A new study has revealed fascinating insights into the evolution of anglerfish (Lophiiformes), a peculiar group of deep-sea creatures renowned for their unusual adaptations.
The research delves into how these remarkable fish have managed to thrive and diversify in the almost inhospitable depths of the bathypelagic zone – an expansive, forever dark region of the ocean that lies 1,000–4,000 metres beneath the surface.
The study examined the evolutionary journey of anglerfish (Lophiiformes) as they transitioned from life on the seafloor to the vast open waters of the deep sea.
By combining cutting-edge genetic analysis with 3D imaging of museum specimens, the researchers constructed an evolutionary tree of anglerfish. Their findings – published in Nature Ecology & Evolution – highlight the morphological innovations that have enabled these fish to flourish in one of Earth’s most challenging environments.
Lure of the deep
Anglerfish are perhaps best known for their bioluminescent lures, which dangle enticingly from their heads to draw in prey within the never-ending darkness of the deep sea. Yet, as this study shows, there's more to these animals than simply their eye-catching fishing rods.
The work reveals that today’s pelagic anglerfish, known as ceratioids, evolved from ancestors that once lived on the seafloor along the continental slope.
These ancestors made a dramatic ecological shift, moving into the bathypelagic zone and sparking a wave of evolutionary change. This transition saw ceratioids develop adaptations such as larger jaws, smaller eyes and streamlined, laterally compressed bodies – features made for surviving in an environment with few resources and no sunlight.
Darkness = diversity
What’s particularly surprising, say the researchers, is the variability in body shapes observed among ceratioids. While environmental constraints might be expected to limit evolutionary diversity in the deep sea, the opposite appears true.
From the round, globular forms typical of many anglerfish to the wolftrap species with their jaw structures resembling a sprung trap, ceratioids exhibit a remarkable array of body shapes.
The finding challenges traditional assumptions about the limits of evolution in resource-poor habitats. In fact, ceratioids demonstrate greater morphological disparity than their relatives living in shallow or deep seafloor habitats. This suggests that, rather than being hindered by their extreme environment, these fish have embraced evolutionary experimentation, developing varied body forms and hunting strategies.
“With their striking adaptations, such as bioluminescent lures and cavernous jaws, deep-sea anglerfish could be one of the few known examples of adaptive radiation in the nutrient-starved bathypelagic zone,” explains Dr. Kory Evans, assistant professor of biosciences at Rice University in Texas, and co-corresponding author of the study.
"These features likely gave them an edge in exploiting scarce resources and coping with their extreme environment. However, we can’t rule out the influence of random mutations or relaxed selection in driving this diversity.”
Fossils, genes and more
The study also found that coastal anglerfish species, such as frogfish that inhabit vibrant coral reefs, evolved at much slower rates than their deep-sea counterparts. This contrasts with the expectation that more complex habitats, rich with features such as corals and rocks, would foster greater evolutionary change.
“The idea that a seemingly barren, homogenous environment – water stretching endlessly in every direction – could produce such diverse body shapes is truly counterintuitive,” says Faucher, co-first author of the paper alongside Elizabeth Christina Miller, a postdoctoral fellow at the University of California, Irvine.
"In more structurally diverse habitats, we expect to see greater morphological variation. Instead, we’re witnessing this explosion of forms in deep-sea fish surrounded by nothing but water.”
To uncover these insights, the researchers used a variety of tools and techniques. They analysed genetic data from over 1,000 locations, across 132 species, representing nearly 40% of described species. This was complemented by fossil evidence and genomic data to trace divergence timelines and ancestral habitats, Faucher exaplains. Detailed morphological data were gathered from museum specimens, including linear measurements and micro-CT scans of skulls. Using advanced statistical methods, the team reconstructed ancestral habitats, visualised evolutionary trends, and quantified morphological variation among anglerfish groups.
Why are these findings important?
“Anglerfish epitomise how life can innovate and thrive under extreme constraints,” says Dr. Evans. “This research not only deepens our understanding of deep-sea biodiversity but also showcases the resilience and inventiveness of evolution.”
Beyond their specific focus on anglerfish, the study has broader implications for understanding life in extreme environments, says the researchers.
The deep sea, still one of the most mysterious ecosystems on Earth, plays a pivotal role in global biodiversity and the planet’s carbon cycle. Dr. Evans says that exploring how organisms adapt to its harsh conditions provides crucial insights into the mechanisms of evolution and resilience. It also sheds light on how life might respond to changing environmental pressures, including those driven by climate change, and underscores the potential for surprising evolutionary innovation in even the most unlikely of settings, Dr. Evans concludes.
Find out more about the study: Reduced evolutionary constraint accompanies ongoing radiation in deep-sea anglerfishes
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