Two Eyes One Side: The Flatfish
Our fascination with flatfish dates back to 8,000 BC, when cave artists in Spain’s Cave of Pileta drew a 1.6 meter creature with eyes on both sides of its head. Modern-day flatfish undergo an anatomical change, transforming from pelagic larvae that swim in open water, to benthic adults that recede into the sand for camouflage. During this metamorphosis, the fish’s skull contorts, forcing one eye to migrate to the opposite side. Most animals display internal asymmetry (your heart is on your left), but flatfish may be the most lopsided vertebrate to ever live on earth. What fuels this migration?
Researchers Bao et al found that flatfish have a disproportionate amount of proliferating cells along the dorsal/ventral axis, compared to the anterior/posterior axis. This reinforces the ‘flattening’ effect on the body that makes it easier to burrow into the sand. These proliferating cells are also responsible for pushing the eye across the cranium. The fish’s skull is contorted as a secondary effect of eye migration. An injection of colchicine in the suborbital blind side inhibits cell proliferation, preventing both the eye displacement and cranial transformation. Biologists recently discovered that thyroid hormones are the first signal to kickstart these proliferating cells. In most flatfish species, these hormones are unevenly distributed as early as the larval stage.
The discovery of the Amphistium fossil in 2008 unearthed a missing link between between modern-day flatfish and their symmetrical relatives. This extinct spiny-finned fish from the Eocene epoch had a mispositioned eye that gave it a slight advantage when adapting to a ground-dwelling lifestyle. A similar fossil Heteronectes has a warped skull but retains other features from pelagic fish. Looks like the flatfish secured its place on the sea floor at least 50 million years ago.