#plaice

Can it be both Flat Fuck Friday and Wet Beast Wednesday on the same day? This post would be the plaice to find out. Today's sole topic is the flatfish, a group of asymmetrical fish that are hard to overlook in the halls of weird fishdom. I was originally going to cover just one group of flatfish, but it turns out there's tons of overlap between common names. Therefore, I decided to turbot through all of them, just for the halibut. Now let's stop floundering around and get to it.

(Image: a European plaice (Pleuronectes platessa) swimming just above a sandy bottom. It is a very flat fish with an oval-shaped body and large dorsal and anal fins that emcompass each edge. The head is small and has both eyes on the same side. The eyes are bulbous and protrude from the face. Its body is a muddy brown with small orange spots. End ID)

Flatfish are members of the suborder Pleuronectoidei. They were formerly classified as comprising the order Pleuronectiformes, but there's now debate over how they should be classified. The non-flat threadfins may actually be part of the order while the flat spiny turbots may not be. It's confusing. Anyway, flatfish are well-named, for they have very flat bodies. Unlike other flattened fish like stingrays, who lay on their bellies, flatfish lay on one side. Flatfish have a top and bottom where other fish have a left or right. Most notably, both eyes are on the same side of the head in adults. Flatfish species are classified as right-eyed or left-eyed, based on the side of the body that faces up and has the eyes. The side that has they eyes is usually colored to bled in with sediment, though some species feature adaptations like eyespots to scare off predators. The side that faces down usually lacks pigment and therefore appears white. Some species have pigment-containing cells called chromatophores on their top sides that can be used to alter their coloration. Flatfish are the most dramatically asymmetrical of all vertebrates. They range in size from a few centimeters long to up to 4.7 m (15 ft) and 320 kg (710 lbs) in the Atlantic halibut (Hippoglossus hippoglossus).

(Image: an Atlantic halibut lying on sand. It is a large, diamond-shaped flatfish with a mottled dark gray and brown coloration. It is facing the camera and both eyes are looking off in different directions. End ID)

Flatfish have a large variety of common names, many of which are polyphyletic, meaning the same common name is used for multiple species that are not in the same taxonomic group. For example, the word flounder is used for multiple loosely-related groups but some flounders are alternatively called plaices, halibut, and dabs. There are also multiple distinct groups called soles, some of which are more flounder, and dabs, some of which are flounders and others of which are whiffs. And then there are the spiny turbot, not to be confused with the turbot, who have a lot of the same adaptations as flatfish but aren't placed in Pleuronectoidei. It's an absolute nightmare trying to figure out what fish goes in what group and why I gave up and just covered the whole subfamily.

(Image: a hogchoker (Trinectes maculatus) American sole held in hand. It is an alost circular flatfish with prominet crosshatching fron its scales. Its body is brown, with black stripes. The head is indistict, with very small eyes and mouth. End ID)

Flatfish are primarily marine, but some species can be found in estuaries and some will swim up river into fresh water. They are one of the largest groups of groundfish, species who spend most of their lives on the seafloor, comprising around 680 species. The majority of species live in relatively shallow water, but there are deep-sea species. One species of tonguefish Symphurus thermophilus, is notable for live only around hydrothermal vents in places with high sulfur content. They are primarily ambush predators who eat invertebrates and smaller fish. The flatness and coloration of the fish allows them to blend in with the seafloor, making it hard for both predators and prey to spot them. Many species will additionally bury themselves in sand, leaving only their eyes and mouths exposed. When prey passes, they will lunge and eat it. Some species will additionally swim up into the midwater to hunt. They swim on their sides, with the eyes facing upwards. Typically, species with more developed teeth include fish in their diets more and are more likely to swim to hunt while species with less developed teeth, like the soles, are more strictly benthic and eat more invertebrates. Swimming requires more effort for flatfish as adults lack swim bladders, requiring them to actively fight gravity. When swimming slowly, they will undulate their dorsal and anal fish, which usually are elongated to cover each side of the body. When swimming fast, a flatfish will undulate its whole body.

If you can't see them, the camouflage is working. (Image: a group of Symphurus thermophilus lying on the sdiment. They are oval flatfish with indistinct heads and no noticeable tail fin with mottled white and brown coloration. They are lying amongst bacterial mats which look like white blobs. End ID)

Flatfish typically reproduce seasonally in offshore habitats, though some will reproduce in estuaries. They are sexually dimorphic, with males typically being smaller and often possessing features like larger fins and spines on the head. When mating, the male will usually position himself beneath the female. They then release sperm and eggs into the water. Females release large quantities of eggs, up to 3 million in the largest halibut. Most species have buoyant eggs that drift in the water. The larvae are symmetrical when they hatch and look no different than most fish larvae. Larvae are planktonic, drifting along with the currents. Once they reach a certain level of maturity, the larvae will go through a metamorphosis in which the body flattens, the swim bladder and (in some species) pectoral fins are absorbed, and one eye will migrate to the other side of the head, accompanied by the skull twisting and becoming asymmetrical. At this point, the juvenile will sink to the seafloor and begin its adult lifestyle. Several species can hybridize even if the parents are in different genera. For example, in Puget Sound, Washington, USA, the English sole (not actually from England) Parophrys vetulus and starry flounder Platichthys stellatus hybridize so much their offspring were formerly thought to be an independent third species.

(Image: a series of artistic and photographic images of flounder larval development from hatching to 50 days old showing how the larva starts out looking like a nirmal fish larva before flattening and having one eye migrate to the other side of the head. End ID. Source)

A combination of fossil evidence and larval embryology helps scientists understand the evolution of flatfish. You can get an idea of an animal's evolutionary history by examining its embryonic and larval development. Larvae and embryos often demonstrate features of ancestral species that have been secondarily lost in their modern descendant. For example, human embryos grow a tail and then absorb it. This reflects how our monkey ancestors had tails, but humans and the other apes then lost their tails. Similarly, despite echinoderms being radially symmetrical as adults, we know they are in the clade of bilaterally symmetrical animals (Bilateria) because their larvae are bilaterally symmetrical. Since flatfish larvae start out symmetrical, we can infer that their ancestors were also symmetrical and the asymmetry is a newer trait. The ancestor of flatfish probably looked similar to a perch. Over many generation, the ancestral flatfish began to adapt the behavior of lying flat on the seafloor. Eventually, the trait of one eye moving to the other side of the head developed so the ancestral flatfish wouldn't have to lift its head up to see out of the downward eye. We have fossil flatfish who still have eyes on both sides of the head, but the eye on the downward side is placed closer to the top of the head, indicating that these species were in the process of evolving eyes to go all the way around. Presumably the placement of the downward eye made it so the fish didn't have to lift its head as much to see out of it, allowing it to lie flatter.

(image: a turbot (Scophthalmus maximus) lying on sand. It is a diamond-shaped flatfish with a sandy body with darker spots. End ID)

Large species of flatfish are commercially fished, usually using bottom trawl nest that scoop up large amounts of sediment and bottom-swelling species. I'm not a fan of trawls as they can destroy fragile benthic ecosystems that can take decades if not centuries to regrow. Some species can also be caught on lines and can put up a good fight, making them popular amongst anglers. Overfishing is a conservation concern for many species, such as the various halibut species. In halibut, overfishing has seen a general reduction in size as larger individuals are targeted and it is becoming increasingly rare for halibut to live long enough to reach monstrous sizes. Flatfish are fished for food as their meat is regarded as high quality. Their fillets are not particularly thick (unless its a big enough flatfish, like a halibut) but are flaky and have a delicate flavor. I'll leave you off with the story of an international conflict over flatfish. In 1995, a Canadian coast guard ship fired warning shots at, boarded, and held hostage a Spanish fishing vessel they claimed was illegally overfishing Greenland halibut. Spain naturally didn't like this and it led to the Turbot War (Greenland halibut are also called turbot, but aren't actually turbot, see my above paragraph about flatfish naming being a nightmare), a bloodless and political slapfight between Canada and the EU over territory and fishing rights. Several ships were boarded, Britain broke ranks with the EU by supporting Canada and arresting a French ship, a Canadian politician got nicknamed the Turbotonator, it was a clusterfuck. The whole mess ended when the EU agreed with Canada's terms and pressured Spain into agreeing. And that's how a bunch of countries got into a fight over a big, flat fishy fuck.