Zoologger: Flashmob gathering of world's largest fish

Species: Rhincodon typus

Habitat: all tropical and warm temperate seas, except the Mediterranean, being large and enigmatic

On 12 August 2009 a small team of scientists took a light plane over the Caribbean Sea, just off Mexico's Yucatán peninsula. To their astonishment they saw 420 whale sharks swimming together in 18 square kilometres of ocean. It was by far the largest group ever seen (see photo).

Whale sharks are the biggest fish alive, regularly reaching 10 metres long and sometimes growing to 12 metres – and possibly even more. They dwarf the largest bony fish, the ocean sunfish, which does not even reach 2 metres.

But despite their size they remain mysterious. No one even knows how or where they breed.

Whale shark of a time

Alistair Dove of the Georgia Aquarium in Atlanta led the survey that spotted the whale shark flashmob. The team had found similar but smaller gatherings in 2006 and 2008, but seeing more than 400 whale sharks together came as a surprise, because they normally swim alone and only come together in groups of a few dozen.

The animals spent almost all their time together feeding on dense clusters of eggs laid by Atlantic little tunny, a fish related to tuna. The sharks mostly fed at the surface, swimming forwards with their top jaws above the water and sucking in floating tunny eggs. Confronted with particularly rich patches, they stopped and hung almost vertically with their heads up, sucking in huge gulps of water and food.

It's unclear how the sharks found the feeding ground. Rather than detecting it themselves, they might have followed other fish.

Certainly they are no strangers to long-distance travel, often crossing entire oceans. In 2007 70 whale sharks from around the world had their DNA analysed, and the most common genetic marker was found in fish from every quarter. Far from sticking to just one ocean, whale sharks are cosmopolitan.

Getting together

But whereas marine mammals like whales take advantage of gatherings to socialise and find mates, the whale sharks were peculiarly antisocial. "It appears they were not gathering to interact with each other," Dove says, "but were simply converging on a particularly rich food source."

Dove has studied captive whale sharks and says there is little evidence of any social system, such as a dominance hierarchy. But he says different sharks do have identifiable behaviour patterns, which suggests that, like many animals, they have at least the beginnings of personality.

A recent study showed that whale shark brains are relatively small for their body size, suggesting they are fairly dim and don't have complex social lives – social behaviour tends to be correlated with brain size in other animals. However, they also have a fairly large pallium – the part of the brain that, in humans, develops into the cerebral cortex and is the key to our intelligence.

Even if their social lives are simple, they must get together at some point to mate – though no one has ever seen them do it. The females give birth to live young, and one pregnant female nicknamed "megamamma" was found to be carrying 300 embryos.

Dove suspects they mate in the same way as other sharks: the mating pair stop swimming and partially entwine their bodies while the male inserts his genital clasper into the female from the side. As Dove says, "Two adult whale sharks sinking into the depths locked in a breeding embrace must be quite a sight."

Journal reference: PLoS One, DOI: 10.1371/journal.pone.0018994