A clasper is a specialized anatomical structure that serves as the male reproductive organ in cartilaginous fishes (Chondrichthyes), which includes sharks, rays, and chimaeras. This distinct feature facilitates internal fertilization, a reproductive strategy that sets these fish apart from most bony fish species. They are elongated, tube-like appendages absent in females, making them the most reliable external characteristic for determining a shark’s sex.
Identification and Location
Claspers are always found as a pair, extending from the inner margin of each pelvic fin on the male’s underside, near the cloaca. When not in use, the two claspers lie flat against the body, making them a discreet but noticeable feature.
These structures are composed of cartilage, not bone, which allows them to be both flexible and sturdy for mating. This cartilaginous composition is consistent with the skeletons of all Chondrichthyes. Claspers are modified fins, meaning they are not homologous to the mammalian penis, which developed from different embryonic tissue.
Role in Internal Fertilization
The primary function of the clasper is to act as an intromittent organ, ensuring the transfer of sperm directly into the female’s reproductive tract. Cartilaginous fishes rely entirely on internal fertilization, unlike most bony fish that release eggs and sperm into the water column.
During copulation, the male maneuvers alongside the female, often grasping a pectoral fin with his teeth to hold position. This behavior is why female sharks sometimes have thicker skin than males, serving as a defense against these “love bites.” The male inserts only one of his two claspers into the female’s cloaca, the common opening for the digestive, urinary, and reproductive systems.
The inserted clasper anchors the male to the female and provides a channel for sperm delivery. Anchoring is important because mating often occurs in the dynamic marine environment.
Specialized Anatomical Structures
The clasper’s effectiveness is due to specialized anatomical structures that enable the precise delivery of sperm. Running along the length of the clasper is the clasper groove, a channel that receives sperm from the male’s urogenital papilla. This groove acts as a conduit for the sperm to travel to the female’s cloaca.
Propulsion is provided by the paired siphon sacs, muscular structures located at the base of the pelvic fins. Before copulation, the male allows seawater to enter the siphon sac through an opening called the apopyle. The siphon sac then forcefully contracts during mating, expelling the stored seawater to flush the sperm, often packaged in bundles called spermatophores, along the clasper groove and into the female.
To maintain a secure connection, the tips of the claspers are often equipped with anchoring features. These features, such as hooks, spines, or barbs, unfold once inserted. This anchoring mechanism prevents the clasper from being dislodged, allowing the siphon sac to complete the transfer of genetic material.