Every
shark species has its own expected life span and so it is wrong to
set an average for sharks as a whole. However, very broadly
speaking, we can say that most sharks live for between 20 and 40 years. The Spiny
Dogfish lives for over a century, and Whale Sharks are expected to
be similar.
Unlike
most other animals, sharks do not care for their young. As soon as
pups are born into the water, they swim away and care for
themselves. Some sharks give birth to litters of up to 100 pups at
a time, while many species may have as few as two or three. In
almost all cases, all of the pups are healthy and well-developed.
This is in stark contrast to many other animals (such as several
fish species and some turtles), which give birth to large numbers of
ill-developed young (in effect, producing quantity over quality).
This means that sharks are defined as
K-selected reproducers.
In
ecology,
r/K selection theory relates to the
selection of combinations of
traits in an organism that trade off between
quantity, or quality of offspring. The focus upon either increased
quantity of offspring at the expense of individual
parental investment (r selection), or reduced
quantity of offspring with a corresponding increased parental
investment (K selection), is varied to promote success in particular
environments.
Mating
amongst sharks is somewhat elusive, rarely seen by researchers and
scientists. Fertilisation of the female egg occurs internally. The
male’s pelvic fin (the fin on the underside of the body, close to
the tail) has evolved into claspers, a pair of organs that can best
be compared to a penis in mammal species.
Once
fertilised, sharks will carry their young in one of three ways:
Oviparity – Some sharks lay eggs,
protected within a leathery egg case.
Viviparity
– This term is used to
describe the sharks that have a placenta from which to feed and
sustain their young inside. The young are born alive and at full
term.
Ovoviviparity
– The majority of sharks lay eggs inside their body. These hatch and
the young remain inside for some time. Eventually, they are born
alive and fully developed. Some shark species display a form of
cannibalism in that the first pup to hatch eats the other eggs or
pups while still inside its mother. This is called
oophagy.
The
gestation period for ovoviviparous species is not known for sure.
However, it is thought to be anywhere from several months to over
two years.
There
have also been isolated cases of asexual reproduction, meaning that
the female has managed to conceive and birth pups without any input
from a male. The cases have all been observed in sharks in
captivity. The incidence of asexual reproduction in the wild remains
unknown and is believed to be a last resort method of reproduction
in the absence of males.