Aeglidae anomuran crabs originated in the sea, but invaded and diversified in southern South American freshwater (FW) streams. We here aimed at examining their tolerance of increased salinity, after a long time of evolution in FW (~33 million years). Aegla schmitti were exposed to FW and dilute seawater of salinities 15, 20, and 25‰ for 1, 5 and 10 days. Mortality in 35‰ was also assessed. Hemolymph osmolality, Na+, K+, Cl-, and Mg2+ ions, and hydration levels of the abdominal muscle were assayed. The activities of the Carbonic Anhydrase (CA), Na+/K+-ATPase (NKA) and V-H+-ATPase (VHA) were also assayed in the gills. A. schmitti preserves osmoregulatory mechanisms of its marine ancestors. It is able to survive in high salinities (25‰) for at least 10 days. Mortality in 35‰ was of 56% after 1 day, and of 100% after 7 days. In 25‰, NaCl is apparently hyporegulated at all times, while hemolymph osmolality rises after 5 days. CA and NKA activities remained unchanged in all experimental conditions, while VHA activity decreased after 10 days in 25‰. Hemolymph NaCl data was compatible with either hyporegulation and/or putative influx of NaCl into cells for regulatory volume increase (RVI). Further studies should deepen the understanding of the roles of low permeabilities and saturation of high affinity uptake systems in truly FW decapods, in their responses to high salinities. Moreover, the fate of extracellular NaCl as secretion in true hypo-regulation and/or influx into cells for RVI should also be investigated.
Keywords: Aeglid; Anomura; Decapoda; Freshwater crustaceans; Gills; Hololimnetic; Osmoregulation; Salinity.
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