Changes in the pattern of osmoregulation in the brackish water amphipod Gammarus duebeni Lilljeborg (crustacea) during embryonic development

Abstract

AbstractEggs of the gammarid amphipod Gammarus duebeni Lilljeborg were cultured in sterilised in vitro conditions over a range of external seawater concentrations. The osmoregulatory ability of embryos and newly hatched individuals cultured in these conditions was examined over a wide range of external concentrations (75‐1,250 mOsm·kg−1 at 15°C). For all of the developmental stages examined the isosmotic point of the periembryonic fluid, P.F. (fluid from the space between the embryo and the vitelline membrane) or haemolymph from newly hatched individuals was similar (500‐550 mOsm·kg−1). Regulation of the P.F. by early stage 2 and 3 embryos, characterised by reddening cephalothorax and limb bud development, and of the haemolymph by new hatchlings followed a hyper‐isosmotic regulation pattern. In dilute external concentrations (between 50 and 100 mOsm·kg−1) P.F./haemolymph was maintained at a concentration of approx. 400 mOsm·kg−1): This difference diminished with increasing external concentration so that at and above 500‐550 mOsm·kg−1 P.F./haemolymph and external concentrations were isosmotic. Maintenance of a hyperosmotic P.F. in dilute media maintains cell function and establishes the osmotic gradient required for the uptake of water that results in the characteristic swelling of embryos. P.F. osmolality was regulated before the appearance of the coxal gills possibly via the vitelline membrane and/or the dorsal organ. In contrast, P.F. from later, stage 5‐7 embryos (eye pigmentation and cardiovascular development but still pre‐hatch) showed a distinct hyper‐hypo‐osmotic pattern of regulation: In dilute media the P.F. was maintained at levels between 400 and 550 mOsm·kg−1, and at external concentrations above 550 mOsm·kg−1 the P.F. was less concentrated than the medium (a difference of > 350 mOsm·kg−1 in the most concentrated external media). The functional significance of this hyper‐hypo‐osmoregulatory pattern is unknown although its appearance may be associated with the appearance of the coxal gills and the subsequent degeneration of the dorsal organ. This study has demonstrated that osmoregulatory function changes with ontogeny in amphipods and that the most “complicated” pattern (hyper‐hypo‐regulation) is associated with late embryo rather than the most “complex” (adult) stage. © 1995 Wiley‐Liss, Inc.