Deep Sea Anglerfish – Ceratiidae © E.Widder ORCA


In the deep, dark depths of the worlds oceans life can be a difficult experience. Even finding enough food can be a huge challenge for many organisms.

Animals that survive in the twilight zone, the depths of the ocean so deep that not even light can penetrate, must have a number of adaptions to help them survive. These can come in a multitude of strange appendages, unusual sensory organs or something as simple as a giant mouth.

Beyond merely surviving, all animals have the need to reproduce, to further their lineage and pass on their genes. In an environment with no light and mostly devoid of life, finding a mate can prove a difficult task.


The Ceratioids, deep sea anglerfish named for the lure on their head that is in fact a highly specialised fin ray, have developed a novel strategy to counter this. The male spends his entire adult life searching for a female and once he does he gets very attached. The male will bite her and attach himself, whereupon he will begin to fuse to her, his body becoming little more than an extension of hers.

In 1922, Bjarni Saemundsson discovered what we now know as a male Ceratoid anglerfish attached to his female mate. He was baffled. Assuming it to be the female’s offspring he wrote ”I can form no idea of how, or when, the larvae, or young, become attached to the mother. I cannot believe that the male fastens the egg to the female”.

This puzzle did not take long to solve. In 1924 Charles Tate Regan also found a smaller fish attached to a Ceratoid female. On first observation he came to the correct conclusion that the smaller fish was a male parasitic upon the female before dissection proved this hypothesis to be true. This was the first, and remains, the only example of intraspecies sexual parasitism in vertebrates.


Melanocetus – A large female angler hunts for prey. Image source: MBARI


So how do they do it? The first problem for any young male is finding his mate. In an environment as dark and dangerous as the bathypelagic zone that they live in, the male must use everything available to him. In most Ceratioid families the males have exceptionally well developed olfactory systems. The males sniff out their potential mates by following species-specific pheromone trails (source).

The Ceratioids are most easily recognised by their unique, bio-luminescent lures named esca. These serve the function of luring potential prey, but in addition to this it is thought that they help the males to recognise females in the dark (source). This is not true of all species, but for most the free living male has much larger eyes than the parasite form of the same species.

Whilst specifics can vary between species, once the male has found his potential mate it is thought he will generally proceed by attaching himself to her, in most examples on the right side of her stomach (source). To do this he uses special teeth developed at the end of his metamorphosis to an adults (source). At the site of the bite the female will begin to grow a ‘plug’, a site for the male to attach himself to more permanently known as a papilla. At this stage both individuals skin will begin to merge with one another. Eventually the male’s mouth is almost entirely fused to the female. He will continue to draw water through his mouth for respiration until it is completely closed off. Thereafter he uses the opening and closing of his opercula. Whilst the pair are growing more attached, the male’s eyes begin to degenerate (source). The level of atrophy is a good way of determining the length of the partnership.


Photocorynus spiniceps. Male is attached on dorsal surface. Photo: Theodore W. Pietsch


As finding a mate for these fish can be particularly tough, even if the partners are not sexually mature they will still form a relationship. After that it is merely a waiting game for both individuals to become sexually mature enough to mate. Some evidence points toward the fact that angler fish may not even become sexually mature until the fusion has occurred.

There is debate within the scientific community as to whether all Ceratioids must reproduce in this manner. Pietsch hypothesised that some species would only form temporary attachments based upon the fact that no attached males had been found. More recent evidence suggests that all species will only attach to one another temporarily given the chance. If both individuals are far from sexual maturity the chance of the fusion becoming permanent increases.

As with much in science the debate is yet to be settled. Much is still not known about these creatures, particularly on the subject of potential human impacts. The deep sea remains an ecosystem shrouded by mystery, partly because of the difficulty exploring it but also due to lack of governmental interest. Research into anglerfish mating is thought to have potential biomedical implications through studying the immunology of the act of fusion.


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