A flatfish is a member of the ray-finned demersal fish order Pleuronectiformes, also called the Heterosomata, sometimes classified as a suborder of Perciformes. In many species, both eyes lie on one side of the head, one or the other migrating through or around the head during development. Some species face their left sides upward, some face their right sides upward, and others face either side upward.
Flatfish
Flatfish are asymmetric, with both eyes lying on the same side of the head
Fossil of Amphistium
The European plaice is the principal commercial flatfish in Europe.
Actinopterygii, members of which are known as ray-finned fish or actinopterygians, is a class of bony fish that comprise over 50% of living vertebrate species. They are so called because of their lightly built fins made of webbings of skin supported by radially extended thin bony spines called lepidotrichia, as opposed to the bulkier, fleshy lobed fins of the sister class Sarcopterygii. Resembling folding fans, the actinopterygian fins can easily change shape and wetted area, providing superior thrust-to-weight ratios per movement compared to sarcopterygian and chondrichthyian fins. The fin rays attach directly to the proximal or basal skeletal elements, the radials, which represent the articulation between these fins and the internal skeleton.
Actinopterygii
Anatomy of a typical ray-finned fish (cichlid) A: dorsal fin, B: fin rays, C: lateral line, D: kidney, E: swim bladder, F: Weberian apparatus, G: inner ear, H: brain, I: nostrils, L: eye, M: gills, N: heart, O: stomach, P: gall bladder, Q: spleen, R: internal sex organs (ovaries or testes), S: ventral fins, T: spine, U: anal fin, V: tail (caudal fin). Possible other parts not shown: barbels, adipose fin, external genitalia (gonopodium)
Tuna are streamlined for straight line speed with a deeply forked tail
Cod have three dorsal and two anal fins, which give them great maneuverability