Super Shrimp believe-it-or-not dossier...

Another instalment from the

Lying in bed aboard at night can sometimes sound like you’re floating in a bowl of rice bubbles at breakfast time.

Some people imagine electrolysis happening all around them, or that the boat is being attacked by the mythical fibreglass-eating teredo worm. But the real reason for those nocturnal snap, crackle and pop noises is even weirder than that.

The noises are caused by the fearsome feeding habits of snapping shrimp or kowhitiwhiti moana.

Several species of snapping (or pistol) shrimp are hard at work munching in various oceans round the world but New Zealand has a native version – alphaeus novazealandiae – which grows about 2-3cm long and is camouflaged to blend into its reef and rocky habitat.

The snapping shrimp’s biggest departure from other shrimps is its huge claw, like a giant boxing glove, which makes up about half its body weight. The claw has two parts – a flat anvil on one side and a concave scoop on the other. The shrimp snaps these together with enough force to super-compress the water caught between them.

The compressed seawater heats to about 4000°C and expands, forcing out a low-pressure cavitation bubble at speeds up to 100km/h. As the bubble de-pressurises, it expands and bursts – causing the loud popping noise which keeps seafarers awake and wondering in their bunks.

Scientists say the noise can reach 218db (a rock concert would be around 150db) and that the force required to make it is greater than any man-made device can produce, weight for weight.

The bursting bubble and the accompanying pressure wave stuns the shrimp’s prey – small fish and marine organisms – which it then drags back into its burrow to eat at leisure.

World War II submariners first recognised the phenomenon because the shrimp chorus interfered with sonar echoes and they would seek out concentrations of snapping shrimp to create an underwater hideaway from surface trackers.

New Zealand was already known to have some of the noisiest reef life in the world; kina make popping noises, gurnard growl, grunt and groan and john dory bark like a dog. But it wasn’t until the Royal New Zealand Navy set up a hydrophone on Great Barrier