Anglerfish are among the ocean’s most bizarre creatures. This group of deep-sea dwellers are best known for the bioluminescent lures dangling from their foreheads to attract predators, but they have also evolved some traits that have helped them defy evolutionary odds. They likely adapted larger jaws, smaller eyes, and more compressed body shapes to survive in the ocean’s harsh bathypelagic zone–3,300 to 13,000 feet below the ocean’s surface. The findings are detailed in a study published November 27 in the journal Nature Ecology & Evolution.
“Anglerfish are a perfect example of how life can innovate under extreme constraints,” study co-author and Rice University evolutionary biologist Kory Evans said in a statement.
These strange and spiny creatures have perfected the art of deep-sea fishing with their dangly lures, but also open up a window into how evolution works in such an inhospitable and unexplored place.
In the study, a team of biologists studied how anglerfish–or Lophiiformes–transitioned from habitats on the seafloor and into the open waters of the deep sea. They used museum specimens to analyze their DNA and took 3D images to build the anglerfish evolutionary tree. They ultimately used the genetic data from 132 species, representing approximately 38 percent of described anglerfish species. The genetic data was complimented by fossils that were analyzed using micro-CT scans. These images and family tree allowed the team to pinpoint the physical changes and innovations that allowed these animals to thrive in one of Earth’s most inhospitable locations.
The team found that the deep-sea pelagic anglerfish–called ceratioids–originated from one seafloor-dwelling ancestor. This ancestor lived on the floor of the ocean’s continental slope before it transitioned into the open waters of the bathypelagic zone. This move then set the stage for rapid evolutionary changes. The ceratioids then developed features including larger jaws, smaller eyes, and laterally compressed bodies. These adaptations are all tailored to living in a place with limited food resources and no sunlight.
Despite these changes, ceratioids also showed a large variability in body shapes. They range from the more familiar round and sphere-like anglerfish to the long “wolftrap” ceratioid with a jaw that looks like a trap. According to the team, this diversity of body shapes was the most surprising part of the study because the harsh bathypelagic zone did not constrain evolution as expected, despite the lack of ecological diversity among other living things in this zone. To the contrary, these ceratioids look quite different from one another, more so than their bottom-dwelling relatives. This suggests that instead of being limited by the deep sea’s environmental challenges, ceratioids explored new evolutionary possibilities through diversifying their body forms and how they hunt.
“With their unique traits like bioluminescent lures and large oral gapes, deep-sea anglerfish may be one of the few documented examples of adaptive radiation in the resource-limited bathypelagic zone,” said Evans. “These traits likely gave anglerfish an edge in exploiting scarce resources and navigating the extreme conditions of their environment, although we don’t have strong evidence directly linking this diversity to this kind of resource specialization.”
According to Evans, the research also leaves room for the possibility that other nonadaptive forces such as random mutations may have also contributed to the observed variability in anglerfish.
[Related: This rare ‘Finding Nemo’ fish mysteriously washed up on a California beach.]
When the team compared anglerfish clades–a group of organisms believed to have evolved from a common ancestor–across different habitats they found some more unexpected results. They looked at coastal species like frogfish, who live in coral reef environments with their counterparts in the deep sea. The coastal frogfish had much lower rates of evolutionary change than their relatives living in the deeper sea.
“The idea that a resource-poor, homogenous environment–like being surrounded on all sides by nothing but water–would produce diverse body and skull plans is really counterintuitive in this field,” study co-author and University of California, Irvine postdoctoral fellow Rose Faucher said in a statement. “When fish have different features to interact with, like corals and plants in shallow water or sand and rocks on the seafloor, that’s when we would expect fish to have a lot of variation in shape. Instead, we’re seeing it in these deep-sea fish who have nothing but water to interact with.”
According to the team, this study provides valuable insights into how all life–not just anglerfish–can adapt to extreme environments. The deep sea is one of the least understood ecosystems on Earth, but it plays a critical role in the planet’s biodiversity and carbon cycle. A better understanding of how organisms survive in these conditions can help scientists predict how life elsewhere may respond to environmental changes. The research also shows that even resource-poor environments like the bathypelagic zone can spur significant evolutionary changes, opening up new paths for studying evolution. Or more simply, life finds a way.
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