The male shark circles, grabs onto the female’s pectoral fin with his mouth and bites down. He keeps a tight grip with his sharp teeth while she remains motionless. He slides in alongside her and curls his body in an arch. The act is complete when he uses one of two claspers to copulate and impregnate the female.
What is a clasper?
Similar to a penis, claspers are an external appendage found on male sharks, skates, and rays that are designed to deliver sperm inside of a female. However, they are dissimilar to a penis in that they are not an independent appendage, but rather a deeply grooved cartilaginous extension of the sharks pelvic fins.
Unlike a majority of fish that release eggs and sperm into the water for external fertilization, sharks fertilize their eggs internally. A major disadvantage of internal fertilization is the limitation on the number of young one can have at one time because growing young internally takes up space. Conversely, a major advantage of internal fertilization is the extended period of parental care that increases long term survival.
In order for female sharks to give birth to live young or fertilized eggs, males must be able to deliver sperm internally to the female. The clasper, just like the penis, is another of evolution’s unique methods for this delivery.
But why are there two claspers?
Male sharks have two claspers because sharks have two pelvic fins. The claspers are simply a modified portion of the pelvic fin, and since there are two pelvic fins, there are two claspers.
Do they use both at the same time?
Interestingly, research suggests that sharks only use one clasper at a time. Shark mating has rarely been observed, but when observed, often involves the clasper on the opposite side of the body from where it has sidled up to the female. Perhaps this allows a better range of motion.
Are there other fish with penis-like appendages?
Another group of fish that are not closely related to sharks have also developed a penis like appendage strictly for the purpose of sperm delivery. Subsequently, the females in this group also give birth to live young. The group (Poeciliidae) as a whole are called live-bearers because of this distinguishing trait. Unlike sharks, the penis-like appendage on these fish is not a part of the pelvic fins. Rather, this appendage, called a gonopodium, is part of the anal fin. Since these fish only have one anal fin, male live-bearers only have one gonopodium instead of two claspers like sharks.