Developmental ecophysiology of the beachflea Orchestia gammarellus (Pallas) (Crustacea: Amphipoda) .1. Female control of the embryonic environment

The survival of divided broods of eggs of the semi-terrestrial beachflea Orchestia gammarellus was compared between in vitro culture and when returned to the female marsupium in 4 external sea water dilutions (25-100% sea water), Mortality of cultured eggs was only significantly greater than marsupium-reared eggs on the most dilute medium (25% sea water). Measurements of the concentration of the marsupial fluid (MF) showed that this was significantly greater than the concentration of the external medium in the 25% seawater treatment (median MF concentration = 621 mOsm . kg(-1), median external concentration = 264 mOsm . kg(-1)). This was also true in 50% sea water (MF = 664 mOsm . kg(-1), external concentration = 500 mOsm . kg(-1)) and 75% sea water (MF = 801 mOsm . kg(-1), external concentration = 742 mOsm . kg(-1)), whereas in 100% sea water the marsupial fluid was significantly less concentrated than the external medium (MF = 964 mOsm . kg(-1), external concentration = 1007 mOsm . kg(-1)). The MF concentration was always intermediate between external medium concentration and the haemolymph (and by inference, the urine) concentration of the female. It is suggested that female control cf MF concentration is important to the survival and development of embryos, especially when exposed to dilute media. Evidence is provided to support the hypothesis that the MF is partly supplied by direction of urine from the antennary gland into the ventral chamber, This is the first demonstration of maternal control of the osmotic environment of developing embryos in amphipods. Results are discussed in relation to the selection pressures acting on the embryonic stages and the ontogeny of osmoregulation.