Developing rainbow trout (Oncorhynchus mykiss) lose branchial plasma accessible carbonic anhydrase expression with hatch and the transition to pH-sensitive, adult hemoglobin polymorphs

Salmonids possess a unique respiratory system comprised of three major components: highly pH-sensitive hemoglobins, red blood cell (RBC) intracellular pH (pHi) protection, and a heterogeneous distribution of plasma accessible carbonic anhydrase (paCA), specifically with absence of paCA at the gills. These characteristics are thought to have evolved to enhance oxygen unloading to the tissues while protecting uptake at the gills. Our knowledge of this system is detailed in adults, but little is known about it through development. Developing rainbow trout (Oncorhynchus mykiss) express embryonic RBCs containing hemoglobins that are relatively insensitive to pH; however, availability of gill paCA and RBC pHi protection is unknown. We show that pre-hatch rainbow trout express gill paCA, which is lost in correlation with the emergence of highly pH-sensitive adult hemoglobins and RBC pHi protection. Rainbow trout therefore exhibit a switch in respiratory strategy with hatch. We conclude that gill paCA likely represents an embryonic trait in rainbow trout and is constrained in adults due to their highly pH-sensitive hemoglobins.