Early marine invertebrates like the Branchiopoda began their sojourn into dilute media some 500 million years ago in the Middle Cambrian. Others like the Mollusca, Annelida and many crustacean taxa have followed, accompanying major marine transgressions and regressions, shifting landmasses, orogenies and glaciations. In adapting to these events and new habitats such invertebrates acquired novel physiological abilities that attenuate the ion loss and water gain that constitute severe challenges to life in dilute media. Among these taxon-specific adaptations, selected from the subcellular to organismal levels of organization, are reduced body permeability and surface (S) to volume (V) ratios, lowered osmotic gradients, increased surface areas of interface epithelia, relocation of membrane proteins in ion-transporting cells and augmented transport enzyme abundance, activity and affinity. We examine adaptations in taxa that have penetrated into fresh water, revealing diversified modifications, a consequence of distinct body plans, morpho-physiological resources and occupation routes. Contingent on life history and reproductive strategy, numerous patterns of osmotic regulation have emerged, including intracellular isosmotic regulation in weak hyper-regulators and well-developed anisosmotic extracellular regulation in strong hyper-regulators, likely reflecting inertial adaptations to early life in an estuarine environment. Our analyses show that across sixty-four freshwater invertebrate species from six phyla/classes, hemolymph osmolalities decrease logarithmically with increasing S: V ratios. The arthropods have the highest osmolalities, from 300 to 650 mOsmoles/kg H ₂ O in the Decapoda with 220 to 320 mOsmoles/kg H ₂ O in the Insecta; osmolalities in the Annelida range from 150 to 200 mOsmoles/kg H ₂ O, the Mollusca showing the lowest osmolalities at 40 to 120 mOsmoles/kg H ₂ O. Overall, osmolalities reach a cut-off at ≈200 mOsmoles/kg H ₂ O, independently of increasing S: V ratio. The ability of species with small S: V ratios to maintain large osmotic gradients is mirrored in their putatively higher Na ⁺ /K ⁺ -ATPase activities that drive ion uptake processes. Selection pressures on these morpho-physiological characteristics have led to differential osmoregulatory abilities, rendering possible the conquest of fresh water while retaining some tolerance of the ancestral medium.

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