Vampire squid
Their development goes through three forms: the very young animals have a single pair of fins, an intermediate form has two pairs, and the mature form again has one. As the animals grow and their surface area to volume ratio drops, the fins are resized and repositioned in order to maximize gait efficiency. Whereas the young propel themselves primarily by jet propulsion, mature adults find flapping their fins to be the most efficient means. This unique ontogeny caused confusion in the past, with the varying forms identified as several species in distinct families (Young 2002).