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.

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Extant Palaemonidae occupy aquatic environments that have generated physiological diversity during their evolutionary history. We analyze ultrastructural traits in gills and antennal glands of palaemonid species from distinct osmotic niches, and employ phylogenetic comparative methods to ascertain whether transformations in their osmoregulatory epithelia have evolved in tandem, driven by salinity. Gill pillar cells exhibit apical evaginations whose surface density (Sv, μm 2 plasma membrane area/μm 3 cytoplasmic volume) ranges from 6.3-7.1 in Palaemon, and 0.7-38.4 in Macrobrachium. In the septal cells, Sv varies from 8.9-10.0 in Palaemon, and 3.3-21.6 in Macrobrachium; mitochondrial volumes (Vmit) range from 43.3-46.8% in Palaemon and 34.9-53.4% in Macrobrachium. In the renal proximal tubule cells, apical microvilli Sv varies from 27.0-34.3 in Palaemon, and 38.3-47.8 in Macrobrachium; basal invagination Sv ranges from 18.7-20.0 in Palaemon and 30.8-40.8 in Macrobrachium. Septal cell Sv shows phylogenetic signal; evagination height/density, apical Sv, and Vmit vary independently of species relatedness. Salt transport capability by the gill and renal epithelia has increased during palaemonid evolution, reflecting amplified membrane availability for ion transporter insertion. These traits underpin the increased osmotic gradients maintained against the external media. Gill ultrastructure and osmotic gradient have evolved in tandem, driven by salinity at the genus level.