conquest of fresh water by the palaemonid shrimps: an evolutionary history scripted in the osmoregulatory epithelia of the gills and antennal glands

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, μm²plasma membrane area/μm³cytoplasmic volume) ranges from 6.3–7.1 in Palaemon, and 0.7–38.4 in Macrobrachium. In the septal cells, Sᵥvaries 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 Sᵥvaries from 27.0–34.3 in Palaemon, and 38.3–47.8 in Macrobrachium; basal invagination Sᵥranges from 18.7–20.0 in Palaemon and 30.8–40.8 in Macrobrachium. Septal cell Sᵥshows phylogenetic signal; evagination height/density, apical Sᵥ, and Vₘᵢₜvary 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. © 2015 The Linnean Society of London, Biological Journal of the Linnean Society, 2015, 114, 673–688.