The research led by Elizabeth Santos, an assistant professor at The Ohio State University, is the first of its kind. This study shines light on the amazing, counterintuitive evolutionary trajectories of deep-sea fishes. The study focused on animals that live at least 200 meters or deeper in the ocean. This area accounts for almost 90% of the ocean volume on our planet. The results show that each of these fishes adapted their body shapes differently depending on where they live.
The research team looked at the shape traits of 2,882 deep-sea fish species. They then used these characters to compare to well known shallow-water pelagic species such as tuna. Primary investigator Maria Santos and her colleagues wanted to know what evolving under these conditions was like for marine life. Their impressive body of research has uncovered a surprising variety of body types among deep-sea fish. This research highlights the importance of understanding how place matters in determining evolutionary trajectories.
Exploring the Depths of Evolution
Santos pointed out that there’s a huge contrast between the deep-sea environment and that of shallower waters. The research shows that virtually no sunlight passes deeper than 200 meters. Intensive light and extreme heat at the surface transitions to near-freezing temperatures and darkness. Without this essential light exposure, it changes the way fishes at these depths seek out prey and even interact with other species.
“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,” – Elizabeth Santos
As Santos points out, the deep sea is not one, homogenous habitat. It is unique in its variety of habitats deeply shaping fish evolution. That diversity opens up many possible paths for evolution to take. Consequently, it contributes to a more diverse body form than in fishes residing in benthic environments.
“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,” – Elizabeth Santos
In their study, the researchers used previously published datasets with measurements of fish body shape, evolutionary relationship, and habitat type. It was only with this broad approach that they were able to make strong conclusions about the evolutionary trends happening within the deep sea.
Unique Adaptations in Body Shape
One of the most interesting things we discovered from this study was this key finding. It turns out that different body types in deep-sea fishes developed at varying evolutionary speeds and through different organismal mechanisms. Even in fish analysis, it was found that pelagic fishes exhibit more different body shapes than benthic species.
Santos noted that fish deep in the water column experience special environmental stressors. As a consequence, they impart little necessity to exhibit all the traits that go along with elite-level swimming prowess.
“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,” – Elizabeth Santos
Instead, for most species it makes sense to be a passive sit in the water that waits for food to come to it. This approach celebrates many kinds of bodies, from gooey amorphous blobs to willowy figures. Yet it continues to serve as a universal blueprint, illustrating how inhabiting an extensive deep-sea pelagic realm cultivates adaptive innovations.
“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,” – Elizabeth Santos
Implications for Conservation and Future Research
The study is immensely helpful in informing our understanding of body shapes. Perhaps most importantly, it highlights the critical need for incorporating habitat-specific evolutionary processes into conservation planning and efforts. Santos concluded that understanding how evolution works in different environments can inform where research and conservation efforts should be focused in the future.
“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,” – Elizabeth Santos
In addition to these findings, Santos highlighted some of the disparity between pelagic and benthic on what they found. She found that a smaller number of lineages are successful in benthic habitats. This might account for why there is a much lower diversity of body forms as opposed to those seen in the pelagic realm.
“Colonization of the deep pelagic seems to be a more typical route for achieving diversity than the benthic,” – Elizabeth Santos
From their results, the researchers determined that evolution has put different pressures on how fish bodies are shaped depending on the environments they live in. This knowledge will greatly influence how scientists study and conserve marine biology in the future.