The gilltail replaced its ancestor the silverling. They have 4 main subspecies, northern and southern Ladym ocean species and northern and southern Jujubee ocean species. Each population will go to the equator during their winter to breed and then back to the poles to feed and lay their eggs on the icebergs. Over time some got lost and would either stay too long and migrate the wrong way thus spreading over all the oceans. When in the polar regions the microbes bloom and the small filter feeders populations get huge for them to eat.
They have, over time, become much more streamlined in shape and their dual coloring helps hide them when in schools. The shiny silver can confuse predators who might want to eat them. Their most interesting new adaption is their new breathing system. Before now they would have to either passively absorb oxygen in the water though their skin or in their mouths. But now they have developed a strange new gill system. Under their beaks they have an intake hole which bring in water. This hole leads to many gills and pass along the bottom of the body. Then on either side of the tail are 2 gill holes in which the water can exit after oxygen is taken from the water. This allows them to absorb more oxygen the faster they go thus helping increase their speed and stamina even in the very cold waters of the polar coasts.