Sharks can detect a drop of blood from a kilometre away, but apart from this jaw-dropping fact, little is known about how these animals smell.
Using the most accurate and detailed model ever made of a hammerhead shark鈥檚 head, of the University of Bath, UK, and colleagues at the University of Cambridge, found that when the shark swims 鈥� usually sweeping its head from side to side 鈥� it pushes water into its olfactory centre, the area of the brain dedicated to smell. This had been suggested before, but Cox鈥檚 team are the first to prove it.
The researchers created their shark鈥檚 head model using CT scan images of a hammerhead preserved at London鈥檚 Natural History Museum. The images were converted into a solid 3D replica using rapid prototyping, which transforms a computer model into a 3D object by building up layers of material.
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The model, complete with internal cavities accurate to 200聽micrometres, was then dunked into a flow tank, while researchers observed the flow of water through its head. 鈥淭o simulate the shark鈥檚 style of swimming, we changed the angle of the head model in the tank and observed the flow at each angle,鈥� says Cox.
Using a red dye, Cox and his team found that water fully circulated throughout the nasal chamber despite its changing angle. According to Cox, this shows 鈥渢he hammerhead shark smells as it swims forwards, propelling water through its nose鈥�.
Hairpin bend
Cox describes the nasal cavity of the hammerhead as a labyrinth of pipes with a hairpin bend at its centre. Shooting from the hairpin are tiny channels that house the key to the shark鈥檚 smelling prowess: olfactory receptors that transform smells into chemical signals.
While the model used in this study wasn鈥檛 detailed enough to replicate these channels and show how water moves to olfactory receptors, Cox hopes the team鈥檚 next model, accurate to 50聽micrometres, will be.
The detailed model did, however, find a valve in the shark鈥檚 head that is not seen in other shark species. According to Cox, the valve guides water into the nasal chamber but prevents too much flooding in and damaging the olfactory receptor channels.
鈥淚t is a very good paper showing how water, and thus odour, passes through the curious and elaborate hammerhead nose,鈥� says , a marine biologist specialising in marine sensory behaviour at Boston University.
, a shark researcher at the University of Queensland in Brisbane, Australia, agrees that the model is helpful. But, without knowing how odour molecules reach olfactory receptors, 鈥渋t does not really address the ability to sense blood in the water鈥�, he says.
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