In many ways the reproductive modes of elasmobranchs are more akin to those seen in mammals than in most other fish species. All elasmobranch species practice internal fertilization, which is accomplished with the aid of the claspers. It is important to distinguish that the clasper is only a rolled edge of the pelvic fin, it is not the penis seen in mammals.
Courtship may last for a considerable period before the female allows a suitor to mate with her. During the courtship, the male(s) often swim with its nose close to her cloaca, suggesting that the female may emit some form of chemical communiqué of her oestrous, and bite at her tail. Immediately prior to mating, the male will seize the female’s pectoral fin and rotate his body, allowing him to bend one of his claspers forward and insert it into the female’s cloaca; which clasper is used may depend on which of the ovaries is functional. In some species, only one of the female’s ovaries is receptive – often the right-hand one, according to Rodney Steel in his 1995 book Sharks of the World). The male shark has a small sac located just underneath the skin at the base of each clasper, filled with seawater that’s used to flush the seminal fluid out of the clasper. Indeed, some authors have suggested that these sacs may be used to give the female a contraceptive douche to wash out any competitor sperm. Batoids have a muscular gland at the base of the clasper that serves to force out the sperm.
The sperm fertilizes the mature ova as they travel along the oviduct to the uterus. Once in the uterus, the ova begin their development, and the male’s job is effectively complete. In some species, the female may retain packets of sperm (referred to as spermatophores) in her uterus, and release the sperm at a time that is best suited to her breeding cycle. Multiple paternity is known from sharks and one study of nurse sharks (Ginglymostoma cirratum) living in the Florida Keys identified at least four different fathers among a brood of 32 pups from one mother.
Sharks and rays operate two main modes of reproduction. Some lay eggs (a process called oviparity), encasing their developing young in a tough shell that also contains an egg sac to nourish the developing pup. The shark egg cases are composed of a protein molecule similar to collagen, whilst skate appear to be composed of six major structural proteins. Upon comparison of the structural robustness of skate and catshark eggs, the shark thecas appear considerably more rigid than those of the skate. The egg cases must protect the developing embryo from the corrosive effects of seawater, whilst also staving off attacks by the many predators of the eggs. Elasmobranch thecas are very telolecithal (rich in yolk) and as such represent a delectable source of carbohydrate and protein for a potential predator. David Cox and Thomas Koob have written several papers documenting the predation of elasmobranch eggs by several species, especially buccinids (whelks).
When the yolk sac has been exhausted, the pup breaks out and goes it alone. The length of time the young elasmobranch spends in the egg case varies greatly according to species and water temperature; the smallspotted catshark spends about eight to ten months in its theca (in UK waters), whilst the heterodontids (horn sharks) can spend between five months and a year in the capsule. Some species develop a special tooth or spine that they use to pries their way free of the egg capsule; the tooth then falls out a few days post-hatch. It is typically the bottom-dwelling species, such as the catsharks, wobbegongs, horn sharks, etc. and most batoids, that lay eggs. A few whale shark eggs have been found, but more recent discoveries suggest that these may have been aborted prematurely.
The second mode is that elasmobranchs give birth to live young. The picture is, however, somewhat more complicated than that because there are two methods of live birth. The first is aplacental viviparity (also referred to as ovoviviparity) and involves the pups being retained in the uterus, without a placental connection, until they are sufficiently developed to be able to fend for themselves. There are two forms of aplacental viviparity: yolk sac aplacental viviparity, where the embryo is nourished by the yolk of the ovulated egg; and aplacental viviparity with oophagy, where the developing embryos feed on the stream of unfertilised eggs coming from the ovaries.
In many batoid species, the aplacental viviparity with yolk sac is modified to the extent that embryo absorbs a rich uterine secretion called histotroph. In the sandtiger shark (Carcharias taurus), the phenomenon of oophagy (“egg-eating”) is taken a step further, to a scenario referred to as adelphophagy (“brother eating”). As the name suggests, the largest embryo in each of the two uteri fights and consumes its siblings – hence only a single embryo is usually born from each uterus. Whilst adelphophagy is only known from the sandtiger, oophagy is known from many but suspected in all lamnoids (i.e. great white, porbeagle, basking shark, etc.), is also found in two members of the Psueudotriakidae (a hound shark and a catshark) and also in the tawny nurse shark (Nebrius ferrugineus).
The second method of live birth is one that’s much less common, found in only a handful of species such as lemon sharks and hammerheads, is true viviparity, whereby the developing embryo is nourished via a placental connection in the womb. At parturition (birth), the pups exit the cloaca, pulling against the umbilical cord until it snaps, and they swim away.
It is worth mentioning that these modes of reproduction are not necessarily as well defined as I have made them appear here. In reality, these reproductive traits may blend to the extent that it is possible to differentiate as many as eight variations on these three primary modes.
Whatever the mode of reproduction, most elasmobranchs have a hemimetabolous life cycle. In other words, the pups look like small adults (albeit there may be colouration differences in some species) and the are ready to feed on solid food immediately after birth. Being independent from birth, they receive no parental care, although in some species the female appears to stop feeding shortly prior to giving birth (perhaps a mechanism to reduce the likelihood she’ll eat her pups). There are anecdotal reports of young and adult sharks swimming together, most notably in the basking shark (Cetorhinus maximus), but these are rare. Pups may, however, hang out together for a year or more after birth and some areas, with some areas having been identified as shark nurseries.
Studies on shark life history in nurseries is lacking for most species, but that of the lemon shark has been very well studied by now-retired University of Miami biologist Sam Gruber and his students. Lemon sharks mate during the spring and early summer and, following a 10-12 month gestation, the females move into the shallow waters of the Bimini mangrove swamps in the Bahamas to give birth. The females can give birth to anywhere up to 18 pups, depending on the mother’s body size, which range in size from about 50 to 70cm (20-28 in.). These shallow waters offer relative protection from predators, being too shallow for large sharks to enter, and offer an abundance of food, which helps the pups practice their hunting skills and grow. Indeed, the young sharks will remain in the mangroves for two or three years before venturing out into open water, having reached a length of about 90cm (35 in.) that makes them much less vulnerable to predators.