Flatfish (Vertebrata: Pleuronectiformes)

Pleuronectiformes (flatfish) is an order composed of almost 800 species in 11 families (Eschemeyer & Fong, 2011), distributed cosmopolitanly in mostly marine waters, although some can also be found in freshwater. They’re most well-known from the edible flounders, turbots, halibuts, and soles, all of which have well-established aquaculture schemes. For example, 9067 tons of turbot were produced in Europe in 2008, a number that pales in comparison to the 60000 tons produced in China.

They have a very derived morphology as is made clear from the above picture (Lee et al., 2009), and a correspondingly small genome that’s half the size of other fish. Flatfish are a classic example of asymmetry in a taxon that is characterised by symmetry, the Bilateria. The larva is a bilaterally symmetric, free-swimming larva. On metamorphosis to the benthic adult, the body can be skewed to the left or to the right: they become flattened, undergoing a series of drastic changes whereby one side stays facing the bottom, while the other (including the eyes) faces the water.

Below is a Youtube video of the inimitable David Attenborough explaining the development of flatfish.

The eyes-on-one-side-of-the-adult-head is considered to be an autapomorphic feature for the Pleuronectiformes (Friedman, 2008). It should be noted that some molecular analyses don’t recover the Pleuronectiformes as a monophyletic grouping (Smith & Wheeler, 2006), but this is most probably due to the shortfalls of molecular phylogenetics.

As Palmer (1996) points out, the side to which the basalmost flatfish are skewed isn’t genetically set, but is determined environmentally. Only in more derived flatfish has one side been developmentally favoured over the other, e.g. Cynoglossidae (tonguefish) are always left-skewed.

Other changes associated with the benthic lifestyle include the loss of the swim bladder – they don’t need to float around, after all.

They have basic camouflaging abilities, being able to mix up to three patterns to camouflage their body according to their current location (Kelman et al., 2006). I don’t imagine they need the more complex camouflaging abilities found in cephalopods (where the chromatophores allow an almost unlimited arrangement of patterns), given that they only live on the benthos (sometimes for over 20 years, which is a pretty impressive lifespan). The way the colour-changing works is by having special pigment-containing organelles in the chromatophores that are pushed around in the cell by the cytoskeleton to form the different colours.

References:

Eschemeyer WN & Fong JD. 2011.Pisces. In: Zhang Z-Q. (ed). Animal biodiversity: An outline of higher-level classification and survey of taxonomic richness.

Kelman EJ, Tiptus P & Osorio D. 2006. Juvenile plaice (Pleuronectes platessa) produce camouflage by flexibly combining two separate patterns. JEB 209, 3288-3292.

Friedman M. 2008. The evolutionary origin of flatfish asymmetry. Nature 454, 209-212.

Lee M-Y, Munroe TA & Chen H-M. 2009. A new species of tonguefish (Pleuronectiformes: Cynoglossidae) from Taiwanese waters. Zootaxa 2203, 49-58.

Palmer AR. 1996. From symmetry to asymmetry: Phylogenetic patterns of asymmetry variation in animals and their evolutionary significance. PNAS 93, 14279-14286.

Smith WL & Wheeler WC. 2006. Venom Evolution Widespread in Fishes: A Phylogenetic Road Map for the Bioprospecting of Piscine Venoms. Journal of Heredity 97, 206-217.

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