Each species of shark has its own conventional life span, and it is challenging to set an average for sharks as a whole. However, conventionally, most sharks live for between 18 and 34 years. The Spiny Dogfish lives for over a hundred years and a few Whale Sharks have been identified to do the same.
Let’s explore the life cycle of sharks in today’s NYK Daily Nature Special.
Unlike other bony fish, sharks are identified as K-selected reproducers, meaning that they generate a small amount of well-developed young instead of a large number of defectively developed young. Productivity in sharks ranges from 2 to over 100 young per reproductive cycle. Sharks mature gradually relative to other fishes. For instance, lemon sharks reach sexual maturity at 15.
Sharks perform internal fertilization. The dorsal part of a male shark’s pelvic fins is mutated into a couple of intromittent organs called claspers, similar to a mammalian penis, of which one is used to delivering sperm into the female shark. Mating has seldom been seen in sharks. The smaller catsharks usually mate with the male curling encompassing the female. The two sharks technically swim parallel to each other in less flexible species while the male inserts a clasper into the female’s oviduct.
There have been several recorded cases in which a female shark who has not been in touch with a male has conceived a pup independently through parthenogenesis. This process’s details are not well explained, but genetic fingerprinting showed that the pups had no parental genetic contribution, ruling out sperm accommodation. The scope of this behavior in the wild is unexplored. Mammals are now the only primary vertebrate group in which asexual reproduction has not been seen.
Modern researchers confirm that asexual reproduction in the wild is incredibly rare and perhaps a last-ditch attempt to reproduce when a male is not present.
Most sharks present three ways to bear their young, varying by species, viviparity, oviparity, and ovoviviparity.
Viviparity is the development of young without using a traditional egg and results in a live birth. Viviparity in sharks can be aplacental or placental. Young are born self-sufficient and fully formed.
A few species are oviparous, placing their fertilized eggs in the sea. In most oviparous shark species, an egg case with the texture of leather shields the developing embryo. These cases may be adequately corkscrewed into crevices for extra protection.
Most known sharks are consistently ovoviviparous, which means that the eggs hatch in the oviduct within the female’s body. The egg’s yolk and fluids are secreted by glands in the walls of the oviduct to properly nourish the embryos. The newborn continues to be nourished by the remains of the yolk and the oviduct’s nutritious fluids. As in viviparity, the young are fully functional and are born alive.