National Sun Yat-sen University study reveals why flounder eyes settle on one side

For most flounder species, individuals develop eyes on the same side of the body. The bigmouth flounder is different: its eyes appear on the left or right side in roughly equal numbers.

That variation led Yu-Chia Lin, an associate professor at National Sun Yat-sen University’s Institute of Marine Affairs, to investigate whether eye placement affects growth, reproduction or survival. The study has been published in the international journal Scientific Reports.

Flounder hatch with eyes positioned like those of ordinary fish, one on each side of the head. As a young fish grows and begins turning toward life on the seafloor, one eye gradually moves across the head. The process produces the flat, asymmetrical body associated with flounder.

Lin’s team analyzed 215 bigmouth flounder collected in the Gulf of Oman between 2020 and 2022. The sample included 109 females, 96 males and 10 immature fish. Their lengths ranged from 13.2 to 65.2 centimeters, and the oldest fish was 15 years old.

The results split almost perfectly: 107 fish were left-eyed and 108 were right-eyed. Their growth curves largely overlapped, offering no evidence that one orientation helped fish “live better” than the other. The findings suggest that eye placement may instead reflect molecular and developmental mechanisms controlling metamorphosis, eye movement and left-right body development. Lin said identifying the genes and developmental pathways that direct eye movement could help resolve the evolutionary question.

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