This is one of the reasons why species are sometimes difficult to identify. As temperature decreases, numbers of fin rays, vertebrae and scales increases in fish such as herrings, minnows, killifish, and darters. Other fish show an increase in meristic counts at high or low temperatures relative to an intermediate temperature. Larval stage begins when fish breaks free from egg and switches from reliance on yolk to external food source. Many features continue to develop during larval stage.
Circulatory system is initially open and blood lacks corpuscles.
Gill filaments are lacking and larvae respire cutaneously. Critical feature of development is switching from yolk to external food. This period appears to be critical. If larvae are unable to find food for a period they may irreversibly starve even if they find food. Feeding success 1.