Adaptation to hypoosmotic challenge in brachyuran crabs: a microanatomical and electrophysiological characterization of the intestinal epithelia.

Besides its role in digestion and nutrient absorption, the crustacean gut participates in osmo/ionic regulation. We investigate microanatomy, ionic permeability and transepithelial electrophysiological parameters in the mid- and hindguts of three hyperosmoregulating crabs that inhabit estuarine waters (Chasmagnathus granulata), brackish mangrove swamp (Sesarma rectum) or freshwater (Dilocarcinus pagei). The abdominal hindguts are cuticle lined, the single-layered epithelia consisting of narrow, columnar cells exhibiting apically dense, unvesiculated cytoplasm. In the saltwater species, the thoracic midgut epithelium consists of tall, narrow, columnar cells displaying numerous, apical microvilli above dense apical cytoplasm. However, the corresponding gut segment in the hololimnetic species, D. pagei, consists of squat cells lacking apical microvilli, overlain by a heavy cuticle, constituting a thoracic or anterior hindgut. The midgut/thoracic hindgut epithelia in all three crabs, and abdominal (posterior) hindgut of D. pagei, exhibit similar, small, lumen-negative voltages when perfused symmetrically with hemolymph-like salines. The hindguts of the saltwater species show similar, small, lumen-positive voltages. Small short-circuit currents are detectable after voltage clamping. Washout and/or addition of luminal glucose or amino acids do not alter current or conductance, suggesting the absence of active, electrogenic nutrient absorption. Ion substitution did not disclose active, electrogenic absorption or secretion of Na+ and/or Cl-. The midguts of the saltwater species exhibit similar conductances, greater than in D. pagei, but no ion selectivity; hindgut conductance is low, the epithelia showing moderate anion selectivity. The thoracic (anterior) and abdominal (posterior) hindgut epithelia of D. pagei, the freshwater species, exhibit similar, low conductances, and are ion selective. These findings reveal that active, electrogenic, salt and nutrient transport is undetectably low or absent. The reduced transepithelial conductances and notable ion selectivities in the abdominal and thoracic hindguts of D. pagei may reduce passive salt losses in fresh water, contributing to osmotic and ionic regulation.