Ion Regulation in Crayfish: Freshwater Adaptations and the Problem of Molting

SYNOPSIS. Crayfish have a long evolutionary history in temperatefresh water (FW). Ion regulation is challenged by low externalconcentrations of Na, Cl, and Ca (<1 mM). In intermolt theprimary concern is Na and Cl balance; around ecdysis the emphasisswitches to Ca regulation as the cuticle is decalcified/calcified.Compared with marine crustaceans, intermolt crayfish maintaina reduced extracellular (EC) osmolality and have lower permeabilityto both ions and water. Hyperregulation involves active branchialuptake of Na and Cl and the unique ability to produce a hypotonicurine. Ion uptake involves apical electroneutral ion exchange(Na^$ for H^$; Cl^– for HCO₃–; counterions providedfrom CO₂ via carbonic anhydrase) followed by active basolateraltransport of Na via the Na pump, with Cl following passively.Reabsorption of 95% of filtered electrolytes at the antennalgland (kidney) involves similar subcellular mechanisms in amorphologically differentiated region of the distal tubule.Intermolt crayfish exhibit negative Ca balance (passive effluxunopposed by uptake) tolerable in view of the large cuticularCaCO₃ reserve. In premolt, cuticular Ca is reabsorbed. A smallamount is stored as gastroliths, the remainder is lost via branchialexcretion and in the discarded exuviae. At ecdysis, FW uptakegenerates the physical force for shedding, leaving the crayfishwith dilute hemolymph and a Ca deficiency. Levels of EC Na andCl are restored by intensive postmolt branchial uptake. Mineralizationof the soft exoskeleton involves remobilization of stored Caand branchial uptake of Ca and HCO₃. Transepithelial Ca transportinvolves Ca^2$ ATPase and Ca^2$/Na^$ exchange. The importance ofexternal electrolytes and pH in postmolt ion regulation is explored,as are some allometric considerations.     

Integrated Responses to Salinity Fluctuation

SYNOPSIS. Continuous and discontinuous intertidal waters bothexperience fluctuations in salinity frequently accompanied bychanges in temperature, levels of dissolved gases and pH. Amultidimensional approach to environmental fitness may thereforebe more representative of natural conditions in the intertidalzone. Various combinations of environmental factors are discussedincluding salinity/temperature, and salinity/O₂ tension. Salinitywill directly affect osmotic and ionic regulation and indirectlyaffect acid-base balance and various components of the respiratorysystem including ventilation, gas exchange, perfusion, O₂ transportby the respiratory pigment and utilization at the tissues. Additionalareas covered in this review article are osmotic problems facedby air-breathing aquatic intertidal animals, ontogenetic changesin osmoregulatory capabilities and a comparison of steady state(square wave) versus sinusoidal (cyclical) salinity changes.     

Interaction of Respiration, Ion Regulation, and Acid-Base Balance in the Everyday Life of Aquatic Crustaceans

Extracellular and intracellular acid-base balance is necessaryfor the maintenance of normal metabolic processes. The primarysource of acid is metabolically produced CO₂, and the CO₂/HCO₃^–system is the most significant buffer. The regulation of acid-basebalance is complex, involving the interaction between respiratorygas exchange and ion transport. In aquatic crustaceans respirationis governed by the need to extract oxygen from water, an O₂-poormedium; thus, acid-base balance is maintained primarily throughion transport mechanisms. These mechanisms include Na⁺/H⁺ andCl^–/HCO₃^– exchange processes that are sensitiveto the extracellular acid-base status of the animal. In marinecrabs, ion regulation and acid-base balance are accomplishedby the posterior gills, while in freshwater species all gillsand the antennal gland perform these functions. Intracellularacid-base balance appears to be maintained primarily by iontransport across the cell membrane. Hemolymph pH varies inverselywith acclimation temperature and salinity. In both cases Pco₂remains nearly constant, and the pH change is a result of changesin hemolymph HCO₃^– concentrations brought about by ionexchange mechanisms. Environmental hypercapnia or hyperoxiainduces a repiratory acidosis characterized by increased Pco₂,low pH, and elevated HCO₃^–; this is partially compensatedfor by ion exchange processes that bring about a further increasein hemolymph HCO₃^–. Exercise causes a mixed respiratoryand metabolic acidosis with compensation via H⁺ ion excretionand hyperventilation.