The Reproductive Cycle of Some British Freshwater Gammaridae

The general facts of the reproduction of Amphipoda have been described by various workers (e.g. Valle 1887-9; Haempel 1908; Embody 1911; Sexton & Matthews 1913; Sexton 1924; Heinze 1932; Le Roux 1933; Clemens 1950), and the sequence of events is now well understood. Briefly, the adult male takes hold of the female some time before she is ready to oviposit, and the pair remain together in precopula for a few days, until the female moults. Copulation proper then occurs, often several times. The male turns the female round and places his sperm on her ventral surface. The female then breaks away from the male, and before her cuticle hardens the eggs are laid into a brood-pouch made of four pairs of oostegites. The eggs are at first enclosed in a gelatinous .sac which slowly dissolves and at about this time fertilization takes place. Hatching occurs after some days and the young leave the mother's pouch after a day or two. She becomes attractive to males again at, or slightly before, the time of hatching of her eggs, and is often to be found in precopula while unhatched eggs remain in her pouch. In most species each female is capable of producing several batches of eggs in succession, and at each moult she grows slightly. The males moult at much longer intervals than the females. The number of eggs laid varies considerably, but on average in any one species larger females lay more eggs (Cheng 1942; Berg 1948; Heinze 1932; Kinne 1952a). In some genera it appears that copulation is not preceded by precopula (Watkin 1941). The young mature after a definite number of moults (Sexton 1924; Le Roux 1933; Kinne 1953a) which differs in each species. Several instars before maturity it is possible to distinguish the sexes because of the appearance in the male of the genital papillae, which have been figured for Gammarus pulex (L.) by Heinze (1932), and in the female of the oostegites, the first appearance of which has been noted in various species by several authors. These appear as small simple plates which enlarge at each moult, and which acquire serrated edges-at the last moult before maturity, at which stage the developing eggs in the ovaries are clearly visible. At maturity the oostegites become fringed by long bristles, which interlace with one another and form the brood pouch, figured by Heinze (1932). No breeding females are ever without these bristles. Legueux (1924) and Heinze (1932) noted, in Gammarus duebeni Lillj. and G. roeselii (Gervais) respectively, that breeding females can lose these bristles at a subsequent moult and return to the condition of serrated edges, although the oostegites remain larger than those of immature specimens. Le Roux (1933) further found that this effect could be produced by sterilization with radium, and was connected with ovarian activity. Kinne (1953b) has made a detailed