Sex is a staple of wildlife documentaries, to the point that the ins and outs of the love lives of a multitude of mammals, birds, reptiles and invertebrates have been revealed in all their intimate detail. And yet the private lives of fish have remained largely just that – private.
There are at least two good reasons for this. First, most fish don’t actually mate, and second, those that do seem to have a knack for doing it when nobody’s pointing a camera at them. Sharks belong to the second category.
How do sharks mate?
Together with closely related skates and rays, sharks form a group called the elasmobranchs, one of the two main lineages of modern fishes – the other being the bony fishes, which include the majority of freshwater and marine species. The two groups went their separate ways about 450 million years ago and differ significantly in terms of both anatomy (elasmobranchs’ skeletons are made of cartilage rather than bone, for example) and reproductive strategies.
Most bony fish go for quantity over quality, producing large numbers of small offspring, each of which are highly unlikely to survive to maturity. With few exceptions, they eject their eggs and sperm straight into the water, leaving fertilisation and the entirety of their progeny’s development to chance.
Sharks and other elasmobranchs, though, focus their efforts on a smaller number of offspring. This starts with fertilisation of the eggs within the body of the female. Males don’t have a penis as such, but evolution has equipped them with something (or, rather, two things) just as effective – elongated extensions of the paired pelvic fins, called claspers, which channel sperm from the male’s genital opening into the female’s.
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Mating itself is rarely seen in sharks, let alone filmed. But it seems to be a lively business that involves the male biting the neck and flanks of the female to hold her in the right position for the insertion of a clasper. The females of some species have extra-thick skin in these vulnerable areas.
Following fertilisation, the developing young are nurtured to independence in various ways. In about 60 per cent of species, including great whites, hammerheads and blue sharks, the embryos develop inside the female until they are born as fully formed and independent pups.
The other 40 per cent of species, including catsharks and bullheads, lay eggs. These leathery structures, commonly known as mermaid’s purses when they are washed up on beaches, are often adorned with tendrils that anchor them to vegetation. The corkscrew-shaped eggs of Port Jackson sharks are designed to twist their way into rock crevices for protection from predators.
Either way, embryonic development is very slow. Eggs can take 11 months to hatch, and gestation lasts up to two years. Combined with their production of small numbers of offspring, this means that sharks are slow to recover from population declines.