Angler Fish Reproduction: When Two Become One

By Curtis Winter

It may be unexpected to find the classic Spice Girls lyric and a deep-sea monster in the same sentence, however, the Angler fish life cycle has demonstrated surprising parallels, particularly during reproduction. 

The Angler fish is a bony fish notorious for its bioluminescent lamp and has subsequently taken centre stage as the Florence Nightingale of the sea. The term Angler fish encompasses 16 different taxa and suborders, all characterised by the presence of an “angler” in the female variant. The lamp is the evolutionary equivalent of a biological lure, used to entice unsuspecting fish into the carnivorous Angler fish’s grasp. The bioluminescence itself is the product of a symbiotic relationship, whereby bacteria living within the lamp provide fluorescence in exchange for nutrients and protection (Young, 1983).  

The harrowing appearance of the Angler fish can be seen in mainstream media, particularly its almost demonic portrayal in Finding Nemo. However, it is rarely touched upon the fact that only females look this way, as male variants are small and feeble in comparison. The Angler fish is therefore an extremely sexually dimorphic creature, which is crucial to its reproductive strategy; the lamp is a lure not only to prey but males also. In the deep dark abyss over 1000m below the ocean’s surface, how are you supposed to attract a mate without a light? The female Angler fish has evolved to avoid this issue. Meanwhile, the male has the largest nostrils in proportion to head size in the animal kingdom, allowing heightened olfactory sensitivity when scouring a female counterpart. Once they find each other, the real chaos begins. 

In 1922, two smaller fish were discovered to be attached to a larger female, assumed only to be a mother and young. However, this discovery was a major insight into the mechanics of the Angler fish mating strategy, which occurs more specifically in ceratioid Angler fish. This totals around 25 species capable of such phenomena. 

After trawling through the unknown following nothing but a simple species-specific pheromone, the male Angler fish bites his female companion. Their bodies fuse:  vessels join like the colliding roots of trees, and tissues form one shared body. The male is a mere appendage, hanging onto the female like some vestigial limb. What he does not need, he no longer has; eyes and fins disintegrate in atrophy and her blood supply is now his (Duarte et al, 2001). He is merely a sperm bank. Their bodily processes are coordinated by a shared hormonal network, so when she is ready, he is ready. Once ejected, the gelatinous sheath of eggs is fertilised in almost perfect timing by the male, and this cycle occurs over several seasons, sometimes with the aid of several male suitors (Afonso-Dias et al, 1996). 

This phenomenon of sexual parasitism is intriguing for many reasons. The immunological complication of two bodies fusing is so vastly complex that some Angler fish no longer have an adaptive immune system (Swann et al, 2020). The MHC II pathway has been completely lost in Lophius picatorius, which would otherwise defend against extracellular threats through its role in T cell antigen presentation (Dubin et al, 2019). This unusual compromise exhibits the competition in the sexual economy of deep-sea species, and how it has skewed selection and evolution. Without such an adaptation, the foreign cells would be attacked by the immune system as if they were pathogenic, and so its absence enables a relatively simple fusion with little possibility for rejection or an immune response. It is worth questioning whether this may pose any potential translations into the world of transplants, and the subsequent immunological issues in our own organ “fusions”. The variability between MHC molecules within even the same species is the main antagonist in the battle of human organ and bone marrow transplantation. Ideally we can learn more than just the horrors of their reproduction from the surprising strategies of the Angler fish.

References: 

Afonso‐Dias, I.P. and Hislop, J.R.G., 1996. The reproduction of anglerfish Lophius piscatorius Linnaeus from the north‐west coast of Scotland. Journal of Fish Biology49, pp.18-39.

Duarte, R., Azevedo, M., Landa, J. and Pereda, P., 2001. Reproduction of anglerfish (Lophius budegassa Spinola and Lophius piscatorius Linnaeus) from the Atlantic Iberian coast. Fisheries Research51(2-3), pp.349-361.

Dubin, A., Jørgensen, T.E., Moum, T., Johansen, S.D. and Jakt, L.M., 2019. Complete loss of the MHC II pathway in an anglerfish, Lophius piscatorius. Biology letters15(10), p.20190594.

Swann, J.B., Holland, S.J., Petersen, M., Pietsch, T.W. and Boehm, T., 2020. The immunogenetics of sexual parasitism. Science.

Young, R.E., 1983. Oceanic bioluminescence: an overview of general functions. Bulletin of Marine Science33(4), pp.829-845.

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