HISTOLOGY AND TERATOLOGY OF PINK SALMON LARVAE NEAR THE TIME OF EMERGENCE FROM GRAVEL SUBSTRATE IN THE LABORATORY

During salmonid larval development, emergence from gravel substrate is a major event marked by final yolk absorption and transition to exoge nous feeding. The gross and histological changes associated with this fundamental biological process were investigated in pink salmon (Oncor hynchus gorbuscha) 4 weeks before emergence (alevins), at emergence (s wim-up fry), and 2 weeks after emergence with or without exogenous fee ding. Larvae were derived from artificially spawned eggs reared in gra vel-filled incubators in the laboratory. Less than 1% of emergent larv ae had gross lesions. These included opercular hypoplasia or dysplasia (semioperculum), spinal anomalies (lordosis and kyphosis), conjoined twins, and ophthalmic dysplasia. Apoptosis, or programmed cell death, was a common cellular change, and several cell types were involved: (i ) intragonadal supporting cells, (ii) gastric submucosal gland epithel ial cells, and (iii) midventral skin cells. Semiquantitative scores fo r apoptosis were usually highest in pre-emergent larvae and lowest in postemergent larvae. Larvae that were not fed during the first 2 weeks after emergence had slightly higher scores for apoptosis and hepatoce llular megalocytosis than did fed controls.