Changes in sense organ morphology and behaviour with growth in the flounder Paralichthys olivaceus.

Ontogenetic development of the sense organs of the flounder, from newly hatched larvae to 72-day old juveniles was examined by light- and scanning electron-microscopy and correlation with changes in behaviour was made. The larvae are capable of horizontal orientation when the otic epithelium is ciliated at 6 hours from hatching. Newly hatched larvae have at least two types of olfactory receptor cells and a pair of free neuromasts with well developed cupula. At 9 hours from hatching, the larvae avoid obstacles by mechanoreception. At 3 days from hatching, eyes are deeply pigmented, the visual system is morphologically complete, and the larvae initiate feeding and are positively phototactic. Differentiation of the taste buds occurs 9 days from the onset of feeding, at this time, food preference is exhibited by the larvae in the sea. Retinal rod cells differentiate at 25 days and twin cones form at 29 days from hatching. Positive phototaxis disappears at 30 days and the larvae become nocturnally active at 33 days from hatching. The shif from pelagic to benthic habit occurs at this stage.