In a recent study of sharks caught by fisheries in Papua New Guinea (PNG), we measured rates of multiple paternity in grey reef sharks and scalloped hammerheads.

The challenge

Understanding multiple paternity

Some species of sharks use a breeding strategy called multiple paternity, meaning litters of baby sharks may have more than one father and include both full and half siblings.

Multiple paternity is thought to be more common in shark species whose females can store sperm after mating for later use and in species that gather together at breeding time, which increases the number of available mates. It may have advantages for both sexes, increasing genetic diversity within a litter, countering the risks of inbreeding if the population is small and isolated, and increasing mating opportunities for males.

Mating systems in sharks can be location-specific as well as unique to a particular species. The extent to which multiple paternity occurs is not well known and solving this is important for conservation. Like most marine species, sharks can't easily be counted. Instead, population sizes are estimated, usually by using data from fisheries. Knowing the extent that multiple paternity occurs in a shark species or population helps with this estimate.

Our response

Working with shark fisheries in PNG

We measured multiple paternity in two species of sharks that are caught by fisheries in PNG, the grey reef shark, Carcharhinus amblyrhynchos, and the scalloped hammerhead, Sphyrna lewini. The sharks in the study were caught on board commercial fishing vessels in the Bismarck and Solomon Seas.

One of the commercial fishing boats we worked with in Papua New Guinea.  © National Fisheries Authority

We took tissue from 11 pregnant females and their pups and used microsatellite markers, which are short repeating DNA sequences, to determine whether the pups in each litter were full or half siblings.

The results

A better understanding of PNG sharks

We discovered that multiple paternity is common in these two species. All five litters of scalloped hammerheads included half siblings, with an average of 17.2 pups per litter sired by between two and eight different fathers. Four of the five litters of grey reef sharks included half siblings, with a much smaller average litter size of 3.3, fathered by between one and three different males.

Two of the scalloped hammerhead litters had significant paternal skew, meaning that different fathers contributed unevenly to the number of offspring. The cause might be the order in which males mated with the female. But females of some polyandrous sharks, including scalloped hammerheads, are able to store sperm for months or even years after mating, releasing bundles of sperm from their oviducal glands. This could suggest that female sharks are making choices about the use of sperm from different males. Another possible cause is sperm competition, meaning females mate with different males in order to let the sperm fight it out, hopefully resulting in fitter offspring.

We’re now combining our observations of grey reef sharks and scalloped hammerheads with information on size and sex of sharks caught by PNG fisheries to find out if and where populations of these species are at risk of declining.

The sharks in this study were sampled as part of an Australian Centre for International Agricultural Research project led by the National Fisheries Authority of PNG and CSIRO to assess shark and ray catches in PNG’s commercial and artisanal fisheries.

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