Sea Stickleback (Gastrosteus spinachia) is a much larger and more slender species than those mentioned; it attains to a length of 7 inches, and is armed with fifteen short spines on the back. It is extremely common round the British coasts, but never congregates in large shoals. At suitable localities of the coast which are sheltered from the waves and overgrown with sea-weed, especially in rock-pools, one or two males establish themselves with their harems, and may be observed without difficulty, being quite as fearless as their freshwater cousins. Harbours and shallows covered with Zostera are likewise favourite haunts of this species, although the water may be brackish. The nest is always firmly attached to sea-weed, and sometimes suspended from an overhanging frond. This species inhabits only the northern coasts of Europe.