A new approach for estimating larval settlement timing of a flatfish based on the shift in lapillus growth direction at settlement

Flatfishes undergo a substantial transition from pelagic to benthic phases during settlement in their early life history. Estimating the timing of settlement is crucial for understanding post-settlement growth and survival. However, settlement checks have only been reported for a limited number of species. In this study, we propose a novel method for estimating settlement timing using Japanese flounder Paralichthys olivaceus as a model species. An ontogenetic change in the morphology of the left (ocular-side) lapillus was observed between 30 and 35 days post-hatching (dph) in hatchery-reared individuals, which coincided with settlement. Up to 30 dph, the lapillus exhibited a single maximum radius along a specific direction (early growth axis), whereas from 35 dph onward, the lapillus formed a projection at approximately 90° to the early growth axis. This morphological change was also observed in the lapillus of wild juveniles, and their settlement timing was estimated to be 35.5 ± 5.5 dph (mean ± SD). These findings suggest that projection formation in the lapillus can serve as a reliable benchmark for settlement in flatfishes.