Yes... you read that right!... Two! Calm down, boys... don't get intimidated! Sharks have indeed evolved to have two organs which they use during mating, called "claspers". If you ever see a shark when you are diving or at an aquarium, and wonder how scientists tell the difference between male and female sharks, it is actually quite clear, if you know what to look for! So how can you tell male and female sharks apart? Why do sharks have two penises? And what do they look like!?
What is a Clasper?
Shark claspers are quite different to human genitalia. They are similar, in that they are projections that deliver sperm into a female (knowns as "intromittent organs"), but they are not actually an independent appendage, like the human penis. In fact, they are modified, grooved fins. Over many millions of years, the two anal fins which could be found on the sharks' ancestor, have slowly changed and adapted into the sexual organs that we see today... As the fins were paired, so are the claspers (Ebert et al, 2021; O’Shaughnessy et al, 2015).
When the males are born the claspers are comparatively small and soft, but as they grow to maturity, they become larger and increasingly stiff (yes... I am rolling my eyes imagining your laughter as I write this!) because they become "calcified". This means more calcium is laid down within the internal structures of the claspers, to make them more efficient for mating (Ebert et al, 2021; Shadwick et al, 2016).
Clasper calcification can also be useful for scientists, because the size and rigidity of the claspers can be a handy way to identify whether a male shark is "sub-adult" (that is to say, quite large, but not yet sexually mature) or "mature" (of an age that they are able to breed) (Ebert et al, 2021; Shadwick et al, 2016).
What is interesting, is that females sharks also have two uteruses (aka wombs)! Yet, as they only have one opening in the cloaca, this does not mean that the male penetrates each uterus with each of his claspers. In fact, the male only uses one clasper during mating (Ebert et al, 2021).
Every Clasper is Unique!
As if this wasn't weird and wonderful enough, claspers are different across different species... in fact, some sharks' claspers are totally bizarre!
For example, the claspers of the milk shark (Rhizoprionodon acutus) have a chicken-foot shaped tip, which splays at the end, to ensure that the clasper remains within the female during mating. Similarly, the spadenose shark (Scoliodon laticaudus) has an extremely splayed tip to the clasper, like webbed toes (Ebert et al, 2021; Sen et al, 2020).
Claspers were Vital to Shark Evolution
It might all seem quite strange and funny, but the evolution of these organs is actually one of the reasons that sharks have been so successful; allowing them to roam our oceans for millions of years. This is because the evolution of the claspers allows for "internal fertilisation" (when sperm meets egg inside the female). In other species of fish, fertilisation is external; with males spraying their sperm into the water over eggs which the female has already laid (known as "spawning"). The strategy of mating and fertilising the egg internally, allows the eggs a higher chance of survival and means that sharks can invest heavily in just a few, well-nourished offspring, which are more likely to survive to adulthood (Ebert et al, 2021).
Isn't evolution amazing!
References
Ebert DA, Dando M& Fowler S (2021). Sharks of the World: A Complete Guide, Second Edition. Princeton University Press: UK. IBAN: 978-0-691-20599-1.
O’Shaughnessy KL, Dahn RD & Cohn MJ (2015). Molecular development of chondrichthyan claspers and the evolution of copulatory organs. Nature Communications, 6:6698. Access online.
Sen S, Kizhakudanb SJ, Zachariac PU & Dasha G (2020).Reproductive adaptation: a description of claspers of the spadenose shark and milk shark from Gujarat. Indian Journal of Geo Marine Sciences, 49:07, 1238-1241. Access Online.
Shadwick RE, Farrell AP & Brauner CJ (2016). Physiology of Elasmobranch Fishes: Structure and Interaction with Environment. 34A Elsevier.
By Sophie A. Maycock for SharkSpeak
