Its dark down there in the depths of the ocean, which is lucky for the Anglerfish, as it is unlikely that these fish will be winning a beauty competition anytime soon. But there rather ugly fish are beautifully adapted for life in the darkness; they have evolved their own light to act as a lure for prey.
Living so far from human reach, Anglerfish were only discovered when one female washed up on the shore of Greenland in 1833. For years, the only way to research these deep-sea denizens was to keep an eye out for any caught up in fishing nets or washing up on beaches. Only in recent years have scientists been able to investigate Anglerfish using remote operated vehicles and cameras that can visit the deepest oceans and observe Anglerfish and other deep-sea species in their natural habitat.
You will likely think of the iconic deep-sea Anglerfish with its wide jaw bristling with needle like teeth and its dangling light, but there are many species of Anglerfish, and not all of them live at the bottom of the ocean. Some live in shallower seas, where they are camouflaged as rocks and act as ambush predators, waiting on the seabed for their hapless prey to swim past. But their relatives live at depths of up to 6660 feet, drifting through the dark. All Anglerfish are part of the bony fish order Lophiiformes, containing 200 species including such wonderfully named fish as the frogfish, coffinfish, the handfish, sea mice and sea toads.
Evolution of Anglerfish is hard to track. There isn’t much of a fossil record, and scientists don’t exactly have their hands full with specimens, although deep ocean ROVs can now capture species and bring them to the surface in specially pressurised containers. From genetic analysis of individuals found, it is likely that deep sea Anglerfish evolved very quickly around 130-100 million years ago. In comparison to their shallow water relatives, deep dwelling species evolved very rapidly in the resource poor depths, where they had to adapt quickly or simply die out.
Light it up: the unique biology of the Anglerfish
Like many deep ocean species, Anglerfish have unique (many would say ugly!) looks and adaptations for living in such an extreme environment. Perhaps their most famous feature is their lure, a luminescent organ called the esca at the end of a dangling extension of their skin. This formed from an outgrowth of epithelial (skin) cells that eventually evolved a small pore at the end. The light that Anglerfish produces is created by species of symbiotic bacteria that live inside the esca. No one knows exactly how this relationship formed, but these bacteria have never been isolated from larval species, so its likely the fish somehow acquire the microorganisms from the surrounding seawater. The pore at the end of the esca is the likely entry point and is also a way for cell waste from the bacteria to leave. This lure serves a dual purpose: one to lure in prey, usually small shrimp, worms and lanternfish, towards those gaping jaws and huge needle teeth. Anglerfish tend to drift through the water rather than swim to conserve energy as food can be pretty thin on the ground (or should that be in the water) and they are opportunistic predators, taking whatever swims across their path. In a study sampling deep ocean species, the researchers found that 52% of the Anglerfish sampled had empty stomachs, indicating that they are incredibly low energy consumers.
Once the unfortunate prey has swum into the mouth of the Anglerfish, the thin flexible jaw bones will snap shut and those needle teeth, previously turned inwards, will snap forwards to prevent escape. Anglerfish must take food where they can find it, as prey are few and far between in such a vast marine desert, so Anglerfish can swell their stomachs up to swallow prey twice their size. Who knows when their next meal will drift towards them out of the dark.
The second reason for the lure is to attract mates from the dark desert of the bottom of the ocean. Only female anglerfish have a lure, and they are definitely the boss in the relationship. Most Anglerfish are around the size of a human fist, although some, like the warty sea devil, can be as large as 2 feet. However, males are tiny, often smaller than 1cm! They don’t have a lure but instead all their evolutionary energy went into a highly developed sensory system, which they use to find females, no small task at the bottom of the ocean. Once a male finds a female, he bites into the female, injecting an enzyme that digests her skin and releases sperm to fertilise her eggs. For some species, after this, the male lets go and swims back into the gloom, hot on the tail of another female. But some species of Anglerfish go even further than this and the male will actually parasitise the female, even joining his blood stream to hers. The stakes are incredibly high for some species; the males will lose their digestive tract, so it’s find a female or starve! Some female Anglerfish have even been found with up to 6 males attached! Talk about a tough dating scene.
This is a very unusual reproductive strategy and it likely evolved due to the very low density of Anglerfish and the pressures of finding a mate in such a harsh environment. Recent genome studies have revealed even more amazing evolution. In a genetic comparison of 13 Anglerfish species, researchers found that in those species where fusing occurred, the males had effectively lost their adaptive immunity. They looked particularly at immune genes encoding the major histocompatibility complex, which is a complex of cell surface molecules that recognise what are an individual’s own cells and what are foreign cells. These genes were so mutated that it made the major histocompatibility complex effectively non-functional, removing the fishes’ ability to detect foreign cells. In most of the animal kingdom, this would be fatal, leaving individuals open to infection without an immune response. But not in the anglerfish! In fact, some male anglerfish rely on this strange adaptation to survive by parasitising the female; without it, they would starve to death in the vast marine desert.
Deep ocean, deep problems
Currently, most species of Anglerfish are considered least concern by the IUCN or haven’t been fully assessed as they are simply so hard to find. Some species of Anglerfish, like North-western European Lophius species are classified as ‘outside safe biological limits’ by the International Conference on Environmental Systems (ICES) due to overfishing. Some species are considered a delicacy in Japan and South Korea, and these species should be closely monitored to make sure they don’t become overexploited stocks. Occasionally El Nino climate events lead to dead anglerfish floating to the surface and being caught in fishing nets, although no one is exactly sure why.
Although Anglerfish are likely doing fine, the deep sea, long thought to be a biodiversity dead zone, is actually an incredibly rich but poorly understood ecosystem in need of protection. Unless you have a spare remote operated vehicle, it is very difficult to study the deep ocean, and we don’t have good baseline data for what should be down there and whether there have been declines or not. Many species are highly adapted for such extreme conditions, and this can put them at increased risk of disturbance. Oil and gas drilling and deep-sea mining are big concerns, with many scientists concerned that fragile habitats, like sea mounts, which many species rely on for nurseries and food, will be destroyed. Arguably the greatest scourge of our oceans, plastic pollution, if affecting even the bottom of the ocean, with bags and bottles found in the Marianas Trench, the deepest point on the planet. On recent surveys of biodiversity in the Marianas Trench, researchers found that all amphipods (small marine crustaceans) tested had ingested high levels of plastic. Since plastic is getting into the very base level of the food chain, its very likely that larger species further up, like the Anglerfish
Overfishing is a major problem and often one that loses prominence in the discussion behind climate change and deforestation. Currently it is estimated that 29% of the worlds fish stocks are overfished, with illegal and unregulated fishing especially threatening the sustainability of the marine ecosystem so many people rely on for food and economic stability. Huge fishing fleets are increasingly fishing deeper and deeper depths to find fish as they exhaust the stocks closer to the surface. This practice has already pushed species like the orange roughy almost to extinction, when overfishing led to the collapse of stocks in the 1990’s. Just like the orange roughy, many deep-sea species are slow growing and don’t reproduce quickly, putting them at high risk of extinction from overexploitation. With the seas above them becoming increasingly bare, there are worries deeper species like the Anglerfish will be next on the plate.
Blue Marine Foundation is an NGO that specialises in ocean management, lobbying to improve governance of the ocean and aiming to secure 30% of the world’s oceans under Marine Protected Areas. They also develop models of sustainable fishing and have worked to secure a ban on bottom trawling in several key areas.
You can check out their work at bluemarinefoundation.com.
We can all think about the fish we eat and put pressure of governments to fish more sustainably to help not only the oceans, but also the fisherman who rely on the security of fish stocks for their future jobs. The WWF has a guide to more sustainable fishing at https://www.wwf.org.uk/what-we-do/addressing-unsustainable-fishing-and-seafood.
Monterey Bay aquarium has a seafood guide at https://www.seafoodwatch.org/. Here you can search fish in supermarkets and determine where it has come from and how sustainable it is, and they rank every commercial fish species with a best choice, certified, good alternative or avoid ranking.
Do you want to see deep ocean species up close and personal? On top of their seafood guide, the Monterey Bay Aquarium has a deep ocean exhibit, although they sadly don’t have any Anglerfish. If you’re in the area, take a trip to walk along the seafloor with the amazing species held at the Aquarium, like deep sea isopods and corals, Japanese spider crabs and the dazzling Abyssal comb jellies, that gleam in the dark just like the lure of the Anglerfish.
Espinoza, M. and Wehrtmann, I.S. (2008) ‘Stomach content analyses of the threadfin anglerfish Lophiodes spilurus associated with deepwater shrimp fisheries from the Central Pacific of Costa Rica.’ Revisita de Biologia Tropical
Miya, M. Pietsch, T.W. Orr, J.W. Arnold, R.J. Satoh, T.P. Shedlock, A.M. Ho, H.C. Shimazaki, M. Yabe, M. and Nishida, M. (2010) ‘Evolutionary history of anglerfishes: a mitogenomic perspective.’ BMC Evolutionary Biology.
Munk, O. Hansen, K. and Herring, P.J. (2009) ‘On the development and structure of the escal light organ of some melanocetid deep sea anglerfishes.’ Journal of the Marine Biological Association of the United Kingdom.’
Paulus, E. (2021) ‘Shedding light on deep-sea biodiversity- a highly vulnerable habitat in the face of anthropogenic change.’ Frontiers in Marine Science
Pietsch, T. (2005) ‘Dimorphism, parasitism, and sex revisited: modes of reproduction among deep sea ceratoid anglerfishes.’ Ichthyological Research
Swann, J.B. Holland, S.J. Petersen, M. Pietsch, T.W. and Boehm, T. (2020) ‘The immunogenetics of sexual parasitism.’ Science.
MASSIVE THANK YOU TO RACHAEL DA SILVA FROM THE UK FOR THIS WRITE UP! PLEASE FOLLOW HER ON INSTAGRAM AND HER WILDLIFE ARTWORK AT TILLY_MINT08