Mating and Reproduction
A shark’s existence revolves mostly around eating, avoiding being eaten, and making more sharks. Adult sharks get together to mate at specific times of the year.
For female tiger sharks, that time comes once every three years. In Hawaii, mating occurs in January. Sperm is stored until ovulation takes place from May to July. Pups are then born 15-16 months later, peaking in September and October of the following year.
Some other pelagic (open ocean) species, such as silkies, oceanic whitetips, and threshers, reproduce year round. Most coastal sharks give birth in the summer.
It is thought that female sharks make it easy for the males to find them by giving out chemical signals, or pheromones, when they are ready to mate. Then the sharks engage in complex behavior as the male attempts to internally fertilize the female’s eggs. Often, the male must bite the female’s back, flanks and fins, sometimes inflicting serious looking wounds, in order to get into a position to mate. For this reason, some female sharks have skin nearly twice as thick as males.
Near the end of this courtship ritual, the male usually bites the female’s pectoral fin, rolls on his side, and inserts a single clasper into her cloaca. All male sharks have claspers, extensions of their pelvic fins used to transfer sperm. Once inserted, the male expels sperm into the female along with seawater, and the eggs are fertilized inside the female’s body.
Some sharks are egg-laying, while others are live-bearing. Egg-laying sharks are known as oviparous, and the reproductive strategy is called ovipary. The only example in Hawaiian waters is the deep-water spongehead catshark.
All other Hawaiian sharks give birth to live pups, but there are two variations on this approach, depending on what the developing embryos use as a food source. In some sharks there is a connection between the embryos and the uterine wall, so the young are nourished in a manner similar to that of placental mammals. This reproductive strategy is called placental vivipary.
In most live-bearing sharks there is no connection between the embryo and the uterus. The young get their nutrition from a yolk sac. This strategy is called aplacental vivipary. In some sharks, particularly the lamnids (threshers, makos, whites, and others), things can get very interesting once the yolk sac is used up. The developing pups may eat unfertilized eggs released by the mother to feed them. Threshers and a few other species even eat younger embryos, their own siblings, in what is sometimes known as “intrauterine cannibalism.”
It’s not known how long it takes a spongehead catshark’s eggs to hatch. The eggs of most oviparous sharks hatch in about six to 12 months, but it’s 27 months for the brown catshark, another deep-sea catshark. For live-bearing species, the gestation period ranges from nine to at least 22 months. Once born (or hatched), shark pups receive no parental care; they’re on their own. They emerge as small versions of their parents and start to feed immediately.