Fundulus heteroclitus acutely transferred from seawater to high salinity require few adjustments to intestinal transport associated with osmoregulation

The common killifish, Fundulus heteroclitus, has historically been a favorite organism for the study of euryhalinity in teleost fish. Despite the species' large range of salinity tolerance, studies of osmoregulation in high salinity are rare, with most previous studies focused on fish transferred between freshwater and seawater. Similarly, while branchial transport properties have been studied extensively, there are relatively few studies investigating the role of the intestine in osmoregulation in killifish. This study sought to characterize the fluid and ion transport occurring in the intestinal tract of killifish adapted to seawater, and furthermore to investigate the adjustments that occur to these mechanisms following acute transfer to high salinity (70ppt). In vivo samples of blood plasma and intestinal fluids of seawater-acclimated killifish indicated absorption of Na ⁺, Cl ^-, and water, the relative impermeability of the intestine to Mg ²⁺ and SO ₄ ^2-, and active secretion of HCO ₃ ^- into the intestinal lumen. The details of these processes were investigated further using in vitro techniques of isolated intestinal sac preparations and an Ussing chamber pH-stat titration system. However, these methods were discovered to be of limited utility under physiologically relevant conditions due to tissue deterioration. Results that could be validly interpreted suggested that there are few changes to intestinal transport following transfer to high salinity, and that adjustments to epithelial permeability occur in the first 24h post-transfer.