So let’s talk about the birds and the bees. Or would that be the shrimps and the inverts? Let’s talk about shark sex! Elasmobranchs are absolutely incredible when it comes to sex. As I mentioned before, there are over 450 species of sharks and over 500 species of rays and skates, with so many species, there is a lot of room for variation among species (Parker, 2008). And their methods of reproduction are no exception. Sharks and rays actually have 4 different methods of reproducing! Wow! Let’s start with the basics: Anatomy
Shark Reproductive Anatomy
All species of elasmobranchs reproduce and fertilize their eggs through internal fertilization (Skomal, 2016). That means that all males have external anatomy specially suited for penetrating the female to transfer sperm and fertilize eggs. In elasmobranchs, the males have modified pelvic fins called claspers, if you’ve ever seen what look like a pair of legs sticking out just before a shark’s tail, that is a male shark. That’s right ladies, I said pair, sharks get two penises- although only one is used at a time in mating. Males have a pair of internal testes that produce a sperm packet called a spermatophore that travel through genital ducts to the claspers during mating (Skomal, 2016).
Because sharks do not have hands, males often bite onto the females over the pectoral fins or gills to stabilize the female during intercourse (Parker, 2008). This often leaves females with wicked looking mating scars over their sides. In some species the females actually develop thicker skin to help protect against the male’s sharp teeth (Skomal, 2016)! Once the male latches onto the female, he twists himself around her for penetration.
Once mating has occur, it’s all up to the female. Eggs are produced in the ovary and transported down the oviduct to the shell gland. The shell gland will envelop the egg in either a membrane or a shell, depending on the species. Inside the shell gland the egg will also be fertilized and sperm can also be stored (Skomal, 2016). The egg then travels down to the uterus and here’s where the craziness begins!
The Types of Elasmobranch Reproduction
As I mentioned before, sharks and rays can go through 4 different types of reproductive methods. Their basic anatomy is the same, however once the fertilized egg is in the womb, they can go through several different variations.
Some shark species, like horn sharks (Heterodontidae), wobbegongs (Orectolobidae), and catsharks (Scyliorhinidae), lay eggs (Skomal, 2016). These egg cases come in a variety of shapes, sizes, and colors. They are all laid along the bottom of the ocean floor, usually in rocky corals, kelp beds, or sea grass beds, where the egg cases, often referred to as mermaid purses, can attached for several months to a year while the young sharks incubate (Skomal, 2016; Parker, 2008). Once the mermaid purse is laid, the mother shark provides no further maternal care for her young.
All other sharks are vivparous, meaning they give birth to fully developed pups. These mothers incubate their young within their womb from a few months up to 2 years depending on the species! Sharks which develop their young within the womb through a direct placental connection from the pup to the mother are true vivparous sharks. Lemon sharks (Negaprion brevirostris) give birth to their pups with the pups still attached to mom. As the pups swim away, the cord breaks and the pups are ready to begin hunting. About 10% of shark species are truly vivparous. The rest fall into our next category of reproduction: ovoviviparous.
Discovery (30 January 2008). Shark Week 2007 – Lemon Shark Gives Birth. Retrieved from https://www.youtube.com/watch?v=LfQgRCg1bNA
The other 90% of vivparous sharks have what is called aplacental- or no placenta- birth, where there is no attachment between the mom and pup while in the womb. Instead the pups are nourished by a yolk sac during their development. This is sometimes referred to as aplacental yolk vivparity, or Ovoviviparity.
To provide extra sustenance for the growing embryos, some species of sharks like makos (Isurus oxyrinchus) and great whites (Carcharodon carcharias) actually produce additional eggs which those growing baby sharks chow down on! This process is called oophagy (Skomal, 2016). There is one species that takes this process to a frightening extreme. In the womb, sand tiger sharks (Carcharias taurus) have been known to eat their brothers and sisters! The largest of the embryos actually consumes the other smaller embryos in a nightmarish scenario called intrauterine cannibalism or intra meaning inside and uterine referring to the uterus, so cannibalism inside the uterus (Skomal, 2016). It is also sometimes referred to as embryophagy, the consuming of embryos. How’s that for a nightmare moms?!
Megabeeach [Megabeeach]. (13 December 2008). Shark eats siblings in womb! Retrieved from https://www.youtube.com/watch?v=ZrqgPjZ07Ts
Parathenogenesis (Asexual) Reproduction
So elasmobranchs lay eggs, they give birth to live pups by way to placental or aplacental birth. What could possibly be left?! Did you know they are capable of asexual reproduction? That’s right! Parathenogenesis has been documented in several species of elasmobranchs over the last ten years (Dudgeon, Coulton, Bone, Ovenden, & Thomas, 2017; Harmon, Kamerman, Corwin, & Sellas, 2016;Chapman, Firchau, & Shivji, 2008; Chapman et al., 2007). Parthenogenesis is a type of asexual reproduction where the offspring are produced from an unfertilized egg (Oxford University Press, 2001). It has been documented in fish, amphibians, reptiles, birds and even plants, however it has never been documented in mammals. The first case of parathenogenesis in sharks came ten years ago in 2007 when Chapman et al. confirmed it in the smallest species of hammerheads, the bonnethead (Sphyrna tiburo). Since then parathenogenesis has been documented in:
- Blacktip Sharks (Carcharhinus limbatus)
- Spotted Eagle Rays (Aetobatus narinari)
- Zebra Sharks (Stegostoma fasciatum)
- Bonnethead Sharks (Sphyrna tiburo)
So there you have it! The wonderfully captivating world of shark sex! It’s amazing what 450 million years of evolution will produce. Just imagine what will come about in another 450 million years! Remember you can make a difference for marine ecosystems by calling your Congress man or woman and tell them that you care about the quality of our waters! Sharks, rays, and other marine organisms cannot speak or be represented in Congress. They need your voice. Get involved and stand up for sharks.
Featured Image Source
Discovery News: How Do Sharks Have Sex (2015). Censored [Screen capture]. Retrieved from https://i.ytimg.com/
Chapman, D. D., Firchau, B., & Shivji, M. S. (2008). Parthenogenesis in a large-bodied requiem shark, the blacktip Carcharhinus limbatus. Journal of Fish Biology, 73(6), 1473–1477. http://doi.org/10.1111/j.1095-8649.2008.02018.x
Chapman, D. D., Shivji, M. S., Louis, E., Sommer, J., Fletcher, H., & Prodöhl, P. A. (2007). Virgin birth in a hammerhead shark. Biology Letters, 3(4), 425–427. http://doi.org/10.1098/rsbl.2007.0189
Dudgeon, C. L., Coulton, L., Bone, R., Ovenden, J. R., & Thomas, S. (2017). Switch from sexual to parthenogenetic reproduction in a zebra shark. Scientific Reports, 7, 40537. http://doi.org/10.1038/srep40537
Harmon, T. S., Kamerman, T. Y., Corwin, A. L., & Sellas, A. B. (2016). Consecutive parthenogenetic births in a spotted eagle ray Aetobatus narinari. Journal of Fish Biology, 88(2), 741–745. http://doi.org/10.1111/jfb.12819
Oxford University Press. (2001). Parthenogenesis. Retrieved July 03, 2017, from http://www.encyclopedia.com/science-and-technology/biology-and-genetics/biology-general/parthenogenesis
Parker, S. (2008). The encyclopedia of sharks (2nd ed.). Buffalo, NY: Firefly Books Ltd.
Skomal, G. (2016). The shark handbook: The essential guide for understanding the sharks of the world(2nd ed.). New York, NY: Sterling Publishing Co Inc.