Ontogeny of osmoregulation in the brackishwater amphipod Gammarus chevreuxi

Abstract

Osmoregulation is a key regulatory function in animals inhabiting brackish waters or areas subject to considerable salinity change, such as estuaries. While our understanding of osmoregulation in adult crustaceans is relatively good, our knowledge of how osmoregulatory ability develops during ontogeny is not well documented. In indirect developers, improvement in osmoregulatory capacity during ontogeny appears to coincide with a major metamorphosis. This is consistent with the ‘incomplete adult hypothesis’, which assumes that early developmental stages are ‘incomplete individuals’ operating less efficiently than individuals at the older stages. Evidence for this is not clear in direct developers. Consequently, we tested the ‘incomplete adult hypothesis’, by characterising the ontogeny of osmoregulation of the euryhaline amphipod, Gammarus chevreuxi, a species which undergoes direct development. We investigated the structure and function of putative osmoregulatory tissues, together with the regulation of key osmoregulatory genes. Embryos were examined at key developmental stages: before the dorsal organ (DO), a putative osmoregulatory structure, appeared (<48 hpf), before the gills appeared but the DO was present (9 dpf), and after both DO and gills were present (14–18 dpf). Adult G. chevreuxi exhibited a pronounced hyper-hypo-osmoregulatory pattern, matched by a strong pattern of haemolymph ion regulation. At a salinity of 35, eggs were hyposmotic to the external medium (989 mOsm Kg−1) in the DO and gill stages (mean ± SD = 584 ± 80.5 and 744 ± 103 mOsm Kg−1 respectively) with less of a difference with the medium before DO development (mean ± SD = 810 ± 91 mOsm Kg−1). At a salinity of 2, eggs from all stages were hyperosmotic to the external medium (52 mOsm Kg−1), with the pre-DO stage being closest to the isosmotic line (mean ± SD = 330 ± 29, 510 ± 55 and 502 ± 55 mOsm Kg−1 for pre-DO, DO and gill respectively). Differences between the stages diminished at salinity 15. Thus, the adult hyper-hypo-osmoregulatory pattern was present before the ontogeny of the DO and gills, although it improved during ontogeny. Expression of Na+/K+-ATPase transcripts was detected throughout ontogeny, further supporting the idea that ion transporting activity may occur before the formation of osmoregulatory organs. The ontogeny of osmoregulatory function in the gammaridean amphipod G. chevreuxi is therefore consistent with the incomplete adult hypothesis.