With eels that have heads shaped like tweezers and anglerfish that look like swimming light bulbs, the deep sea is host to a strange menagerie of fish body forms. The alien conditions at the bottom of the ocean may be the cause, making the dark depths a hotspot of body-shape evolution.
Previous research has shown that fish metabolism, muscle enzymes and swimming strength decreased with depth, says at the University of California, Davis. But there has been no comprehensive comparison of the shapes that fish bodies take at different ocean depths.
Martinez and his colleagues were part of an effort to unveil environmental patterns in fish evolution, in which researchers measured the physical proportions of about 6000 fish species in the collections at the Smithsonian National Museum of Natural History in Washington DC.
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Using this vast new database, Martinez鈥檚 team compared the body shapes of more than 3000 fish species that come from habitats ranging from surface waters to the abyss.
The team found that fish in the deep sea are nearly twice as diverse as shallow water fish when it comes to body shape. There are many more species in the shallows, but so much of that diversity is concentrated in fish with spindle-like shapes, such as tuna, or those with more compressed shapes, such as butterfly fish.
Martinez suggests this is because the turbulent, light-rich environment closer to the surface is full of active prey and predators, which creates an evolutionary pressure to develop a strong, manoeuvrable body. This funnels shallow water fish into a handful of streamlined shapes.
In the slow, quiet deep, athleticism isn鈥檛 as important for survival, he says. Released from the constraints of natural selection, surreal proportions emerge.
at the University of Louisiana at Lafayette says the findings 鈥渟erve as a good reminder that [traits] are not always necessarily evolving to some sort of selective pressure. It can be a release from those pressures, as well, that causes diversification.鈥�
Despite the impressive scope of this study, it covers just 10 per cent of fish species and is 鈥渟till only scratching the surface鈥�, says Kane.
Martinez says evolutionary influences related to ocean depth only account for a fraction of the diversity that we see in fish body shape. Both the physical complexity of habitats and the ways in which fish fit into food webs can mould fish鈥檚 bodies.
鈥淭his study tells a small part of a much larger story that we hope to continue revealing,鈥� he says.
Ecology Letters
