Why it evolved

As oxygen levels decline in its habitat, the Abyssal Heat-Gill will be selected for traits that maximize oxygen uptake. Expanded gill surfaces, resembling feathery filaments, will dramatically increase the surface area available for gas exchange, allowing it to survive in the oxygen-poor water of Verglas's under-ice ocean. This adaptation comes at the cost of increased energetic expenditure to maintain these delicate structures and a reduced capacity for bursts of speed, making it more vulnerable to predation and limiting its ability to compete with the more streamlined Veridian Swift-Fin in areas with slightly better oxygenation.