Ontogeny of feeding and respiration in larval Atlantic codGadus morhua (Teleostei, Gadiformes): I. Morphology

Cranial development in larval Atlantic cod Gadus morhua was studied throughout ontogeny using specimens treated by staining and clearing, scanning electron microscopy and histology. Newly hatched cod larvae have closed mouths, no operculii, five well-developed branchial arches, and transversii ventralis muscles. During the endogenous feeding (yolk-sac) stage, viscerocranial structures remain simple and nonarticulated. Six days after hatching at 5°C, articulation occurs between the quadrate/Meckel's cartilage and the hyomandibula/cranium. Integration of skeletal elements results in a functional jaw that facilitates the transition from endogenous to exogenous feeding. During later ontogenetic stages, the opercular apparatus and levator-operculi coupling develops, facilitating the transition of cutaneous to branchial respiration. Overall, feeding and respiratory needs are met by changes in form (including composition) and function during larval fish growth and are correlated with demands of energy acquisition essential to survival.