Sharks - Ovoviviparity2013
By Amelia Meyer
By Amelia Meyer
The majority of shark species are ovoviviparous. In basic terminology, this means that, after fertilisation, eggs are formed and hatched inside the mother before live young are born into the surrounding water.
During mating, the male shark inserts the clasper (similar to the mammalian penis) into the female’s oviduct and releases sperm. This sperm then fertilises several of her eggs. At this point, some sharks either lay their eggs (known as oviparity), nourish these young pups off a placenta (called viviparity) or, as is the case with ovoviviparous species, hatch the eggs within the oviduct before giving birth at a much later stage.
Ovoviviparous pups are sustained and fed by the unfertilised egg yolk as well as by special secretions by the glands in the oviduct walls. Once the pups have developed sufficiently inside their eggs, they hatch into the oviduct. This usually occurs after approximately three months. They continue to feed off the remaining egg yolk as well as the gland secretions.
For additional protection, some sharks push these cases into rock crevices using a corkscrew method. The leathery exterior is hardy enough to handle the pressure as well as the wear and tear, while the eggs are nestled safely inside. This makes it almost impossible for other fish to access or penetrate the eggs’ cases. These cases often wash up on beaches and shores and appear black and elongated.
The eggs are filled with a particularly nutritious yolk, ideal for the development and sustenance of beable shark pups. Other fish eggs do not have this yolk, which plays a role in the resulting poorly-developed babies, many of which die before maturing. In contrast, shark pups are hatched strong, beable and independent of their mother’s care and nursing. It is not known exactly how long it takes for the pups to develop to the point of hatching, but it is certainly somewhat extended to allow for sufficient maturation. Once hatched, the pups will each go their own direction and will begin their life of hunting, mating and (in some cases) migrating.
Well known oviparous shark species include the Horn Shark, Port Jackson Shark, Zebra Shark and Bullhead Shark.
In some shark species, the first pup to hatch will eat the other eggs in the oviduct before they have the opportunity to hatch. This is referred to as intrauterine cannibalism. The Grey Nurse Shark pup will even eat the other embryos of its siblings. This establishes a lifetime of hunting and displays the strong survival instincts present in these beautiful killing machines.
Being ovoviviparous means that the pups are able to develop better and for longer before being born into the ocean, where
they will have to fend for themselves from the moment that they leave their mother’s oviduct. It is also for the pups’ protection that these species most often give birth in sheltered bays or river mouths. These areas are usually free from other sharks and dangerous predators and have more abundantly available food sources than the open ocean.
Dogfish Sharks, Angelsharks and Mako Sharks are common examples of ovoviviparous fish.
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