The deep ocean is home to many strange looking fish and other animals. A new study shows that the cold, dark depths of the ocean contain a wider variety of body shapes for fish than shallower waters. That supports the idea that the deep ocean is a hotspot of evolution for fishes.

Postdoctoral researcher Christopher Martinez and colleagues from Professor Peter Wainwright’s laboratory in the Department of Evolution and Ecology, analyzed more than 8000 specimens representing 3033 species of fishes from the deep ocean, shallow waters and intermediate depths. The project drew on a database developed by the lab from fishes in the collection of the Smithsonian National Museum of Natural History.

They measured body length, depth and width, head depth, mouth width and jaw length, and the depth and width of the “caudal peduncle,” where the tail fin is attached. They used these data to organize the species in a two-dimensional diagram, or morphospace.

The body shapes of shallow water fishes, they found, tend to cluster around a set of proportions somewhat like a red snapper. Deep ocean fishes show a much wider spread of shapes, from extremely elongated to almost globular.

Sight and swimming

In shallow waters, fishes can hunt (or detect predators) by sight at relatively long distances. That puts a premium on powerful swimming to catch food or avoid being eaten. Shallow water fishes also have to cope with surface currents and if they live on coastlines, with rocks and reefs as well. Those factors all favor body shapes that are good for fast swimming and rapid maneuvering.

In the pitch dark of the deep ocean, fishes encounter (or become) prey at very close quarters. That allows animals to evolve for slow, energy-saving swimming, leading to rapid evolution of a wider variety of possible body shapes.

The one area where shallow water fishes showed more diversity was in jaw length. Deep water fishes generally don’t have small mouths – if you do find food, you don’t want it getting away.

While there are clearly other evolutionary drivers at work beyond depth, the findings support a long-standing impression that the deep ocean is a hot bed of evolution and diversification for fishes, the authors write.

The paper was published May 31 in Ecology Letters. Additional coauthors are Sarah Friedman, Katherine Corn, Olivier Larouche and Samantha Price. The work was supported by the National Science Foundation.