How anglerfish defy evolutionary expectations

A Rice University study sheds light on the extraordinary evolution of anglerfish, a group of deep-sea dwellers whose bizarre adaptations have captivated scientists and the public alike. The research, published in Nature Ecology & Evolution, uncovers how these enigmatic creatures defied the odds to diversify in the harsh, resource-poor environment of the bathypelagic zone—part of the open ocean that extends from 3,300 to 13,000 feet below the ocean’s surface.

Led by a team of biologists including Rice’s Kory Evans and his former undergraduate student Rose Faucher, the study analyzed the evolutionary journey of anglerfish (Lophiiformes) as they transitioned from seafloor habitats to the open waters of the deep sea.

Through cutting-edge genetic analysis and 3D imaging of museum specimens, the researchers reconstructed the evolutionary tree of anglerfish and identified the morphological innovations that allowed these animals to thrive in an environment considered among the most challenging on Earth.

Anglerfish are best known for their bioluminescent lures, which dangle from their foreheads to attract prey in the perpetual darkness of the deep sea. However, their evolutionary story goes far beyond this striking adaptation.

The study reveals that the deep-sea pelagic anglerfish (ceratioids) originated from a benthic or seafloor-dwelling ancestor. This ancestor lived on the continental slope before transitioning to the open waters of the bathypelagic zone in a transition that set the stage for rapid evolutionary change. The ceratioids then developed features such as larger jaws, smaller eyes and laterally compressed bodies—adaptations tailored to life in an environment with limited food and no sunlight.

Despite these directional trends, however, ceratioids also displayed remarkable variability in body shapes, from the archetypical globose anglerfish to elongated forms like the “wolftrap” phenotype, which features a jaw structure resembling a trap.

This finding is the most surprising of the study, for the bathypelagic zone did not constrain evolution as expected despite its apparent lack of ecological diversity. Instead, anglerfish achieved high levels of phenotypic disparity, greater than their benthic relatives in both shallow and deep waters. This suggests rather than being limited by the environmental challenges of the deep sea, ceratioids explored new evolutionary possibilities, diversifying their body forms and hunting strategies.

“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, a co-corresponding author on the paper and assistant professor of biosciences.

“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.”

Evans noted that the research leaves room for the possibility that nonadaptive processes, such as relaxed selection or random mutations, could also have contributed to the observed variability.

The researchers also compared anglerfish clades across different habitats and found more unexpected results. Coastal species like frogfish, which live in diverse and productive coral reef environments, exhibited much lower rates of evolutionary change than their counterparts in the deep 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,” said Faucher, who was co-first author of the paper along with Elizabeth Christina Miller, a postdoctoral fellow at University of California, Irvine.

“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.”

The researchers used a combination of advanced methods to conduct this study. They constructed a phylogeny of anglerfish using data from 1,092 genetic loci across 132 species, representing approximately 38% of described species, complemented by fossil calibrations and genomic data to estimate divergence times and ancestral habitats.

Morphological data were collected from museum specimens, including linear body measurements and 3D skull shape analyses via micro-CT scans. To evaluate evolutionary trends, the researchers applied phylogenetic comparative methods to assess phenotypic and lineage diversification, while disparity analyses quantified the extent of morphological variation across anglerfish clades and habitats.

They then employed Bayesian models to reconstruct ancestral habitats, revealing that ceratioids originated from benthic ancestors before transitioning to the pelagic zone. Finally, principal component analyses visualized how anglerfish occupied different regions of phenotypic space, shedding light on evolutionary trends in body, skull and jaw shapes.

“Anglerfish are a perfect example of how life can innovate under extreme constraints,” said Evans. “This work not only enhances our understanding of deep-sea biodiversity but also illustrates the resilience and creativity of evolution.”

This study’s significance extends beyond the evolutionary history of anglerfish. It provides valuable insights into how life adapts to extreme environments. The deep sea is one of the least understood ecosystems on Earth, yet it plays a critical role in global biodiversity and the planet’s carbon cycle. Understanding how organisms like anglerfish thrive in such conditions helps scientists predict how life might respond to environmental changes, including those caused by climate change.

Moreover, the study touches on broader questions of macroevolution: how new species arise, adapt and diversify. By showing that even resource-poor environments can foster significant evolutionary radiation, the research challenges conventional wisdom and opens new avenues for studying evolution in extreme habitats.