Of all the reproductive strategies that have evolved over time, the mating habits of the Ceratoid Anglerfish are by far the most bizarre to have developed. If the Twilight series were to be crossed with Fifty Shades of Grey, we would expect to see something like what many of these fish do everyday: sexual parasitism. As to the question of why many of these fish evolved into such a strange way of copulating, it seems as though to be driven by the pressure of finding a mate in the deep dark sea. If you rarely can a partner in the world, you better make sure that when you do find one, they can never get away.
Anglerfish live so far down in the ocean that it is a challenge for researchers to study them. There have been few occasions for the fish to be filmed. The techniques used to view most species in-situ is so costly that many researchers cannot afford to do them, so they stick with traditional methods such as trawling (Luck and Pietsch 2008). Often, most captured specimens cannot survive the ascent to the surface or they will die shortly after arrival at the surface (Luck and Pietsch 2008). Although it very difficult to study the behavioral characteristics, researchers have discovered much about the fish with the evidence they can gather.
So how do they ‘do it?’ In the case of the obligate sexual parasitism, males will find a female and then bite into her. He then holds on to her with his jaws long enough until their two bodies fuse together. The female’s body basically consumes the male, with only things of him remaining are his circulatory system and his reproductive organs; all of his other organs degenerate (Pietsch 1975). The exact mechanism as to how the two different epidermal tissues fuse together is still unknown.
There are two other types of sexual parasitism in Ceratioids; the males have a temporary non-parasitic attachment or they are facultative parasites. These types do not share all of the same characteristics of obligate parasitism such as undeveloped mechanisms for digestion. Facultative parasitism has only been observed in two families of the Ceratioidei: Caulophryne and Bertella. It was assumed that all Caratioidei were obligate parasites until a pregnant female was discovered that was un-parasitized (Pietsch 2005). This discovery led to two possibilities: either the female could reproduce asexually or there was chance that males could detach themselves after handling their business; facultative attachment.
|Male attached to female host anglerfish, Cryptopsurus. Photo by T. Pietsch (2005)|
In species with facultative relationships, fertilization can either occur during a temporary or permanent attachment, both options are viable. “Males of facultative forms probably attach to females whenever the two meet regardless of sexual readiness. If both partners are in a state of readiness at the time of attachment, spawning and fertilization take place, after which the male releases his hold on the female and is then presumably capable of beginning a new search for another mate” (Pietsch 2005). If partners aren’t ready to begin spawning, the males will remain attached until the time is right, and the longer that a male remains attached, the greater his chances of a permanent attachment (Pietsch 2005). This is believed because to be the case because females have been sighted fully developed sexually without males attached, leading researchers to believe that that sexual development is not a factor of attachment as with obligate parasitism.
The last method of sexual reproduction is non-parasitic temporary attachment. In this method, males do not need to fuse with the females at all, they only chomp down for a short ride. The collections around the world are devoid of examples of temporary sexual couplings because it is believed that males will detach themselves under the stress of fishing gears. The only known examples are two pairs of Melanocetus, which are firmly attached without a fusion of the epidermis (Pietsch 2005). These males are free to float around to other ladies they may find, chomping down whenever they smell something good in the water. I would propose that temporary attachment to females would increase the chances of survival for the species because it would allow for mating to be more selective.
It was originally believed that males locate the females through olfactory cues and visual stimulus. This idea is supported by the presence of well-developed eyes and nostrils in the free-living males. “According to this theory, the males use their large and sensitive nostrils and eyes to pick up upon conspecific female olfactory and visual cues such as pheromones and bioluminescent lures” (Carley et al. 2010). The eyes of the males were driven to develop to due the dark conditions and the olfactory system was refined to search the open waters. Not every species of has the same capabilities of sight and sense; some species with underdeveloped nostrils have well-developed eyes, or vice versa (Pietsch 2005). So it is believed that males either use sight or smell depending on species, but not both.
In the species with obligate sexual parasitism, both the males and females do not develop sexually until attachment has occurred; well-developed sexual organs have never been seen in un-parasitized females and free-living males. It seems that males never mature sexually unless they have become attached to a female, and, likewise, females will not produce eggs until stimulated by the male parasitic attachment (Parr 1930). Unlike most species of fish where sexual development is a factor of size or age, sexual development in obligate types is brought on by the parasitic attachment (Pietsch 2005).
Another strange characteristic of free-living males of the obligate type is the lack of jaws and digestive tracts capable of digesting food. Males that do not find a female to attach to will not only fail to reproduce, but will also die of starvation within the first few months of their lives (Pietsch 2005). The only purpose of the male’s life is to find a female and reproduce. Males have evolved to simply serve as sperm donors to the females and are good for nothing else, not a completely bad life.
How did such a strange love-making strategy start? It was originally believed that Ceratioid were monophyletic based off of morphological-based phylogenies. This was based on characteristics that have been highly conserved such as extreme sexual dimorphism, loss of pelvic fins, repositioning of pectoral fins, reduced density due to loss of bony parts, and infusion of lipids throughout the body (Carley et al. 2010). However, when more molecular data became available, it showed that the phylogeny is most likely paraphyletic, indicating that obligate parasitism has evolved more than once and then lost by other species. “The maximum likelihood molecular reconstruction of the phylogenetic relationships also suggests that temporary attachment to females was a precursor to facultative and obligate parasitism, rather than a reduction of an ancestral parasitic behavior” (Carley et al. 2010).
In a 1926 paper about sexual parasitism in anglerfish, the discoverer of sexual parasitism, C.T. Regan, almost assumed correctly about the suborder when he stated, “The reason why the Ceratioid, alone amongst Vertebrates, have males of this kind is evident. They are necessarily few in numbers in comparison with the more active fishes on which they prey, and they lead a solitary life, floating about in the darkness of the middle depths of the ocean. Under such circumstances 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, soon after they are hatched, when they are relatively numerous, attaching themselves to the females, if they are fortunate enough to meet them, and remaining attached throughout life.” The pressure of living in a place where life is challenging has caused the amazing relationship developed by Ceratioid anglerfish. The anglerfish provide another example of the majesty in the survival of the fittest, that life will stretch the realm of possibility in order to continue living into the future.
Carley, L., Ellington, J., Turvey, M. 2010. Ceratioid anglerfish mating:Sexual dimorphism and parasitism.
Luck, G. T., and T. W. Pietsch. 2008. In-situ observation of a deep-sea ceratioid anglerfish of the genus Oneirodes (Lophiiformers: Oneirodidae). Copeia, 2008 (2): 446-451.
Parr, A. 1930. On the probable identity, life-history and anatomy of the free-living and attached males of the ceratioid Fishes. Copeia, 1930 (4):129-135
Pietsch, T. W. 1975. Precocious sexual parasitism in the deep sea ceratioid anglerfish, Cryptosaras couesi Gill. Nature 256:38-40.
Pietsch, T. W. 2005. Dimorphism, parasitism, and sex revisited: modes of reproduction among deep-sea ceratioid anglerfishes (Teleostei: Lophiiformes). Ichthyological Research 52: 207-236.
Regan, C.T. 1926. The pediculate fishes of the suborder Ceratioidea. Dana Oceanographic Report 2:1–45