Distributed throughout the world’s oceans, at depths greater then 300m, this dark, vast and often food-scarce environment is home to a suborder of anglerfish known as the Ceratioidei, that exhibit a very unique form of reproduction.

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Fig 1. Four female ceratioids with distinctive lures and fang-like teeth. Species A) Melanocetus eustalus; B) Cryptopsaras couesii; C) Cryptopsaras couesii; D)Lasiognathus amphirhamphus. Source.

They are the most species-rich vertebrate group within the bathypelagic zone (1000 to 4000 metres deep) with 162 recognised species from 11 families. Commonly known for their large heads, fang-like teeth and distinctive bioluminescent lure – hanging over the mouth like a fishing rod to draw in prey from the immense darkness (fig 1.). However, these features are only observed from females and, for a while, the only Ceratioids observed were the large females. This left scientists with a mystery as to where all the males were located. As exploration into the deep sea progressed, the males were still not found and the enigma surrounding Ceratioid reproduction continued. It was not until almost a century after the first ceratioid was catalogued that a male was discovered, in a most unexpected spot, fused to the side of a female – living off her as a parasite.

“This remains a puzzle for some future researcher to solve”

Seamundsson in 1922, wondering why, what he thought to be young anglerfish (later discovered to be males) were attached to the female.

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Fig. 2 An 11mm parasitic male attached to the larger 77mm female of the species Bertella idiomorpha. Source

Once a parasitic male was found it was realised that males had been mistaken for the fishes young, miscatalogued, or missed altogether. The smaller size of males, combined with limited number of specimens from the deep sea, meant observation of males were, and still are, infrequent. This is due to the physically extreme differences between the males and females, a phenomenon known as sexual dimorphism. Sexual dimorphism is common throughout the animal kingdom and is seen in other fish, ranging from size difference to markedly different colouration between the sexes. The sexual dimorphism of the Ceratioids is particularly extreme though due to the immensely larger size of the females compared to the males (fig. 2) and their very different structures. One of the most extreme examples is found in the species Ceratias holboelli whose females can be a million times heavier than males, and more than 60 times the length.

“merely an appendage of the female, and entirely dependent on her for nutrition…so perfect and complete is the union of husband and wife”

Regan in 1925, marvelling at the unusual reproductive strategy of the Ceratioids.

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Fig. 3 A large female Ceratioid with two males fused to her belly. Source

Alongside an extreme size difference, the sexes also exhibit very different sensory structures, some of which function to increase the odds of a union between the sexes. The male’s large nostrils relative to body size, help in identifying species-specific pheromones emitted from a female to accurately locate a potential partner in the deep sea. The male’s large eyes are thought to help seek out the female, possibly becoming drawn in by her  bioluminescent lure. Whilst the female’s fleshy lure, called an illicium, sprouts out of a bone at the top of the head. The corresponding male structure is a basal bone that controls movement of the rostral bone that opens and closes the jaw – allowing the male to bite and grasp hold of the soft side of a female’s belly with pincer-like teeth. These equivalent structures draw the sexes together and also enable attachment for reproduction.

Upon finding and attaching to a prospective partner, the males of some species fuse to the tissues of a female – this can be temporary or permanent. In permanent parasitism the eyes and nostrils of the male fish become unnecessary and degenerate. Outgrowths of tissue from the snout and lower jaw connect to an outgrowth of tissue from the female, sometimes extending into the males mouth. The circulatory systems become connected, making the male completely dependent upon the female for nutrients transported through the blood. The male becomes a nutritional parasite of the female; the female becomes a hermaphrodite – obtaining sperm from the male gonads as she requires. Two or three males have been found attached to the same female species simultaneously (fig. 3), and one specimen of Cryptopsaras couesii was found to have an astounding eight males attached to her belly at once.

“It would be very difficult for a mature fish to find a mate, but this difficulty appears to some extent to have been got over by the males”

Regan, in 1926.

This phenomenon has drawn some attention due to its unusual and interesting nature, however, sexual parasitism has not been found in all species of the Ceratioids. Currently sexual parasitism has been found in 24 species comprising 6 families. Other species are thought by scientists to possibly have free-living males that are able to feed and live independently of the female, nipping onto her side to reproduce but not fully fusing. The lack of data on many families and species makes it difficult to determine how common sexual parasitism is within this suborder; for many families the possibility has not been ruled out.

Whether parasitism is a common feature of Ceratioids or not, genetic studies have demonstrated that this reproductive strategy has evolved independently at least 3 times, and possibly as many as 7 times within the Ceratioids. The Ceratioids diversified within the deep sea 100 to 130 million years ago, when many of these unique features arose. Scientists think it is likely that sexual parasitism and dimorphism are advantageous by increasing their survival and reproductive success. The Ceratioid females are thought to have low population numbers and solitary lifestyles, slowly swimming through the water column in the food-poor deep sea environment seeking out prey. In this environment it could be a struggle to find a potential mate. Sexual parasitism is perhaps a successful way to ensure that once the sexes have found each other, they can never lose each other again in the colossal, dark, depths of the ocean.

Video from the BBC’s ‘Blue Planet’, narrated by Sir David Attenborough

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