The origins of gas exchange and ion regulation in fish gills: evidence from structure and function.

Gill function in gas exchange and ion regulation has played key roles in the evolution of fishes. In this review, we summarize data from the fields of palaeontology, developmental biology and comparative physiology for when and how the gills first acquired these functions. Data from across disciplines strongly supports a stem vertebrate origin for gas exchange structures and function at the gills with the emergence of larger, more active fishes.

Ion regulation at gills precedes gas exchange and the origin of vertebrates.

Gas exchange and ion regulation at gills have key roles in the evolution of vertebrates. Gills are hypothesized to have first acquired these important homeostatic functions from the skin in stem vertebrates, facilitating the evolution of larger, more-active modes of life. However, this hypothesis lacks functional support in relevant taxa.

Water pH limits extracellular but not intracellular pH compensation in the CO-tolerant freshwater fish .

Preferentially regulating intracellular pH (pH) confers exceptional CO tolerance on fish, but is often associated with reductions in extracellular pH (pH) compensation. It is unknown whether these reductions are due to intrinsically lower capacities for pH compensation, hypercarbia-induced reductions in water pH or other factors. To test how water pH affects capacities and strategies for pH compensation, we exposed the CO-tolerant fish to 3 kPa  for 20 h at an ecologically relevant water pH of 4.

A solution to Nature's haemoglobin knockout: a plasma-accessible carbonic anhydrase catalyses CO excretion in Antarctic icefish gills.

In all vertebrates studied to date, CO excretion depends on the enzyme carbonic anhydrase (CA) that catalyses the rapid conversion of HCO to CO at the gas-exchange organs. The largest pool of CA is present within red blood cells (RBCs) and, in some vertebrates, plasma-accessible CA (paCA) isoforms participate in CO excretion. However, teleost fishes typically do not have paCA at the gills and CO excretion is reliant entirely on RBC CA - a strategy that is not possible in icefishes.

Hagfish: Champions of CO2 tolerance question the origins of vertebrate gill function.

The gill is widely accepted to have played a key role in the adaptive radiation of early vertebrates by supplanting the skin as the dominant site of gas exchange. However, in the most basal extant craniates, the hagfishes, gills play only a minor role in gas exchange. In contrast, we found hagfish gills to be associated with a tremendous capacity for acid-base regulation.