Overall, the analysis of nearly 3,000 species showed more diversity of body types among the pelagic fishes, those that swim in open water, than among the benthic species spending their life on the ocean floor. Pelagic fish body types span from the round anglerfish to skinny eels, while benthic fishes generally share a common elongated, tapered shape.
"We found that evolution pushes and pulls fish body shape in different directions depending on whether they're benthic or pelagic," said lead study author Elizabeth Santos, assistant professor of evolution, ecology and organismal biology at The Ohio State University.
"We talk about the deep sea as if it is sort of all one thing, when really it is not - it is actually quite diverse," she said. "There are very different types of environments in the deep sea that have their own different effects on evolution."
The study was published recently in the journal Evolution.
Santos and colleagues analyzed shape characteristics among ray-finned fishes living 200 or more meters below the surface, the portion of the world's oceans constituting almost 90% of their volume.
Using previously published datasets of fish body shape measurements, evolutionary relationships and habitats, the researchers included 2,882 deep-sea species in this study.
They found that increasing ocean depth promoted diversification in body shape, and that evolution occurred more rapidly at deeper depths than in shallower waters.
The faster rate of evolution - essentially, a lot of change in a relatively short period of time - was most common in bottom-dwellers, suggesting their similar body shapes resulted from more closely related species adapting in place. In contrast, the higher diversity of body shapes in open-water fishes hinted at the possibility that colonization by invading species has resulted in a community of more distantly related species.
"Colonization of the deep pelagic seems to be a more typical route for achieving diversity than the benthic," Santos said. "In the water column, you see more lineages that are very distantly related from each other that probably colonized that habitat at all different times. And that's why you get a lot of diversity.
"Whereas in the benthic environments, that's true to some extent but not to the same level. You get fewer lineages that have been in that habitat a little longer - and that's potentially why you see less variety in body shape."
The findings also shed light on how living in the dark affects the evolution of marine life. Only trace amounts of sunlight can reach the 200-meter depth, but it's not enough to enable photosynthesis - making the deep sea a dividing line that has an impact on how fishes spend their time hunting for food, one driver of evolutionary change.
For example, Santos said, compare with a well known shallow-water pelagic fish like the tuna, a very active species.
"In the deep water column, you don't see a lot of big, powerful swimmers because it's a very different kind of environment. It's dark," she said. "And so you tend to see fishes that sit in the water and wait for food to find them. Being pelagic in the deep sea seems to be fine for many different types of body shapes, from the blobby all the away to the skinny."
The lack of sunlight and photosynthesis in the deep sea also affects the ocean floor environment.
"In shallow water, oftentimes you'll be in an environment like a coral reef or a kelp forest, which has organisms undergoing photosynthesis. There are a lot of things to interact with, and that creates a diversity of body shapes because fish are designed to interact with different things," Santos said.
"The deeper bottom tends to be more plain and boring and muddy, so the fishes that live there don't tend to be shaped in different ways."
Overall, the study's rare look at life in Earth's largest habitat shows that the different body types of deep-sea fishes are products of multiple evolutionary pathways defined largely by location.
"The one place that humans have not dominated on this planet is the deep sea - and there is still so much to learn about the mystery of what all is there," Santos said. "This paper moves us forward with a recognition that evolution can work really differently depending on where exactly in the deep sea the fishes are."
Co-authors of the paper include Sarah Friedman of the NOAA Alaska Fisheries Science Center and Christopher Martinez of the University of California, Irvine.
Research Report:Distinct evolutionary signatures underlie body shape diversity across deep sea habitats
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