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

Abstract

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.

Structural and biochemical correlates of Na+,K+‐ATPase driven ion uptake across the posterior gill epithelium of the true freshwater crab, Dilocarcinus pagei (Brachyura, Trichodactylidae)

“…Na + ,K + ‐ATPase activity in D. pagei is similar to marine species such as C. ornatus (Garçon et al, 2007) and Clibanarius vittatus (Gonçalves et al, 2006), but is 2‐ to 5‐fold lower than in crabs from fluctuating salinity regimes such as C. danae (Masui et al, 2002) and in freshwater shrimps such as M. amazonicum (Santos et al, 2007) and M. olfersi (Furriel et al, 2000), suggesting that continual intense Na + ‐uptake may not be the main compensatory ion uptake strategy employed. Compared with other freshwater crabs (Harris and Micallef, '71; Schubart and Diesel, '98), D. pagei maintains a lower hemolymph osmolality (≈420 mOsm (kgH 2 O) −1 ) and [Na + ] (214 mmol L −1 ) when in fresh water (Onken and McNamara, 2002; Augusto et al, 2007a), and exhibits reduced transepithelial conductances and prominent ion selectivities in the abdominal and thoracic hindguts (McNamara et al, 2005). These adaptive physiological characteristics may result in lowered passive salt losses and water influx in dilute media, decreasing energy expenditure on active ion‐uptake processes, consistent with a low Na + ,K + ‐ATPase specific activity.…”

“…In contrast to most marine and intertidal crabs whose hemolymph is roughly isosmotic to the surrounding medium, such hololimnetic species hold the osmotic and ionic concentrations of their extracellular fluids far above those of their fresh water medium. To compensate for the strong diffusive gradients of salt loss and osmotic water influx across their body surfaces, these species typically exhibit low osmotic and ionic permeabilities (Shaw, '59; Greenaway, '81; Morris and van Aardt, '98; McNamara et al, 2005) and particularly efficient mechanisms of active, transbranchial salt uptake (Péqueux, '95; Lucu and Towle, 2003; Freire et al, 2008). …”

“…Gill Na + ,K + ‐ and V‐ATPase activities and transporter mRNA expression confirm the posterior gills as the principal sites of ion uptake (Weihrauch et al, 2004b). Dilocarcinus pagei is also an attractive model for studies of intestinal permeability and morphological adaptation to fresh water (McNamara et al, 2005), and of isosmotic intracellular (Amado et al, 2006; Augusto et al, 2007a) and tissue water regulation (Foster et al, 2010). …”

Adaptive shifts in osmoregulatory strategy and the invasion of freshwater by brachyuran crabs: evidence from Dilocarcinus pagei (Trichodactylidae)

“…Dilocarcinus pagei, a hololimnetic freshwater crab, endemic to the Amazon and Paraguay/Paraná river basins of South America (Mello, 2003), maintains strong hemolymph osmotic and ionic gradients in freshwater (Onken and McNamara, 2002). Its mechanisms of gill Na 1 and Cl À transport, including Na 1 / K 1 -and V-ATPase activities and mRNA expression in the uniquely asymmetrical gill epithelia have been investigated in detail (Onken and McNamara, 2002;Weihrauch et al, 2004); passive salt loss is reduced in the peculiar abdominal and thoracic hindguts, which show very low transepithelial conductances and notable ion selectivities (McNamara et al, 2005). Amado et al (2006) have accompanied the effect of lead contamination on hemolymph and tissue osmotic and ionic responses to low salinity exposure.…”

Osmotic and ionic regulation by hololimnetic freshwater crustaceans: a molecular role for gill ion transporters

“…The main processes that effect osmotic and ionic homeostasis in freshwater Crustacea are: (i) Isosmotic Intracellular Regulation, that maintains the composition and volume of the intracellular fluid (Péqueux, 1995; Augusto et al, 2007b; Freire et al, 2013), mediated by the transport of ions such as Na + , K + and Cl − (Freire et al, 2013) and by the synthesis and/or degradation of amino acids and peptides, reducing or increasing intracellular osmolality; and (ii) Anisosmotic Extracellular Regulation, that holds the osmolality, ionic concentration and volume of the hemolymph within species-specific limits, through the action of ion transporting proteins like the Na + /K + - ATPase and V(H + )-ATPase (Towle and Kays, 1986; Tsai and Lin, 2007), and membrane transporters such as the Na + /K + /2Cl − symporter, Na + /H + antiporter and ion channels, located in specialized ionocytes in the gills, antennal glands and intestine (Péqueux, 1995; Freire et al, 2008a; McNamara et al, 2005; McNamara et al, 2015).…”