If you are walking quietly near an estuarine seagrass flat or snorkelling near a rocky shore you may hear a constant crackling noise. This is the combined effect of countless pistol or snapping shrimps. For such small animals they make a lot of noise.

Relatively little was known about them until World War II when underwater noises were playing havoc with the instruments used to detect submarines. American and British naval authorities commissioned research to try to sort the natural noises from those made by the enemy. It was then that pistol shrimps were found to be one of the most abundant and noisiest of all marine creatures.

The local shrimps only grow to about 6cm long. They have claws on the first pair of legs but the claw on one side is much larger than the other. The large claw has a movable finger with a small peg near the hinge. This peg fits into a socket on the fixed finger. When the claw is snapped shut a loud click is produced, said to be like glass cracking, as the peg slams into the socket. The peg also displaces a spurt of water from the socket like that from a water pistol. The sound, perhaps aided by the jet of water, is thought to stun prey, startle predators, signal to mates and warn competitors to keep their distance.

Pistol shrimps behave in the opposite way that good children are supposed to – they are heard and not seen. There are several species of pistol shrimps in local waters. The two most common belong to the genus Alpheus. They prefer to stay hidden during the day, venturing out at night to feed. The estuarine shrimp lives among mangrove roots or rocks in a burrow up to 30cm long that it sometimes shares with a mate. Those that live on rocky shores hide under rocks and among sponges and sea squirts.

The pistol shrimps of our southern waters are usually well camouflaged by their colours. The estuarine shrimp is muddy green with darker bands and the rocky shore species has shades of purple and dull red, which blend with the coralline algae and other marine growths.