Snapping Shrimp Turn up the Volume as Oceans Warm

If you’re on the receiving end of a snapping shrimp’s attack, prepare to be stunned. Also known as pistol shrimp, these little crustaceans shoot lethal rounds at predators and prey at highway speeds—a direct hit can be outright fatal or shock the recipient into submission. It’s not just the force of the attack that’s stunning though, it’s the sound. Snapping shrimp are among the noisiest creatures in the ocean.

Left: Researcher Ashlee Lillis holds a microphone to tank of snapping shrimp. Right: A snapping shrimp, note the large snapping claw. Credit: Woods Hole Oceanographic Institution.

“They are hardly ever seen, and very cryptic in their habitat, but their sounds are ubiquitous,” says to Ashlee Lillis, a scientist leading the coral conservation activities in the US Virgin Islands for The Nature Conversancy. As a PhD student Lillis studied the soundtrack of the ocean and its impact on marine larvae. “I was fascinated by the most common biological sound in all of the recordings, the snapping shrimp, and wanted to know more about what all of their racket was about.”

After earning her PhD, Lillis spent a few years exploring that “racket” at the Woods Hole Oceanographic Institution as a postdoctoral researcher in Aran Mooney’s bioacoustics lab. Earlier this spring, they shared the results of this effort at the Ocean Sciences Meeting 2020, an interdisciplinary conference on ocean-related research. The bottom line: Snapping shrimp colonies are getting louder as the oceans are warming and this noise has the potential to indirectly affect lots of other organisms, including humans.

Snapping shrimp have two claws, a typical pincher and an especially large snapper. To launch an attack, a snapping shrimp pulls back the hammer-like part of its snapper, which stores energy like a spring. Then boom—the hammer is released and smacks into an immobile part of the snapper. But the smack isn’t the danger or the source of the sound, it’s the resulting bubble.

Soft coral snapping shrimp / Synalpheus neomeris. Credit: Christian Gloor from Wakatobi Dive Resort, Indonesia (CC BY 2.0).

During the rapid strike, the hammer pushes water molecules out of its way faster than they can flow back in, essentially launching a high-speed water jet. As this fast-moving jet speeds through relatively still water, the result is a swirling vortex with a void—or cavitation bubble—at its center. The bubble quickly and violently collapses, producing a dangerous shock wave and especially loud sound. (To see a video of this in real time and in slow motion, check out BBC Earth Lab’s Pistol Shrimp’s Cavitation Bubble on YouTube.)

Snapping shrimp use this tactic to hunt and defend their colonies, but researchers also think it’s a form of communication. On recordings, like those studied by Lillis, you can hear colonies of snapping shrimp emitting a continuous, crackling background. The noise is so loud that it can actually interfere with sonar technology and confuse other marine animals that rely on sound for navigation and communication. In this new research, Lillis and Mooney studied how that sound is affected by the temperature of the shrimp’s environment.

Photographs of Lillis and her research subjects in the lab, set to an audio recording of snapping shrimp Credit: AGU / Images and audio copyright Woods Hole Oceanographic Institution.

Using underwater microphones and thermometers, the team tracked the noise level and temperature near snapping shrimp colonies for several months. They also documented the snapping rate of shrimp captured and brought to the lab. The animals lived in seawater tanks and were exposed to one of three temperatures treatments, within the temperature range of their home environment, for 24-hour periods.
The results showed that as water temperature increased, both the sound level of colonies in the ocean and the snapping rate of individual shrimp in the lab increased. Sound and snapping rate appear to be closely tied. According to the researchers, the increase in snapping rate most likely accounts for the overall increase in sound. A +1°C temperature change caused the snapping rate to increase by 15-60%, which corresponded to sound increase of 1-2 decibels.

The temperature dependence isn’t surprising to Lillis. “Crustaceans are ectothermic so their activity levels are dictated by the temperature of the environment,” she says. Ectothermic animals rely primarily on the heat in their environment, rather than energy from food, to control their body temperature. They can survive over a large range of temperatures, but reduce their activity at the lower end of the temperature range to keep heat energy flowing to their most vital processes. Given this, it’s reasonable to theorize that when more heat is available, snapping shrimp have more energy to spend on things like communication and hunting.

Still, lots of questions remain. For example, scientists don’t know the breakdown of why snapping shrimp snap—the percentage of snaps made in pursuit of food, communication, defense, or some other unknown purpose—or whether the snap rate for each function depends on temperature in the same way. “[I]t’s unclear whether the temperature influences one or all of the functions of the snap,” Lillis says.

What is clear is that temperature has some direct impact on snap rate and underwater noise level, and that we need more research to better understand the consequences of this increase. As ocean temperatures rise due to climate change, the crackling background produced by these creatures could become more problematic for marine life and sonar-based underwater technologies like fish finders and submarine mine detectors. “[T]hese incredible little shrimp are full of intrigue,” Lillis says. “No one notices them, not even marine scientists, because they are rarely seen, but if you start listening to the sea, they are a big deal!”

A snapping shrimp in a petri dish. The tiny critters are among the loudest animals in the ocean.  Credit: Woods Hole Oceanographic Institution. –Kendra Redmond