Female whale shark found carrying more than 300 embryos

August 2010: A new study is shedding light on the reproductive behaviour of whale sharks, which seems to suggest that the females will ‘store' sperm, and fertilise their own eggs as they are produced. Remarkably, it was also found that the 300 embryos found in one female whale shark were of different ages, and at different stages of development.

OCEAN GIANT: The whale shark

The study was carried out by biologist Jennifer Schmidt, of University of Illinois at Chicago and reveals new details about the mating habits of this elusive, difficult-to-study fish.

Schmidt determined paternity of 29 frozen embryos saved from a female whale shark caught off the coast of Taiwan in 1995. The embryos, studied in collaboration with Professor Shoou-Jeng Joung at the National Taiwan Ocean University, are extremely rare.

The pregnant shark carried a surprisingly large number of embryos – 304 – which were still in the uterus and represented a spectrum of age and development stages ranging from being still egg-encased to developed, near-term animals.

All embryos had the same father
Schmidt and her colleagues spent several years developing DNA genetic markers to study whale sharks, initially for population genetics, but in this study the tool was used to determine paternity. Shark reproduction is still an emerging science, but what is known suggests that most broods are sired by more than one male. But that is not what Schmidt found with this particular female whale shark.

‘These differently-aged embryos – itself unusual across animal species – had the same father,' Schmidt said. ‘We have to be very cautious in drawing conclusions from a single litter, but the data suggest female whale sharks store sperm after a single mating event, and subsequently fertilise their own eggs as they are produced.'

If the finding can be supported from analysis of other whale shark litters, Schmidt said ‘it would suggest that there is no whale-shark breeding ground where large numbers of animals meet to mate, but rather that mating occurs as an isolated event'.

Whale shark numbers seem to be declining
International protocols protect whale sharks from capture, few are housed in aquariums, and those that are are usually less than 25 years old and not yet sexually mature. Scientists typically study whale sharks at seasonal feeding grounds, but those animals are usually juveniles not mature enough to breed. Rarely are adult females observed in the wild.

‘Protections for whale sharks have increased in many parts of the world, yet shark numbers seem to be declining, and the average size is getting smaller,' said Mark Meekan, principal research scientist with the Australian Institute of Marine Sciences. ‘This is a classic sign of overfishing, where larger, more valuable animals are selectively removed. Targeted fishing of breeding-age animals in a late-maturing species can be devastating for its survival.'