By Hannah Medd

Sharks have been around for a long time, making them evolutionarily successful. One characteristic which has allowed them to survive major extinction episodes is the fact the group has evolved a range of reproductive adaptations. Variety in brood size, ovarian cycles, gestation periods, mating systems, and the differences among nursery habitats have given sharks many opportunities to endure changes in their environment.

Modes of Reproduction

A primitive mode of reproduction is to produce many eggs and sperm and shed them into the water, common in bony fish. In this method, the embryos have little yolk, hatch as larvae and require much time to develop, leaving them vulnerable to predation and environmental factors. Obviously, this mode suffers high mortality. All sharks have developed internal fertilization as a mode of reproduction. This approach to reproduction results in fewer numbers of large young that spend the vulnerable portion of development inside the relatively safe refuge of mom's uterus until they are born ready to evade predators and competitors, enhancing their chance of survival.

Sharks deposit eggs through oviparity or give birth to live young through vivparity. Their fecundity, potential reproductive ability, ranges from 1-2 a year to a possible 300 produced by whale sharks. Studied done on tope and scalloped hammerhead sharks show that fecundity increases with overall body length of the female shark but may not be a clear indicator. Shark embryos receive nutrients from several sources. Embryos can be nourished by solely the yolk, a form called lecithotrophic, or from the yolk and maternal nutrients, called matrotrophic. Gaining nutrients from mom is advantageous, giving the juvenile the opportunity to increase in size at birth therefore increasing survivorship.

The oviparous form of reproduction, egg-laying, is considered a primitive form and common for benthic, littoral, and bathyal sharks, about 40% of shark species. Tough egg cases are laid on the substrate or attached to structures on the sea floor. The embryos within the cases are nourished by the yolk sac but those nutrients are limited so the young emerges small. Incubation in the case can last from a few months to more than a year. Oxygenation and ventilation occurs through the slits in each side of the egg case as the embryo constantly fans its tail, increasing water flow. All eggs are laid in pairs and the development of the embryo is likely determined by the ambient water temperature. There is some evidence that the substrate upon which the eggs are laid is somewhat chosen by the mother. Bullhead sharks have been observed picking up egg cases and wedging them into secure positions in the substrate.

Viviparity is the mode of reproduction when the embryos remain in the uterus for all development. The embryo's nourishment can come from the yolk or the yolk can be supplemented by a connection to the mom. When there is no placental connection between mom and embryo but the embryo remains within the uterus for development, it is called aplacental viviparity, also referred to as ovoviviparity. This type of development has three forms dependent on how the embryo is nourished. The embryo can be dependent on yolk only, nourished by other eggs in the uterus, or nourished through placental analogues.

Yolk dependency is found in Squaliformes, Hexanchiformes, Squantiformes, some Orectolobiformes and some Carcharhiniformes. At ovulation, yolk is deposited in the egg and that is the only amount the embryo will receive so they are typically small at birth.

Oophagy, a form of nutrition found in bigeye thresher, pelagic thresher, shortfin mako, and porbeagle sharks, evolved early in cartilaginous fish and allows the ovary to grow very large, over five kilograms, but the eggs are small, 5-7 mm in diameter. Most of the eggs that exist are for the nourishment of the developing embryos that rely on the yolk for only a short time period. The embryos, at about 5 cm, begin to ingest the other eggs by using temporary tooth structures. Only a few fertilized eggs are produced early on in gestation, like the pelagic thresher that has only one per oviduct, with the exception of the sandtiger that has about 12. The ‘feeding egg cases' are produced by the female after the fertilized eggs.

Intrauterine cannibalism occurs when the embryo reaches about 10-12 cm, seeks out other embryos, kills them by biting, grows larger and ingests the long dead embryo and feeding egg cases. The cannibal embryo ingests so much that it has a large, protruding yolk stomach. All of the yolk consumed means these embryos are born relatively large, like the sandtigers' pups that are sometimes larger than one meter at birth, about one third the adult size.

Rays commonly demonstrate a method of nutrition delivery to their embryos by creating a placental analogue, something that acts like placenta. A part of the uterine lining secretes a nutritive substance called embryotrophe, which is ingested by the embryo.
Placental viviparity refers to the method of nourishment that involves the yolk in the yolk sac and occurs only in the Carcharhiniforme order of sharks. Within that order, placental viviparous species may share a family or genus with other species that are aplacental viviparous. For example, the genus Mustelus has aplacental viviparous species such as the spotted estuary smooth-hound, gummy and starspotted smouth-hound sharks as well as placental viviparous species like dusky smooth-hound, and spotless smooth-hound. The embryo ingests the yolk for the first few weeks and as it runs out, the sac gets long and thin, one side becomes vascularized with blood vessels and grows together with the uterine wall, creating a yolk sac placenta. All the nutrients in the mom's blood are then shared through the placenta, a nearly inexhaustible supply of energy. As long as mom's healthy, baby shark will be, too. This method of nourishment has evolved independently 11-20 different times within the Elasmobranch group, resulting in the diversity of the structure.

Reproductive Anatomy

Male Anatomy
The male reproductive system is complicated and involves many body structures: testes, genital ducts (i.e. efferent ducts, epididymides, ampullae epididymides), urogenital papilla, siphon sacs, and claspers. The testes, like in humans, are paired and symmetrical. They are located at the on top of the liver and are suspended by a fibrous sheath called the mesorchinum. Some species have embedded testes at the front end of the epigonal organ which is part of the shark's immune system. Immature testes can be difficult to identify as they are only a mass of white tissue or a faint streak on the surface of the epigonal organ but adult testes are conspicuous and change in throughout the year. Testes are also involved in the creation and secretion of some steroid hormones. Spermatogenisis, when the immature cells become mature sperm cells, also takes place within the testes.

Elasmobranchs can have three different types of testes; radial, diametric, and compound testes. Radial testes, found in basking and lamniform sharks, are enclosed in the epigonal organ and the cells that will eventually become sperm begin development at the center of the lobe and proceed to the outer edge for further transport. Requim sharks of the family Carcharhinidae and hammerheads have diametric testes that protrude from the surface of the epigonal organ and the developing cells transport from wall to wall. Compound testes are typically found in most batoids and shows a little of both the previous arrangements.

The spermatocyst, the functional unit of shark testis, is a spherical form that contains spermatoblasts, which contain Sertoli cells, cells that nurture the developing sperm. The spermatocyst bursts and the Sertoli cells break up, releasing the sperm cells into the ductus deferns for storage.

Claspers are paired tube-like copulatory organs that are formed from the median edge of the pelvic fins and serve to transport the sperm from male to female. Immature claspers are small and flexible but mature claspers go through some calcification that hardens them and display some articulation with the pelvic fin base. In many studies, it is imperative to identify the life stage a shark is in and maturity is determined by the calcification and rigidity of the clasper and whether or not the rhipidoin, the end of the clasper that contains the spur, can open.

Copulation occurs when the clasper is inserted and transfers the sperm. During copulation, the male determines which clasper to use by the way he has grasped the female. If he grips the females' right pectoral fin, he'll use the right clasper. Most copulatory studies indicate that sharks use only one clasper during mating but there is some evidence that species like the small spotted catshark may use both! During mating the clasper is rotated forward and inserted into the female reproductive tract, held in place by a sharp spur. When the clasper is initially positioned forward, a new channel is formed between the one end of the clasper and a tube called the urogenital papillae of the male. The siphon sacs, paired muscular bladders on the bottom side of the male's body, contract and force sperm from the cloaca to the claspers and into the female by a current of seawater. This flushing action created by the sacs may also serve to clear any other male gametes from the vicinity. Sperm travels as spermatophores, either round, ovoid, or tubular matrices that have a large number of sperm or sperm packets within them. Spermozeugma, globs of sperm, is embedded in the matrix of the spermatophores but are not encapsulated. C. carcharias and C. taurus have relatively large spermozeugma at about 10mm and 300 mm respectively. These are temporary structures that vary greatly and little is understood about their function. The Leydig gland is a branched tubular gland connected to the first part of the epididymis and contains cells for protein production, possibly accounting for the high levels of protein found in sharks' seminal fluid.

Female Anatomy
The female reproductive anatomy has many familiar names, like ovaries and oviducts. The ovaries can be either paired or single at the front end of the body on top of the liver and are responsible for the creation of germ cells, accumulation of yolk, and the creation and secretion of hormones. Both ovaries are functional in ancient shark groups, however, only the right ovary works in ‘galeoid" (Scyliorhinus, Carcharhinus, Mustelus and Sphyrus) sharks. More advanced forms of sharks have only a single ovary that is embedded in the front end of a long epigonal organ. An immature ovary is small and looks like a thin strip of granulated tissue but a mature ovary can be very large and bright yellow. Ovaries can either be characterized as being external or internal. External ovaries are compact and produce few large eggs, around 20-60 mm diameter. Internal ovaries, found in lamnid sharks, produces countless small eggs, 3-5 mm diameter, that are fed to the oophagus embryos. In all sharks, oviducts are paired, tubular, and run the length of the body cavity on either side of the vertebral column. They are joined at the front end and terminate at the ostium where they curve into thinner tubes to the shell or oviduct gland (aka the nidamental gland). The shell gland is well developed in mature females, much larger than the oviduct, and functions to secrete the egg membrane. This gland may also play a part in sperm storage and fertilization as it becomes twice as large immediately after fertilization and during egg passage. The uterus is considered the back part of the oviduct where the embryonic shark develops. Typically, embryonic sharks are in contact with each other, however some carcharinid sharks and hammerheads keep their developing young in separate compartments. The vagina is the location of where the two uteri unite, opening up to the cloaca.

Elasmobranch eggs, or ova, are typically large with a lot of yolk which takes a lot of energy to produce so they usually aren't numerous. However, lamnid sharks do produce large numbers of small eggs, as mentioned above, to feed the growing embryos. The eggs are released from the ovary, pass through the ostium into the oviduct, usually two at a time, one into each oviduct. As the eggs pass through the shell gland, they gain a membrane/egg case. Oviparous sharks' shell gland produces a tough case for the long developmental period in the outside environment, ranging from several months to one year. Viviparous sharks' case is thin and sheer. The eggs are telolecithal, the yolk concentrated at one end, because of their large size. A small part of the animal end of the egg is involved in the cleavage process and the other end becomes the yolk sac. Elasmobranch ova cleavage is typically meroblastic, utilizing only part of the egg because of the large amount of yolk.

Sperm storage within the shell gland has been found in several species including the dusky, blue, Atlantic sharpnose, and scalloped hammerhead sharks. The length of possible sperm storage seems to vary among species. The nervous shark has apparently stored sperm up to four weeks, tope and soupfin for five months, whiskery shark for six months, and blue's for 12 months. In one study, chain catsharks isolated from males, produced eggs that hatched normal embryos after 843 days of isolation. Just because of the space provided by the tubular nature of the shell gland makes it a likely spot for sperm storage doesn't mean that it performs that function. Much more research needs to be completed to unravel the mystery of sperm storage.

Reproductive Cycles
For most sharks species, even the simplest descriptions of their reproductive cycles, including the ovarian cycle and gestation period, is lacking in detailed information. Ovarian cycles are the time period that it takes a female to develop a group of oocytes, immature egg cells, and ovulates the eggs. The gestation period is the length of time it takes between fertilization and parturition, otherwise known as birth. Spiny dogfish oocytes develop in the ovary at the same time a brood is growing in the uterus so the ovarian cycle and the gestation period are happening concurrently. In this instance they both last about two years, therefore called a biennial reproductive cycle. Many carcharhinids, like the blacktip and finetooth sharks, run consecutive cycles and periods. A blacktip shark gets pregnant in May, gestate for one year, give birth the following May, experience a resting period to store lipids in her liver and the oocytes grow fast in late winter so that the next May she mates and ovulates again. So, from ovulation to ovulation, the cycle also takes two years. Other sharks have annual cycles, like the Atlantic sharpnose, smooth dogfish, and scalloped hammerheads. These cycles run concurrently, carrying oocytes and embryos at the same time, giving birth and mating again shortly after. It has been hypothesized that some reproductive cycles may be as long as three and a half years.

Mating
Very little is known about the act of mating among sharks. Before copulation occurs, precopulatory activity is a long process of complex mating recognition and a lot of rejection of males. Studies show the female nurse sharks demonstrate selective behavior like refusing, avoidance, arching and shielding. Acceptance behavior in female sharks is demonstrated by flaring and or cupping their pelvic fins. Male's will then show cooperative behavior towards each other by blocking others, considered a helping behavior and reveals a level of social interaction. Because they all use internal fertilization, the precopulatory actions result in the male grasping the female in some manner in order to line up the insertion of the clasper. Some sharks grasp the female pectoral fins, or bite and hold onto the body. Initial courtship bites may show the males' intent and get the female's attention and are pointedly less vicious than feeding bites, demonstrating less force and males don't fully close their jaws. Female blue sharks have skin twice as thick as that of the male blue sharks to mitigate the damage done by mating bites. Nurse and lemon sharks have been observed mating with more than one male, confirmed by blood tests that show different paternity within a brood, a general strategy to maintain genetic diversity.

Conclusions:
The evolution of sharks' reproductive system shows a progression from modified oviparity to vivparity along a few pathways but with great diversity in adaptations to nourish the young, variation of reproductive cycles and small brood size, etc. There is a lot more work to be done to discover about shark reproduction to facilitate conservation and management models that require such specific data. Another critical area for future research is identifying the critical habitat requirements of coastal species so they can be maintained to ensure successful future reproduction.

References:
Conrath, C.L. 2004. Reproductive biology, p.133-164. In: Elasmobranch Fisheries Management Techniques. J.A. Musick and R. Bonfil (eds). IUCN SSG/APEC, Singapore.

Carrier, J. C., Pratt, H.L., and Castro, J.I. 2004. Reproductive biology of elasmobranchs, pp. 269-286. In: Biology of sharks and their relatives. J.C. Carrier, J.A. Musick, and M.R. Heithaus (eds). CRC Press, Boca Raton.

Compagno, L., Dando, M., and Fowler, S. 2005. A Field Guide to the Sharks of the World. pp. vi-368. Harper Collins Publishers Ltd, London.

Other Literature:

Compagno, L. J. V. 1990. Alternative life-history styles of cartilaginous fishes in time and space. Environmental Biology of Fishes. 28:33-75.

Gilbert, P.W., and W. Heath. 1972. The clasper-siphon sac mechanism in Squalus acanthias and Mustelus canis. Comparative Biochemistry and Physiology. 42A: 97-119.

Hamlett, W. C. 1999. Male reproductive system, p. 444-470. In: Sharks, Skates, and Rays: the Biology of Elasmobranch fishes. W.C. Hamlett (ed.). The Johns Hopkins University Press, Baltimore.

Hamlett, W.C. and T.J. Koob. 1999. Female reproductive system, p. 398-433. In: Sharks, Skates, and Rays: the Biology of Elasmobranch fishes. W.C. Hamlett (ed.). The Johns Hopkins University Press, Baltimore.

Maruska, K.P., Cowie, E.G., and TC. Tricas. 1996. Periodic gonadal activity and protracted mating in Elasmobranch fishes. Journal of Experimental Zoology. 276: 219-232.

Parson, G. R. 1981. The reproductive biology of the Atlantic sharpnose shark, Rhizoprionodon terraenovae (Richardson). Fisheries Bulletin. 81:61-73.
Parson, G.R. and Grier, H. J. 1992. Seasonal changes in shark testicular structure and spermatogenesis. Journal of Experimental Zoology. 261: 173-184.

Pratt, H.L. 1979. Reproduction in the blue shark, Prionace glauca. Fisheries Bulletin. 77:445-470.

Pratt, H.L. 1993. The storage of spermatozoa in the oviducal glands of western North Atlantic sharks. Environmental Biology of Fishes. 38: 139-149.

Teshima, K. 1981. Studies on the reproduction of Japanese dogfishes, Mustelus manazo and M. griseus. J. Shimonoseki Univ. Fish. 29:113-199.

Wourms, J. P. 1977. Reproduction and development in chondrichthyan fishes. American Zoology. 17: 379-410.