Evolution of osmoregulatory patterns and gill ion transport mechanisms in the decapod Crustacea: a review.

Decapod crustaceans exhibit a wide range of osmoregulatory patterns and capabilities from marine osmoconformers to brackish and freshwater hyperregulators to terrestrial hyporegulators. The principal gill salt transport mechanisms proposed to underlie the ability of the better-known taxa to occupy these specific habitats are examined here. Traditional thinking suggests that a graduated series of successively stronger adaptive mechanisms may have driven the occupation of ever more dilute osmotic niches, culminating in the conquest of freshwater and dry land. However, when habitat and osmoregulatory parameters are analyzed quantitatively against the phylogenies of the taxa examined, as illustrated here using a palaemonid shrimp clade, their association becomes questionable and may hold true only in specific cases. We also propose a putative evolution for gill epithelial ion pump and transporter arrangement in a eubrachyuran crab clade whose lineages occupy distinct osmotic niches. By including the systematics of these selected groups, this review incorporates the notion of a protracted time scale, here termed 'phylophysiology', into decapod osmoregulation, allowing the examination of putative physiological transformations and their underlying evolutionary processes. This approach assumes that species are temporally linked, a factor that can impart phylogenetic structuring, which must be considered in comparative studies. Future experimental models in decapod osmoregulatory physiology should contemplate the phylogenetic relationships among the taxa chosen to better allow comprehension of the transformations arising during their evolution.