Deep-sea anglerfish defy evolutionary expectations

A new study reveals the remarkable evolutionary history of anglerfish, deep-sea creatures whose unusual adaptations have long fascinated both scientists and the public. 

Published in the journal Nature Ecology & Evolution, the research reveals how these mysterious animals defied expectations by diversifying in the harsh, resource-poor environment of the bathypelagic zone – the open ocean depths ranging from 3,300 to 13,000 feet below the surface.

From the seafloor to open waters

The study, led by biologists including Kory Evans of Rice University and his former undergraduate student Rose Faucher, investigated the evolutionary progression of anglerfish (Lophiiformes) as they moved from seafloor habitats to the deep sea’s open waters. 

By employing advanced genetic analysis and 3D imaging of museum specimens, the researchers reconstructed the anglerfish’s evolutionary tree and identified the morphological innovations that enabled them to thrive in one of Earth’s most challenging environments.

Unique adaptations of deep-sea anglerfish

While anglerfish are famous for their bioluminescent lures that dangle from their heads to attract prey in the deep sea’s eternal darkness, their evolutionary tale encompasses much more than this notable feature. 

The study revealed that deep-sea pelagic anglerfish (ceratioids) descended from benthic, or seafloor-dwelling, ancestors. 

These ancestors inhabited the continental slope before venturing into the bathypelagic zone’s open waters – a shift that paved the way for rapid evolutionary changes. 

The ceratioids developed characteristics such as larger jaws, smaller eyes, and laterally compressed bodies – adaptations suited to an environment with scarce food and no sunlight.

Surprising diversity of anglerfish 

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

This was the most unexpected finding of the study, as the bathypelagic zone did not limit evolution as previously thought, despite its apparent lack of ecological diversity. 

Instead, anglerfish achieved high levels of phenotypic disparity, surpassing their benthic relatives in both shallow and deep waters. 

This suggests that, rather than being constrained by the deep sea’s environmental challenges, ceratioids explored new evolutionary avenues, 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 pointed out that the research allows for the possibility that nonadaptive processes, such as relaxed selection or random mutations, might have also contributed to the observed variability.

Anglerfish evolutionary trends

The researchers also compared anglerfish groups across different habitats and uncovered more surprising results.

Coastal species like frogfish, which live in diverse and productive coral reef environments, displayed much lower rates of evolutionary change than their deep-sea counterparts.

“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 the 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.”

Methodology and breakthrough findings

To carry out this study, the researchers utilized a combination of advanced methods. They constructed a phylogeny of anglerfish using data from 1,092 genetic loci across 132 species.

This represented about 38% of described species and was supplemented by fossil calibrations and genomic data to estimate divergence times and ancestral habitats. 

Morphological data were gathered from museum specimens, including linear body measurements and 3D skull shape analyses via micro-CT scans. 

To assess evolutionary trends, they applied phylogenetic comparative methods to evaluate phenotypic and lineage diversification, while disparity analyses measured the extent of morphological variation across anglerfish groups and habitats. 

They then used Bayesian models to reconstruct ancestral habitats, revealing that ceratioids originated from benthic ancestors before moving 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.”

Understanding life in extreme environments

The significance of this study extends beyond the evolutionary history of anglerfish. It offers valuable insights into how life adapts to extreme environments.

The deep sea is one of Earth’s least understood ecosystems, yet it plays a crucial role in global biodiversity and the planet’s carbon cycle. 

Understanding how organisms like anglerfish succeed in such conditions helps scientists predict how life might respond to environmental changes, including those caused by climate change.

Moreover, the study addresses broader questions in macroevolution: how new species arise, adapt, and diversify. 

By demonstrating that even resource-poor environments can foster significant evolutionary radiation, the research challenges conventional wisdom and opens new pathways for studying evolution in extreme habitats.

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