Flatfish are an unexpected triumph of evolution , says Karen Boswarva , who explains their intricate life cycles
OCEANSCIENCE
Fun with flatties
Flatfish are an unexpected triumph of evolution , says Karen Boswarva , who explains their intricate life cycles
It was during a Seasearch course that I was asked a thought-provoking question . We were at the species identification part , discussing the major groupings of fish . We were looking at distinct characteristics that tell them apart enough to help you find them in an ID guide .
As a group of fishes , we can all agree that flatfish are quite recognisable . The clue is in the name . A pancake thin , flat fish with boggly eyes on the top . Worldwide , there are more than 700 species inhabiting everything from the deep ocean hydrothermal vents to shallow freshwater rivers . As divers we see flatfish on sandy / muddy bottoms or rocky reefs , while snorkellers may catch a glimpse of a tiny juvenile at the edge of the tide .
ID gets tricky when you want to tell species apart . Colour is never the best indicator . You need good photographs of the overall body and fin shapes , the body patterns , the eyes , and finally the ability to accept that your picture may never look like the ones in a book ( mine rarely do ).
To help , we ’ ve historically taught that there are left sided ( sinistral ) and right sided ( dextral ) flatfish . This sidedness depends on which way up the flatfish is laying down . If the right pectoral fin ( the side fin ) is facing up then the right eye is dominant , and vice-versa with the left . Sidedness tends to be specific to species . So next time you spot one first try to distinguish whether it ’ s a lefty or righty .
There are of course always exceptions to this rule that make ID harder . The life history of flatfish is actually pretty funky ! As larvae , they swim about the same as any other fish in the sea . Upright in the water column with their left and right sides symmetrical , one eye on each side of the head . After a couple of weeks , the larvae drop to the seabed and begin the most remarkable metamorphosis .
The skull completely remodels , the eye closest to the seabed relocates to the top . Pigments starts to develop on the top side . The brain , heart , and gut change shape and move into a more suitable place in the body .
“ The larvae drop to the seabed and begin the most remarkable metamorphosis ”
I watched as a look appeared on the student ’ s face . “ What causes the fish to drop on to the seabed , and if it ’ s not random then why and when do some species go left , and others go right ?” they asked ? A fantastic question that had me immediately searching for answers .
There ’ s some interesting and complex flatfish research out there ! It ’ s widely accepted that the side a flatfish falls on is controlled by genetics , a missing gene results in random side selection and a high percentage of reversed individuals .
But what causes it to fall in the first place ? Metamorphosis is triggered by elevated levels of thyroid hormone , the same process that turns tadpoles into frogs . But does it start while the fish is swimming normally and cause it to fall on its dominant side , or does falling on its side activate the hormones ?
All fish have a dorsal light reflex ; it enables them to always swim upright towards a light source until balance is established through sight and gravity . If the thyroid hormone increases and the eye starts to migrate while the fish is swimming upright , the angle of light would change , causing the fish to gradually tilt further to one side till it falls over – a plausible answer . However , flatfish larvae are notably weak swimmers , and fossils dating back 53 million years show incomplete eye migration . Left / right sidedness was random , and the eyes remained on top of the head . Meaning eye sidedness and species-specific selection of sidedness evolved after flatness .
Being weak swimmers certainly works in the flatfish ’ s favour . These evolutionary adaptations have offered a clear advantage , combining 360-degree vision with camouflage for spotting prey while evading predators – and cameras ! �
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