December 2021
When asked to think of loud animals, we gravitate towards the big ones: the roar of a lion, the trumpet of an elephant, and, underwater, the song of the mighty blue whale. But in many underwater environments, the smallest animals are among the loudest. Snapping shrimp, or pistol shrimp, live in a variety of different shallow water ocean places, from coral reefs to temperate kelp forests to sponge dominated and rocky bottom habitats. They measure in at a couple of centimeters, yet their snapping claw can grow to be half the size of their body. When closed, the claw’s hinge produces a bubble which bursts and makes a loud snap sound. At 183-210 decibels, this makes them some of the loudest sound producers in the ocean.
Little Sentinels
Despite being loud, their small size and love of crevices means that we don’t know much about why they make this sound. Snaps are most commonly recorded during fights between shrimp who are defending territories, while they are digging burrows or in response to changes in their environment, but scientists think they also use the sound to stun prey and deter predators.
The crackling sound that you can hear when you are snorkeling or diving is often the most distinctive and memorable sound in places where snapping shrimp are prevalent. For those who have not experienced this, it sounds a bit like the popping sound Rice Krispie cereal makes when you pour milk over it.
Spectrograms are ways of looking at sound. This 10 second recording in Gray’s Reef National Marine Sanctuary portrays what we hear when we are diving near snapping shrimp: their collective snapping generates the loudest sounds (lighter blue to yellow colors) across most of the tones or frequencies that we recorded (vertical axis). Image and sound: NOAA.
Since many avid divers are so tuned into the sounds they hear underwater, people have started to take notice of changes in what they are hearing after a habitat is damaged by an oil spill, hurricane, coral disease, or bleaching. Anecdotal recognition that we are hearing less snapping in these places than we used to is transitioning to analysis of underwater recordings before, during, and after efforts to restore habitats that are damaged. These studies are showing that the amount of snapping can help us understand how our restoration efforts are doing, or even help us rebuild ecological communities. Because their sound plays an important role in navigation, directing marine larvae and fishes to suitable habitats, their early return to the soundscape can potentially alert other organisms to the suitability of the habitat.
Sounds pretty snappy! But there are big caveats. In order to understand whether changes in snapping relate to how healthy a habitat is, we need to first understand why snapping varies to begin with.
What’s Normal?
We’ve all been taking our temperatures a lot this year. So we know that our temperatures aren’t 98.6 degrees all the time, even when we are healthy. Some peoples’ bodies run warmer than others. To know when an elevated temperature means that you are sick and when it doesn’t, you have to know your own normal body temperature. The same is true of snapping: we expect some differences in the amount of snapping that we hear among different healthy shallow water habitats. So, before we can tell whether less snapping hints at damage to habitats, we have to understand what’s normal for each of them.
During the SanctSound project we have been listening to underwater habitats within national marine sanctuaries all over the country for a few years. Snapping shrimp were big contributors to soundscapes in four of our monitored sites: Florida Keys, Gray’s Reef, Channel Islands and Hawaiian Islands Humpback Whale national marine sanctuaries. We used the same automated tool to quantify how much snapping occurred in each of these places, so that our measurements would be comparable. We found different amounts of snapping at these places, but we also found that snapping changed throughout the day and year as well.
Gray’s Reef National Marine Sanctuary, a hard live-bottom reef habitat off the coast of Georgia, showed the highest amounts of snapping of all of our recording sites when we lumped all the data together, while recordings from Florida Keys National Marine Sanctuary had the least. But when looking at different times of year, it’s clear that the peak times for snapping in the sanctuaries are different: Gray’s Reef, Florida Keys and Hawaiian Islands Humpback Whale national marine sanctuaries had peak snapping in the spring, while snapping at Channel Islands didn’t peak until early summer. This generally correlates well with other studies, which have found that snapping peaks as waters warm above a certain threshold, which happens at different times of year in different places.
Diving even deeper into the data, we see that time of day affects how much snapping we hear as well. Take a look at these plots of shrimp snapping at Gray’s Reef and Hawaiian Islands Humpback Whale national marine sanctuaries. You can see that we hear more snapping during the darker hours of the day in both places. This is similar to other studies that have found that snapping picks up at dusk. But you can see that the pattern is not exactly the same in each place: in Gray’s Reef, twilight has the highest snapping, while in Hawai‘i, snapping picks up strongly once the sun goes down.
Is the Habitat Healthy?
So, can changes in the amount of snapping that we hear alert us to changes in how a habitat is faring? We’ve learned that how we measure snapping really matters—It’s important to use a tool that allows for making comparable measurements across places and time periods. It’s also necessary to listen for a long enough timeframe to account for peaks and troughs in snapping over daily and seasonal cycles. At least one more element is critical: accounting for differences in how well the recorders were positioned to hear snaps to begin with.
To do this, we can use models to estimate the distance over which a snap could be heard around each of the recorders. The models predict how the sound made by snapping shrimp will travel in places with different depths, bottom types, and in different seasons—all of which are factors that affect how sound waves behave underwater. For example, we hear more snapping in Gray’s Reef than we do in Florida Keys. The model results show us that we recorded snaps occurring over a larger area around our recorder in Gray’s Reef than around our recorder in Florida Keys, which we need to account for in our assessment of what’s normal for each place.
However, there are hints that snapping levels in Florida Keys are lower than they used to be. Dr. Jenni Stanley of Waikato University who led this analysis also collected recordings at the same Florida Keys location in 2016, before a coral reef disease outbreak that has affected many habitats in the sanctuary. Comparison of snapping across recordings suggests a decrease in recent years. “The more we understand snapping, the better chance we have of using this behavior as an indicator of habitat health, ecosystem change, and restoration success,” said Dr. Stanley.
Our underwater listening stations have recorded continuously over years in sanctuary habitats—through day and night, and often right through major storm events. By honing in on the snaps of these tiny shrimp we are asking them big questions: is the timing of when you snap the most getting earlier in the year as ocean climates warm? After a hurricane, how much of your snapping do we need to hear before fish begin to recolonize damaged areas? These little sentinels may have hints to offer, we just have to keep listening.
Dr. Leila Hatch is a marine ecologist at Stellwagen Bank National Marine Sanctuary and co-lead of the SanctSound project