How do fish know if they are swimming in the right school?
While fish have big eyes to help them find prey and keep track of each other up close, they rely on their chemosensory system to track other fish of the same species in the vastness of the ocean, says Dr Ashley Ward, a fish biologist at the University of Sydney.
"A fish can smell itself, and recognises others with the same smell," says Ward, who studies the social behaviour of fish.
Fish use smell to sniff out a partner with a strong immune system.
The smell of an individual fish is genetically programmed by major histocompatibility complex (MHC) molecules, which are crucial to vertebrate animals’ immunity to disease.
"As a by-product, MHC affects the way we and other vertebrates smell," explains Ward.
"Even though all the fish in a school may look alike, when it comes to choosing a mate, picking one that smells different, that is, not related, will ensure the resulting offspring will have the best range of immune responses."