#pleuronectiformes

Soleichthys dori

Dor's Sole

Image source

Status: Data deficient

Three-spot Right-eye Flounder Samariscus triocellatus

A flatfish found throughout the western Pacific and Indian Ocean. Inhabits sandy areas in lagoons and on seaward reef slopes at depths to 30 m, and feeds on small benthic invertebrates and fishes.

Least Concern

Strange Symmetries #18: Flat Fish Friday

Modern flatfish are characterized by their highly asymmetrical skulls, with both eyes positioned on just one side of their head. They aren't actually born this way, but instead they undergo "eye migration" as juveniles, twisting up their skulls to bring one eye across the top of the head.

Progressive eye migration in a developing Summer Flounder, Paralicthys dentatus. From Helfman et al (2009). The diversity of fishes. 2nd ed., Wiley-Blackwell.

This bizarre arrangement is the result of flatfish adapting to life laying flat on the seafloor, but instead of slowly widening and flattening themselves out they took an evolutionary "shortcut" by simply tipping their tall narrow bodies over onto one side. Initially this would have left one of their eyes unusable, but random mutations causing slightly asymmetrical skulls would have rapidly become highly advantageous to the earliest members of this lineage – and over time they just got wonkier and wonkier.

We've even found fossils of early flatfish in the "halfway there" stage of their evolution!

Amphistium paradoxum lived in what is now northern Italy during the Eocene, around 50-48 million years ago. About 20cm long (~8"), it had one eye partially migrated towards the top of its head, but not all the way around yet, showing a transitional state between its bilaterally symmetric ancestors and its more twisted-skulled modern relatives.

Unlike most modern flatfish Amphistium came in both "right-eyed" and "left-eyed" forms in equal numbers, suggesting that a genetic preference for a specific side also hadn't developed yet.

———

Carangiformes. This order contains the aggressively predatory dolphinfishes, and remoras, which have a sucker-like dorsal fin that allows them to attach to a larger host animal.

Pleuronectiformes. This order is made up of flatfishes, wuch as the flounder and halibut.

Amphistium

Amphistium — вимерлий рід променеперих риб ряду Камбалоподібні (Pleuronectiformes). Типовий вид — Amphistium paradoxum. Він вважається базальною формою камбалоподібних та перехідною формою від окунеподібних риб. На відміну від сучасних камбалових, у Amphistium очі не повністю зміщені на одну сторону голови, зокрема одне око розміщене на верхній частині голови.

Повний текст на сайті "Вимерлий світ":

Hey everyone! In an attempt to bring a little more fun education, I’m going to start posting a daily fun fact in addition to our usual species-focused posts. Our first fun fact:

Flounders, a group of fish famous for their sideways appearance, don’t start out that way! As hatchlings they have one eye on each side of their head, as most animals do. Then, as they get older and become bottom-dwellers, one eye migrates to the other side of their body! The side that becomes their ‘top’ is based on which way the flounder leans while it swims as a juvenile.