Study reveals how ancient fish colonized the deep sea

A deep-water fish known as a lanternfish gets its name from its capacity to emit light.

A deep-water fish known as a lanternfish gets its name from its capacity to emit light. Monterey Bay Aquarium Research Institute / Steven Haddock

More than 90% of the water in our oceans is found in the deep sea, but only approximately a third of all fish species. Since shallow ocean waters are warm and rich in resources, they provide an ideal environment for new species to emerge and prosper, according to scientists who have long believed that the explanation for this is intuitive. 

But according to a recent University of Washington study led by Elizabeth Miller, there were a number of times throughout Earth's distant past when fish actually preferred the chilly, dark, and desolate waters of the deep sea.

Miller, who completed the study as a postdoctoral researcher in the UW School of Aquatic and Fishery Sciences and is currently a postdoctoral fellow at the University of Oklahoma, said that it is simple to look at shallow habitats like coral reefs, which are very diverse and exciting, and assume that they have always been that way. These findings significantly refute that presumption and shed light on how fish species have adapted to significant climatic shifts.

Anything below 650 feet is commonly considered to be in the deep sea because that is the depth at which photosynthesis can no longer take place due to a lack of sunshine. It is a challenging location to survive because there is much less food and warmth than in the shallows. However, Miller was able to discover an unexpected evolutionary pattern by examining the links among fish using their genetic records dating back 200 million years: the speciation rates, or the rate at which new species originated, changed over time. Tens of millions of years ago, new species were emerging in the deep sea more quickly than in the shallower regions.

This finding, in some respects, left more questions unanswered than it did answers. Why did fish favor some habitats over others? What caused some fish more easily able to travel into the deep sea than others? And how did these earlier changes contribute to the current diversity of species?

a bristlemouth fish from deep water.    Monterey Bay Aquarium Research Institute / Steven Haddock
a bristlemouth fish from deep water.    Monterey Bay Aquarium Research Institute / Steven Haddock

Miller was able to pinpoint three crucial moments in Earth's history that most likely contributed to the flip-flopping speciation rates.

The division of Pangea, which took place between 200 and 150 million years ago, was the first, according to Miller. "This resulted in the formation of new coastlines and oceans, which increased the opportunity for fish to migrate from shallow to deep water. Suddenly, there were a lot more access points.

Next came the Cretaceous Hot Greenhouse Period, one of the warmest periods in Earth's history, which lasted for about 100 million years. Due to sea level rise at this time, many continents were submerged, leading to the creation of numerous new, shallow places all across the planet.

Shallow-water fishes really took off and diversified about this time, according to Miller. A large portion of the species diversity seen in the shallows today can be attributed to this period.

The middle Miocene climatic transition, another significant climatic change that occurred about 15 million years ago, was the third occurrence. This resulted from the continents' continued movement, which changed the ocean's circulation significantly and chilled the globe down to the deep oceans.

Miller noted that deep-sea speciation rates were accelerating at this time. "Cold-water fishes were a major driving force behind this. Off the Alaskan and Washingtonian coasts, a lot of the species you see now underwent diversification during this time.

However, climatic changes by themselves cannot account for how fish originally settled in the deep oceans. Not every animal possesses the ideal mix of characteristics needed to endure in deeper water and make use of the comparatively scarce resources available to them outside of the range of sunlight.

In order to evolve into a new species in the deep sea, one must first travel there, according to Miller. What we discovered was that the appearance of deep-sea fishes also fluctuated throughout time, in addition to speciation rates.

Large jaws were a common feature of the earliest fish that were able to adapt to life in the deep water. These probably increased their chances of catching food, which may be difficult at deep. The scientists discovered that fish with longer, tapering tails tended to be more successful at making the shift to deep water far later in history. They were able to save energy by moving more slowly along the seafloor as opposed to swimming in the water column.

The deep sea is home to a variety of creatures, some of which have tapering bodies and others that have large, menacing jaws with sharp teeth, according to Miller. "Those two body types show two different eras of colonization in the deep sea."

Despite the fact that these occurrences may seem like ancient history, we may learn from them about how life in our waters will be impacted by the current climate change. In order to better understand how modern deep-sea fish may respond to climate change and possibly guide conservation efforts, Miller believes that future studies will build on these findings.

According to this study, deep-sea fish thrive in colder waters, but as a result of climate change, ocean temperatures are rising, she said. "In the upcoming years, we may anticipate that this will have a significant influence on fish in the deep water."

Luke Tornabene from the University of Washington, Christopher Martinez from UC Irvine, Sarah Friedman from the NOAA Alaska Fisheries Science Center, Peter Wainwright from UC Davis, and Samantha Price from Clemson University are the co-authors.


Font Size
lines height