Female control of the embryonic environment in a terrestrial amphipod, Mysticotalitrus cryptus (Crustacea)

1. In amphipod crustaceans the ventral chamber plays an integral role in a number of physiological processes and in the female forms the marsupium in which eggs are brooded. The ventral chamber can be viewed as a pre-adaptation to the colonization of land by the family Talitridae. The hypothesis that the female of the terrestrial species, Mysticotalitrus cryptus, can control the osmotic concentration of the marsupial fluid ([MF]) bathing the eggs, thereby buffering the brood from potential physiological stresses presented by the terrestrial environment, is examined. 2. [MF] was maintained significantly higher than the concentration of the external medium ([Medium]) on both dechlorinated tap-water and 400 mOsm kg-1 media. In each case, [MF] was intermediate to [Medium] and the concentration of the female haemolymph ([Haem]): when [Medium] = 40 mOsm kg-1, [MF] = 277 mOsm kg-1 and [Haem] = 590 mOsm kg-1, respectively, and when [Medium] = 413 mOsm kg-1, [MF] = 516 mOsm kg-1 and [Haem] = 722 mOsm kg-1, respectively. 3. Evidence is provided that females produce urine that is isosmotic with the haemolymph and that the urine is directed, by capillarity, into the marsupium via cuticular channels. It is suggested that this urine plays a role in controlling [MF] in combination with other behavioural mechanisms. 4. Some preliminary observations are presented on the ontogeny of embryonic osmoregulation in M. cryptus which suggest that osmoregulatory ability improves with developmental stage. There is also limited evidence for the ability of the late embryonic stages to hypo-osmoregulate on concentrated media, even though adults lack this capacity. 5. The results are discussed in relation to the colonization of the terrestrial environment by the Talitridae.