Snapping Shrimp

Snapping shrimp, also known as pistol shrimp, are not at all like the little pink prawns that some people like to eat. They have a giant claw that gives them an uneven look (Figure 1). Snapping shrimp produce powerful shockwaves by snapping this oversized claw. The resulting shockwave is used to communicate with other shrimp, to protect themselves and to hunt and kill their prey.

There are over 600 species of snapping shrimp in our oceans. They are part of the group of animals known as crustaceans. Snapping shrimp are the only marine eusocial animal, meaning they live in highly organised colonies, like those of bees, ants and mole-rats [1]. These shrimp can be hard to see because they like to hide amongst sponges, corals, stones and shells. They live in areas of the sea near to land where the water is warm and shallow. Snapping shrimp are found around Australia, Hawaii, the Korean peninsula and the Mediterranean Sea. Recently, due to global warming, snapping shrimp have been discovered on the coasts of the United Kingdom, an area once too cold for their survival.

Snapping shrimp can grow to 6 cm in length but some can be as long as 10 cm. Their giant claw can grow to half their body length [2]. Despite their shy nature, these crustaceans are some of the loudest animals in the ocean! Scientists used to think that the loud snaps that these shrimp make came from the upper and lower parts of the giant claw banging together. But more recently, high-speed cameras and underwater microphones called hydrophones have been used to analyse the snapping claw in more detail [2].

Figure 2 shows a recording of the sound made by a shrimp snap. Scientists found that, at the beginning of the recording, the shrimp is already in the process of quickly slamming its big claw shut—this takes about half a millisecond. This rapid movement forces a jet of water to whoosh away from the claw at high speed, which, in turn, generates a small bubble. The bubble grows to around 7 mm wide. It is the collapse of the bubble that makes a short but loud snapping sound that generates a big spike in sound pressure, which is the pressure caused by sound as it moves through the water. The bursting bubble creates a shockwave that stuns or kills a shrimp’s prey.

A single snap can go unnoticed in the sea, but a large group of snapping shrimp can create a continuous sound resembling the crackling of burning twigs or the hiss of Creepers from Minecraft. Sometimes this sound is loud enough to interfere with the underwater communications of other animals. During the Second World War, American scientists realised that the noise of snapping shrimp was affecting the sonar systems used by Japanese ships. The scientists worked out where large colonies of snapping shrimp lived and used the shrimp noise to hide American submarines from Japanese sonar [3]!

Underwater Noise

Listening to sounds like music or the wind in the trees can be a nice experience. But some sounds are not very nice to listen to, such as that of a fork scratching a dinner plate or a loud, busy road. These unwanted sounds are often called noise, and too much noise can be bad for our health. It can damage our ears and our hearing and can stop us from sleeping. Have you ever been kept awake by noises? If you do not sleep well, you can feel awful the next day, so we all know that noise can be stressful and tiring. It is not just humans that are affected by noise; too much noise can also damage wildlife. For example, loud traffic noise can affect the times that birds start singing in the mornings and evenings.

We know that noises traveling through the air can be damaging, but what about underwater noise? When having a bath or going for a swim in the sea or a swimming pool, you might have noticed how sounds travel differently through water. Many marine mammals, fish and invertebrates use sound as part of their daily lives. Dolphins, whales, walruses, fish, crabs and shrimp, for example, rely on sounds to get around, communicate and hunt. Some fish even make sounds to attract mates [4].

Unfortunately, too much noise can have serious effects on these animals, as it masks other sounds and prevents the animals from thriving (Figure 3). Lots of background noise can interfere with animals’ senses and can cause confusion and changes in behavior. Human activities such as drilling into the seabed, the movement of boats and ships, fishing and dredging can all damage life on the seabed. This excess noise is called noise pollution and can make it harder for animals to communicate, find their way around, hunt or escape predators.

But just how damaging is noise pollution? Scientists can find it difficult to measure the effects of noise on our underwater wildlife, especially invertebrates [5]. Lots of studies are done on marine mammals and some on fish, but what about other, smaller animals? Could studying snapping shrimp help?

Monitoring the Ecosystem Using Acoustics

Biodiversity is a measure of the number of different types of organisms, like plants and animals that live in a particular place or habitat. Areas with a high biodiversity are home to many species. Healthy coral reefs and other marine habitats are famous for their amazing biodiversity, but in some places, these habitats are affected by overfishing or pollution.

One way to measure the biodiversity of marine environments is by diving down to the seabed and taking lots of photographs and notes. This can be expensive, time consuming and sometimes dangerous. We know that snapping shrimp like to live inside sea sponges, which are commonly found on coral reefs and other areas. We also know that the healthier the seabed, the more snapping shrimp will live there [6]. Snapping shrimp can tell us something about the condition of the ocean floor. This means that snapping shrimp are good biological indicators; they can help us work out how healthy our oceans are. But how do we determine how many snapping shrimp live in a particular place? We cannot count them by sight because they often hide and are not always easy to see. This is where acoustics comes in handy!

We can work out how many snapping shrimp there are by listening to the sounds they make. Scientists use hydrophones to measure underwater noise. Placing hydrophones in carefully arranged positions can help locate the origin of a sound. If snapping shrimp are living in an area, their snaps will be heard on the recordings and their position can be detected.

Shrimp snaps are so loud, quick and full of energy that they can be fairly easy to detect. Scientists can also use groups of hydrophones to check that the sounds they hear actually come from the shrimp and not from some other animal or machine. The result is that acousticians can use sound recordings to count the number of snapping shrimp in large areas without disturbing the organisms that live there. This gives scientists an idea of the overall health of the seabed [6]. Acoustics has also been used this way to study other animals including birds and bottlenose dolphins [7, 8].

Recently, scientists have discovered snapping shrimp living in regions that were once too cold for them. The snapping shrimp are telling us, loud and clear, that our climate is changing, and that our seas are getting warmer. Perhaps we should listen to what they are saying!

Glossary

Shockwave: ↑ A wave of high pressure that moves faster than the speed of sound.

Crustaceans: ↑ A group of animals with hard exoskeletons, two pairs of antennae, joined legs and gills for breathing. Includes lobsters, crayfish, shrimp, and true crabs.

Eusocial: ↑ Animals that are highly organised into groups or colonies, in which only a few individuals have offspring and those offspring are cared for by the group.

Hydrophone: ↑ A type of microphone that has been designed to measure underwater sound.

Sonar: ↑ A system of sonic echoes used by ships and some animals for navigation. It helps to detect the size, shape and location of other objects including predators, prey or obstacles.

Noise Pollution: ↑ Unwanted sound that can be disturbing or annoying.

Biological Indicator: ↑ A type of plant or animal that can be studied to show the health of a whole environment or ecosystem.

Acoustics: ↑ The study of sound.

Conflict of Interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Acknowledgments

Our thanks go to Byoung-Nam Kim from the East Sea Environment Research Department of the Korea Ocean Research and Development Institute for providing the shrimp snap recording used in Figure 1; to Sergio Mascolino for Figure 3; to Marco Brunoldi, Daniele Grosso, Giorgio Bozzini, and Alessandra Casale from the Department of Physics at the University of Genova, Italy for providing hydrophone recordings and productive discussion, and to Caryl Hart for editorial advice.