Taking another Angler on Reproduction
When you think of an Anglerfish, is the first thing that comes into your mind their unique ‘lure’ (extension of the first dorsal spine called illicium) with a bioluminescent bulb on the end (esca)? Or perhaps you think of the succulent, meaty monkfish often ordered at a restaurant? One of the most unique adaptations of the Anglerfish, which is found throughout all 162 species of the suborder Ceratioidei, is their sexual dimorphism and sexual parasitism. The deep-sea Ceratioidei are found below depths of 300m and are able to emit light from their esca, which often causes people to overlook their unique mode of reproduction.
Sexual Dimorphism for Ceratioidei
Ceratioid female anglerfish often have relatively large jaws with long, slim, backwards facing teeth. Females are much larger than males, for example Ceratias holboelii (Figure 1) can be up to 60 times the length of males at lengths of approximately 770 mm and about half a million times as heavy as males. For this reason, it is not surprising that the females mature at a much greater size than males (e.g. female Cryptopsaras couesii are attractive to males at just 15 mm). The most distinguishable characteristic of the female is the presence of the illicium which terminates above their heads with an escal photophore.
In contrast, the less numerous male ceratioid anglerfish are dwarfed, with some as little as 6-10 nm in length, such as Photocorynus spiniceps (being the world’s smallest vertebrates) and the largest recorded is Ceratias holboelii at 73 mm in length. As figure 1 shows, all males lack a luring apparatus. However, they possess a set of denticles on both the upper and lower jaws, which grow during metamorphosis. These denticles are created by a fusion of the dermal spines, which are found in front of the teeth. The free-living males own a large, well-developed pair of eyes along with an extensive binocular field and a relatively enormous pair of nostrils. These nostrils are used to ease detection of a conspecific female, who emits a species specific pheromone.
Sexual Parasitism for Ceratioidei
There is very limited research to help us understand this unique mode of reproduction, as ceratioids are rare and often museums are unwilling to give up their specimens for further research. Theodore Pietsch, a leading scientist specialising in the reproductive biology of deep-sea fishes, told Live Science the “problem with the research is the impossibility of keeping specimens alive”. In situ ceratioids are adapted to live at extremely high pressures, so when taken out of their natural environments their physiology is hugely impacted causing mortality.
What is known is that the males use their denticles to securely attach onto the female (shown in Figure 2) followed by a fusion of the epidermal tissues and in some cases a connection of the circulatory system occurs. This leaves the male with complete dependence on the female for nutrients, which is transported through the blood. After the attachment, the male begins to degenerate but remains alive. When the male is fully fused with the female, often its volumes become much greater than that of a free-living male and it serves no other role. The female then becomes a self-fertilising parasitised individual who is able to release both the egg and sperm into the water column for an external fertilisation at her please.
In a study, 2 female Neoceratias spinifer both with a parasitic male attached were examined. In both cases the whole of the flattened dorsal part of the males’ head were fused with their female hosts. For one of the females, the male was also attached by a fold of the female’s skin which protruded into the mouth of the male and extended to the back of the throat.
Another study found that in 2 families of Ceratioidei (Ceratiidae and Linophrynidae), free-living parasitic males and parasitised females have under-developed gonads, suggesting they are only able to mature when they are in parasitic association with the opposite sex. It was discovered that, for some ceratioids, within the first few months of its life, if the male is unable to find a conspecific female it dies. This was supported by finding poor development of the alimentary canal and jaws which were unsuitable for predation. For this reason, it was concluded that the males sole purpose in life is to fertilise a female. This need to for sexual parasitism is known as obligatory parasitism.
The most common of all the ceratioids that have been examined are of the Ceratias species. The same study reported out of over 300 metamorphosed Ceratias females who had been collectively examined only 11.7% had a parasitic male attached. For Cryptopsaras couesii, 6.2% of 600 metamorphosed females were parasitised. If these statistics are representable of the overall population, there are a remarkably low number of mature females suggesting most Ceratias females (90%) live their lives as companionless non-reproducing individuals. If these statistics are true, how are such unique fish not extinct? Only future research will find the answer.