Pleuronectiformes: Flatfishes

Who wouldn't recognize a flounder, both of its eyes strangely located on only one side of its body? This weird order of fishes has discovered a unique benthic lifestyle. Instead of being dorsoventrally flattened like many other groups, the flatfishes are laterally flattend and lay on their side in the sand. They have adapted to this lifestyle by moving the sand-side eye over to the upper side, a transition that is observable from larva to adult. Interestingly, most families lay on either their left or right sides, but usually not a mixture (there are some exceptions). You can watch a video of the eye transition HERE.


Psettodoidei: Spiny Turbots

This suborder contains one family (Psettodidae) and one genus, Psettodes. They differ from the rest of the flatfishes in a few respects. The dorsal fin does not extend onto the head, they have a large mouth full of barbed teeth, and the eyes may be on either the left or right side. The paper I used for my undergraduate research (Near, et al. 2012) actually found that the genetic data grouped this family away from the rest of the flatfishes, possibly indicating convergent evolution towards the flatfish body plan.


The dorsal fin extends at least to the eyes, and the dorsal and anal fins lack spines.

Citharidae: Largescale Flounders

This small family has few characteristics. Mainly, the nostril on the eyeless side is enlarged.


Scophthalmidae: Turbots

This family has their eyes on the left (sinestral) side and has a large mouth with a noticably large lower jaw. Some species may have an unusually tall body, giving them a circular or rhombus shape.


Paralichthyidae: Sand Flounders

These fishes have their eyes on the left side and usually have large teeth (they are also known as large-toothed flounders).


Pleuronectidae: Righteye Flounders

This family is aptly named, because the adults have their eyes on the right (dextral) side of the body. The dorsal fin extends in front of the eye. This family includes the Atlantic Halibut (Hippoglossus hippoglossus), an important food fish and the largest flatfish in the world, reaching over 4.5m and 300kg. I have grouped Paralichthodidae, Poecilopsettidae, and Rhombosoleidae in this family, even though Nelson gives them their own families, as their designation is uncertain.



Bothidae: Lefteye Flounders

These flounders have their eyes on the left side, and supposedly have spines present on the face. There are a couple interesting members:


Achiropsettidae: Southern Flounders

This family has their eyes on the left side of the head. They have extremely compressed bodies, and completely lack pectoral fins as adults.


Samaridae: Crested Flounders

These appear to be right-eyed flounders (no one actually mentions it, but the pictures seem to indicate as such). Similarly to the crested flounder in Bothidae (see above), these fishes have elongated dorsal rays above their eyes, producing a crest.


Achiridae: American Soles

These are right eyed flatfishes. They may lack pectoral fins, and have a fleshy lower lip on the eyed side.


Soleidae: Soles

These right-eyed fishes have tiny pelvic fins that remain independent of the anal fin. The Moses Sole (Pardachirus marmoratus) can secrete a toxin from the fin bases that contains pardaxin. It causes cell lysis and is thought to be a predator deterrent.


Cynoglossidae: Tonguefishes

This family has their small eyes on the left side of the body. The caudal fin is continuous with the dorsal and anal fins. Some lack pectoral fins or lack the pelvic fin on the eyed side. Some species have a hooked snout or a curved mouth on the inferior side.