Why are there no freshwater, longhorn sculpin (Myoxocephalus octodecimspinosus)? Effects of low environmental salinity on gill ion transporter expression

Abstract

The genus Myoxocephalus contains 10 seawater (SW) and 3 freshwater (FW) species; the FW sculpin separated from the SW sculpin ~2–5 mya. The SW longhorn sculpin (M. octodecimspinosus) can tolerate days in low salinity but has never been found in FW. The purpose of our study was to determine why this fish is incapable of surviving in low salinities. The teleost gill contains mitochondrion‐rich cells that are the main site of ion balance, and these cells contain high densities of ion transporters such as Na+, K+‐ATPase (NKA), Na+‐K+‐2Cl−cotransporter (NKCC), and the Cl− channel, CFTR. SW teleosts generally have higher abundances of these transporters than FW teleosts. We hypothesized that sculpin can not survive low salinities because they can not properly regulate ion transporter density. We first sequenced sculpin gill cDNA for NKA, NKCC and CFTR. Next, sculpin were acclimated to 100% (control), 20%, or 10% SW for 24 and 72 hr. Plasma osmolality, Na+, K+ and Cl− did not differ between 100% and 20% SW but were significantly lower in the 10% SW sculpin at 24 and 72 hr. Gill NKCC and CFTR mRNA levels did not differ among treatments. Conversely, NKA levels were significantly higher after 72 hr in 20% or 10% SW, although there were no obvious changes in immunolocalization of the transporters in the gill or changes in protein levels (Western and dot blots). We conclude that the longhorn sculpin is incapable of surviving in low salinities because it can not regulate ion transporter densities to maintain proper ion balance. It may be that variation in this ability initially led to the invasion of FW by other sculpin species. This work was funded by NSF IOB‐0519579 to DHE.