Ontogeny of feeding and respiration in larval Atlantic cod Gadus morhua (Teleostei, Gadiformes): II. Function.

Feeding mechanisms were studied in larval cod from first feeding to metamorphosis. Structural and functional changes that govern jaw movement and control the flow of water through the mouth change in concert with requirements for food energy. Early in development, exogenous food resources supplement the endogenous yolk-sac, viscerocranial structures and functions are simple and nonintegrated, and respiration is cutaneous. Hyoid coupling, which governs mandibular depression via the mandibulohyoid ligament, serves as the major musculoskeletal linkage for opening the mouth. With growth and differentiation of new structures such as the opercular bones, gills, and secondary lamellae, a second musculoskeletal linkage, the levator-operculi coupling, develops. This coupling mediates mandibular depression via the opercular apparatus and interoperculomandibular ligament and supplements hyoid coupling. Changes in musculoskeletal elements become more complex, facilitating obligatory exogenous feeding and branchial respiration. Thus, increases in structural and functional complexity result in the replacement of the simple, less efficient feeding mechanism by a complex combination of feeding mechanisms similar to those found in adult Atlantic cod. © 1996 Wiley-Liss, Inc.

Ontogeny of feeding and respiration in larval Atlantic cod Gadus 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. © 1996 Wiley-Liss, Inc.