This is the first part of what I hope will be a series of posts related to shark reproduction. I’m starting with the male reproductive system, mainly because I read about it first. Future posts will focus on the female reproductive system, gametogenesis (i.e. the production of eggs and sperm), and reproductive behaviors. Although this article specifically concerns sharks, there are more similarities between the reproductive systems of sharks and other chondrichthyes (rays and chimaeras) than there are differences. I may make a future post about these differences if I feel like it.
The first question people often ask about an unfamiliar reproductive system is “what is the penis like?” If one wishes to be truly pedantic, sharks have no penises. Intromittent organs (made to transfer sperm into some internal part of the female) have evolved many times over the years, producing a wide variety of appendages related only in purpose. Only those intromittent organs of the mammals receive the honor of being referred to as “true penises”. Instead, the male shark has a pair of claspers. They are modified pelvic fins that penetrate the female’s cloaca, delivering sperm through an internal tube known as the dorsal grove. The main support structure of the clasper is the stem cartilage. It lends the clasper a certain stiffness that most intromittent organs require to attain any level of effectiveness. Claspers may also boast a number of appendages whose purpose is to prevent them from slipping out of the female’s cloaca during copulation. For example, the clasper of the male spiny dogfish, Squalus acanthias, contains four such appendages. The spur has a sharp tip. It rotates perpendicularly to the stem cartilage upon penetration. The claw and ventral terminal cartilage also rotate perpendicularly to the stem cartilage, but in the opposite direction of the spur, forming a “T” shape. The claw is hooked and sharpened at the end. The ventral terminal cartilage contains the distal end of the dorsal grove through which semen is transferred. A fourth appendage is called the rhipidion. It expends, fanlike, with edges anchoring into the female’s cloacal wall, further anchoring the clasper during copulation. There is significant diversity in clasper anatomy. For example, the blue shark, Prionace glauca, forgoes most of the above-listed appendages, retaining only the rhipidion.
Claspers of Carcharhinus brevipinna (from Wikipedia)
In evolutionary terms, the benefits of such complex adaptations are clear. Most fishes practice external fertilization, and ocean fishes tend to practice a particularly wasteful form of external fertilization known as broadcast spawning, wherein clouds upon clouds of gametes are released into the ocean in vague hopes of reproduction. By contrast, the injection of sperm directly into the female reproductive system greatly increases the chances that sperm and egg will find each other. This means that the animals do not have to waste resources producing gametes that will likely never amount to anything. In addition, the young are able to develop to highly advanced stages before leaving the safety of the womb, greatly reducing both early-life predation and starvation.
You probably weren’t wondering about the internal reproductive physiology of male sharks, which is a shame. But this article wouldn’t be complete without it, so here we go:
Sperm originate in the testes, as you may have guessed. Unlike human testes, shark testes contain seminiferous ampullae rather than seminiferous tubules. These burst upon maturation of the sperm cells (similar to the ovarian follicles in humans). The testes are nested in the epigonal organ, whose other primary purpose is to generate blood cells (remember that sharks have no bone marrow). When the seminiferous ampullae release their sperm, the sperm travel out of the testicles through the ductus efferens, a group of tubules that run through a band of connective tissue called the mesorchium. The ductus efferens transports the sperm cells to the epididymis. Some of them are stored there for a while, while others travel down the small and convoluted tubules of the epididymis. These tubules eventually all converge, forming the ductus deferens. Near its end, the ductus deferens enlarges to form the ampulla ductus deferens. Here, the sperm grouped into packages called spermatophores. A papilla from the ampulla ductus deferens projects into a chamber known as the urogenital sinus, which in turn projects into the cloaca. Normally, sperm that make it this far would end up in the open ocean, but during copulation the male lines the base of his clasper up in such a way that sperm are forced into the opening of the dorsal grove (this basal opening is called the apopyle). Sperm are forced out through the contraction of siphon sacs, bladders located just beneath the skin (but above the muscle). These sacs are filled with seawater as the male swims forward, and emptied through muscular action. This forces the sperm into the apopyle of the clasper, simultaneously breaking up the spermatophore’s membranes before they enter the female. Sperm finally exits the male and enters the female’s cloaca through an opening at the tip of the clasper called the hypopyle. An inexperienced male is unlikely to be able to perform all of these actions correctly, but with any luck, the sperm will travel through the series of tubes that form the female’s reproductive system (to be discussed in the future), where some of them will encounter eggs, and a batch of new sharks will be formed.
The reproductive system of Lamna nasus from the National Oceanic and Atmospheric Administration.
If you read through this article you probably noticed some typos, and also have questions. Please take advantage of the comment utility to share your insight.
Some sources I used for this article are:
The Biology of Sharks and Rays, by A. Peter Klimley, University of Chicago Press, 2013
Shark and Ray Reproduction, by some Unknown Author, published online at http://www.sharksavers.org/en/education/biology/shark-and-ray-reproduction/
I also used Google images to figure out what some of these things looked like.
“I'll bet this author doesn't even know how to write a bibliography in APA format!” --someone who is wrong
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