In March 2018, the University of Washington released footage shot south of São Jorge Island in the Azores, at a depth of about 800 meters. It shows a female Caulophryne jordani swimming with a tiny male fused to her belly. Nobody had ever filmed a living pair like this before.
That pair is a textbook case of sexual parasitism in deep-sea anglerfish, a group of roughly 160 species that live far from any sunlight. The male attaches to the female, then their tissues and blood supplies join. He stays attached for the rest of his life.
This update fixes the earlier version of the page, which credited every species with this way of reproducing and sometimes mixed up deep-sea fish with the anglerfish you find along European coasts. We kept the facts that check out. The rest is gone.
What is the difference between a deep-sea anglerfish and a coastal anglerfish?
Both belong to the order Lophiiformes, but they live in separate worlds. The coastal anglerfish (genus Lophius, the monkfish sold at fish counters) stays on the seabed. Deep-sea anglerfish make up the suborder Ceratioidei and drift in open water, between 300 and 5,000 meters deep according to the University of Washington.
For Lophius piscatorius, FishBase gives a maximum length of 200 cm and a habitat of sand or mud bottoms, sometimes rock, from the shore down to 1,000 meters. It hunts by ambush, half buried in the sediment, and waits for prey to come within reach. Males and females are about the same size. Neither one grafts itself onto the other. Nothing in the monkfish resembles the fusion described below, even though both groups share the lure on the head and the huge mouth.
| Coastal anglerfish (Lophius) | Deep-sea anglerfish (Ceratioidei) | |
|---|---|---|
| Depth | Shore to 1,000 m (FishBase) | 300 to 5,000 m (University of Washington) |
| Lifestyle | On the seabed, half buried, ambush hunter | Drifting in open water |
| Male vs female size | Comparable | Female up to 60+ times longer and about 500,000 times heavier |
| Male fuses to female | Never | In some species only |
| Seen by recreational divers | Sometimes, in the Mediterranean | No |
In short, the word “anglerfish” covers two stories that barely overlap.
In the deep, females are often small and dark, with a head that looks enormous next to the body.
How does the male deep-sea anglerfish fuse with the female?
The very small male finds a female of his own species, bites her and holds on. In species with permanent attachment, his skin then fuses with hers and their blood vessels connect. From then on he depends on her for food, a bit like a graft.
The size gap is dizzying. According to the University of Washington, the female can be more than 60 times longer than the male and about half a million times heavier. The old version of this page described a male “sometimes 10 times smaller.” That badly understated the difference.
Still, the whole group does not work this way. In a 2005 review in Ichthyological Research, American ichthyologist Ted Pietsch separates species where fusion is obligatory from species where it is optional. In others, the male releases his sperm and leaves without fusing. In some, he only becomes sexually mature once attached. A Cryptopsaras female can carry up to eight males, according to a 2015 Live Science summary of this research.
How the male finds his partner in the dark is still poorly documented in the wild.
So we use the phrase “fused for life” with care. It only fits some of the species.
Why doesn’t the female’s body reject the male?
Because these fish have lost part of their adaptive immunity. In 2020, a team from the Max Planck Institute in Freiburg and the University of Washington published a genetic analysis of several species in Science. In the species that fuse, genes linked to graft rejection are missing or no longer work.
The details depend on how the male attaches. In species where he attaches only temporarily, the aicda gene, which helps fine-tune antibodies, is no longer functional. Species with permanent attachment go further. Some no longer have functional rag genes, which removes the diversification of B and T cell receptors, seen as the hallmark of so-called classical adaptive immunity. The University of Washington also notes that some species lack major histocompatibility complex molecules.
In humans, those same tools are what make the body reject a transplanted organ.
One question stays open. How do these fish fight off infections? The authors think another form of immunity has taken over. It still has to be fully described, and this page cannot go further than the study does.
Where does the light in the anglerfish’s lure come from?
The glow of the esca, the fleshy tip of the lure the female carries on her head, comes from symbiotic bacteria. A study published in eLife in 2019 identified two species of the genus Enterovibrio in anglerfish from four of the eleven Ceratioidei families.
These bacteria have a heavily reduced genome. You might expect them to pass from mother to eggs, as many symbionts with such small genomes do. The study concludes instead that the fish pick them up from the environment, without saying at what stage of life. Only the female has a lure. It draws prey in the dark, and may also signal her species to males.
So the light does not come from the female’s own cells.
The basic idea is simple, though. A predator that barely moves lures its prey instead of chasing it, a strategy you will also find among these extraordinary defense mechanisms in marine animals. In the deep-sea anglerfish, the bait just happens to glow.
What did the first video of a living pair show?
It showed a female Caulophryne jordani and her fused male, filmed for about 25 minutes at roughly 800 meters. Kirsten and Joachim Jakobsen were behind the camera aboard LULA1000, the submersible of the Rebikoff-Niggeler Foundation, off São Jorge.
At the time, museum collections held only 14 females of this species. No male had ever been seen. The video revealed another detail never documented in a fish before: points of light that switched on at intervals along her fin rays. Nobody knows what they are for. Ideas range from attracting prey to deterring predators, or mimicking a jellyfish.
“This is a unique and never-before-seen thing,” said Ted Pietsch, professor emeritus at the University of Washington.
The video did not settle everything. A single filmed pair proves these fusions happen in the wild, but says nothing about how common they are in the species or how long an attached male lives.
Can you see an anglerfish while diving?
A deep-sea anglerfish, no. They live below 300 meters, out of reach of recreational diving. You can come across the coastal anglerfish in the Mediterranean on sand or mud, from around 20 meters according to FishBase. No dive guarantees an encounter, though.
Its camouflage makes it hard to spot. Lying on the bottom, it takes on the color of the sand and debris around it, and its flat outline blends in so well that an untrained eye can swim right past. If you do find one, keep your distance and do not touch it.
How deep you can go depends on the site and your certification level, so talk it over with your dive center at the briefing.
For the deep sea, you are left with footage. Deep-water fish often get distorted when brought to the surface, so a video shot on site tells you more than a photo of a specimen.
FAQ
Why do people say deep-sea anglerfish are fused for life?
In some species, the male attaches to the female and their tissues fuse. Their blood supplies join and the male stays attached for the rest of his life. This way of reproducing only applies to some of the species.
How big is the size difference between the male and the female?
According to the University of Washington, the female can be more than 60 times longer than the male and about half a million times heavier. The gap varies between species.
How many species of deep-sea anglerfish are known?
About 160 have been described, spread across eleven families in the suborder Ceratioidei. Not all of them reproduce by male fusion.
Can you see a deep-sea anglerfish while diving?
No. They live below 300 meters, out of reach of recreational diving. In the Mediterranean you may sometimes come across the coastal anglerfish (genus Lophius), with no guarantee.
Sources
- A blind date in the deep sea: First-ever observations of a living anglerfish, a female with her tiny mate, coupled for life (University of Washington, 2018)
- The immunogenetics of sexual parasitism (Swann et al., Science, 2020, Max Planck Society record)
