Channichthyidae - Evolution

Evolution

The loss of hemoglobin was initially supposed to be an adaptation to the extreme cold (The higher solubility of O₂ reduces the demand on hemoglobin and the lack of RBCs decreases the blood viscosity). However, current analysis has shown that the lack of hemoglobin—though not lethal—is still maladaptive. The fish have evolved fairly drastic changes to their physiology to compensate. These compensations include spending twice as much energy pumping blood compared to other fish.

These fish have descended from a sluggish demersal ancestor. The cold, well mixed, oxygen rich waters of the Antarctic ocean provided an environment where a fish with a low metabolic rate could survive even without hemoglobin—albeit less efficiently. During the mid-tertiary period a species crash in the southern ocean opened up wide range of empty niches to colonize. Despite the hemoglobin-less mutants being less fit, the lack of competition allowed even the mutants to leave descendants that colonized empty habitats and evolved compensations for their mutations. Later the periodic openings of fjords created habitats that were colonized by a few individuals. This gave the opportunity for four lines of fish to even lose their myoglobin genes by a similar process.