Sharks - Reproduction2013
By Amelia Meyer
By Amelia Meyer
Mating amongst sharks is seldom seen by researchers and scientists. Fertilisation of the female egg occurs internally. This is rare amongst fish in general, as most species release their eggs and sperm into the water, where fertilisation then takes place. The male’s pelvic fin (the fin on the underside of the body, close to the tail) has evolved into claspers on the posterior side. These claspers are a pair of organs that can best be compared to a penis in mammal species.
They are intromittent, meaning that they are external and are designed for transferring sperm into the body of the female. However, although both are fully functioning, the shark will only ever use one clasper at a time.
While the clasper is used as a channel for the sperm, it also anchors the male to the female during mating. In some instances, females have been found to have significant bite marks as a result of the male holding on to her with his teeth.
In addition, some males bite females to indicate their interest in mating with her, a type of courtship ritual.
During mating, the male shark may swim alongside the female. At this point, he inserts one of his claspers into her oviduct and releases sperm in surges, using the surrounding water to pump the sperm into her body efficiently. This sperm then fertilises several of her eggs, and so begins the development of her litter. Some sharks bear only one or two pups (e.g. the Great White Shark), while others bear over 100 (e.g. the Whale Shark and the Blue Shark) pups a season.
After a gestation period (which remains unknown for most species but can extend for up to two years), live pups are born.
alternatively, in the minority of species, eggs are laid and concealed in protective pouches. In both cases, once pups emerge into the water, they are well-developed, have a full set of teeth (essential to their survival) and are independent.
The mother does not nurse them and may, in fact, eat them if they do not leave the vicinity quickly. So, from birth, the pups are required to hunt, travel and mate with no training, protection or support from their mother. This implies that they need to be born in a well-developed, beable state. This is in stark contrast to many other fish species, which hatch or give birth to hundreds of babies that are not well-developed in the hope that at least a few will survive. The killing and survival instinct begins long before the pups emerge from the protection of their mother’s body. Many species have displayed cannibalism in that pups inside their mother have eaten the eggs or embryos of their siblings.
Once born, sharks grow slowly. Sexual maturity is reached at between 15 and 20 years of age in most species. This is another reason that it is vital that, by the time the pups are actually born, they are strong and developed enough to defend themselves and their population numbers.
Conceiving and rearing a litter before they are birthed takes much energy out of the female shark. She needs considerable breaks between litters in order to regain her strength for the next litter, sometimes several years.
These long breaks and the extended time before sexual maturity is reached have negative implications on the survival of many species, particularly when faced with the onslaught of human invasion and hunting. It is vital that legislative and environmental initiatives be implemented in order to preserve and sustain the awe-inspiring shark species that occupy the oceans of today.
For more information, please view: http://www.marinebiodiversity.ca