The biology of brooding in the amphipods Lembos websteri Bate and Corophium bonnellii Milne Edwards

Abstract

Behavioural aspects of the brooding biology of the amphipod crustaceans Lembos websteri Bate (gonochoristic) and Corophium bonnellii Milne Edwards (parthenogenetic?) were studied at Millport, U.K. Brood survival was better in moving than in still water, with C. bonnellii broods generally surviving better than those of Lembos websteri. Hatching times were ≈ 10 days in both species (at ≈ 17°C). Once hatched, young generally spent < 1 day in their mother's marsupium. Ovigerous female Corophium bonnellii manipulated eggs in the brood pouch by “elbowing” movements of the first gnathopods and can remove eggs and eat them using these limbs. C. bonnellii has a more open mesh brood pouch than Lembos websteri and eggs may be lost accidentally. Such “lost” eggs can be replaced by the mother. Egg loss and replacement was never observed in L. websteri. “Replacement” of eggs of one Corophium bonnellii by those from another occurred, but eggs from a different species were not accepted. All eggs offered to Lembos websteri were eaten; Corophium bonnellii ate eggs (particularly Lembos eggs), but less often. Hatching in both species involved rupture of the egg membrane, probably by the telson spines, and was aided by pleonal flexure. Hatchlings were active in the brood pouch, feeding on detritus from a variety of sources. They did not appear to moult while in the marsupium. Irrigation of the brood by the female was achieved by pleopod beating and oostegite flexure. Brood release in L. websteri lasted < 1 day at both 16.5 and 19.5 °C, and was quickly followed by the female moulting < 1 day later. In Corophium bonnellii the female moulted > 2 days after brood release. Hatchings mostly emerged unaided by the mother. Premature hatchling loss could be rectified in C. bonnellii by the mother assisting reentry of the hatchling into the marsupium using gnathopods. Brood pouch reentry by “full-term” hatchlings was often observed, but only rarely did this occur after hatchlings had vacated the maternal tube; it was not assisted by the mother. Emergent young C. bonnellii sometimes resided in the maternal tube for only a matter of minutes; Lembos websteri stayed rather longer, usually < 1 day. Emerged hatchling L. websteri clustered inside the maternal tube in proximity to the mother rather than to the male attendant on her next moult (for fertilization of her next brood). Such hatchlings fed on detritus in the maternal tube, even taking material from the mother's gnathopods. They were never seen to take food from the male. Eviction of young from the maternal tube by the mother could be, but was not always, active. Female Corophium bonnellii frequently hindered the activity of young in the maternal tube. The differences in brooding biology between these two species are adaptive when viewed against differences in 1. (1) water movement regimes of their respective habitats 2. (2) feeding habits 3. (3) reproductive behaviour (mating vs. nonmating) of the adults.