If you listen closely to San Francisco Bay — in the right place, and with the right equipment — you can hear its heartbeat. And it sounds like the crackling and popping of a campfire.
Some of us at San Francisco State University’s Estuary & Ocean Science Center are listening to that pulse, created by thousands of snapping shrimp, as a way to monitor the health of the Bay.
Snapping shrimp (in the family Alpheidae) have specialized claws that they use to stun prey, defend territory and communicate with each other. During these interactions, the shrimp’s claw shuts rapidly, producing a small bubble that collapses violently with a snapping sound. That snap lasts less than a thousandth of a second, but it’s so loud that when the shrimp are abundant or very active, they can even interfere with ships sonar. In terms of natural noise in the ocean, these shrimps’ snap is second in loudness only to the echolocation clicks of sperm whales, even though the crustaceans grow to only two inches long.
In many different shallow-water habitats where snapping shrimp are present, the number of shrimp pulses detected per minute, or pulse rate, is strongly influenced by environmental conditions. Some research indicates that water temperature, water quality, and habitat quality can all affect the pulse. That means that listening in on these shrimp could be a way to measure the health of the Bay over time, monitoring habitat conditions, the impact of human activities and the effects of climate change, in a way that’s cheaper and less invasive than other techniques.
But first, we need knowledge of how the shrimps’ snaps vary over days and seasons. Making those baseline measurements is the overarching goal of my research with San Francisco State University Assistant Professor of Earth and Climate Sciences Piero Mazzini.
The Bay is a unique and dynamic body of water with lots of oceanographic variability that could affect the shrimp’s behavior. Within a single day, the water temperature can change four to nine degrees Fahrenheit with the tides. I see high tide as the Bay’s tidal in-breath, where the Bay breathes in colder, saltier ocean water. With the Bay’s tidal out-breath, the influence of river water becomes dominant, warming the Bay. In our preliminary data, we can track the behavior of shrimp very closely with each tide and its corresponding temperature change. Like a heartbeat with a large and small peak, the snapping behavior peaks twice every day, at low tide.
The pulse changes with the seasons, too. A shrimp’s metabolism is intimately linked to its environment — so SF Bay’s 18-degree annual cycle in average water temperature makes for more abundant pulses in the summer and sparser pulses in the winter.
But in a crowded place like the Bay, it’s not always easy to make out its heartbeat. I’ve been working with Roger Bland, a recently retired professor in SF State’s Department of Physics & Astronomy who has continuously recorded underwater sound from the EOS Center’s pier in Tiburon since 2006. For this snapping shrimp project, Bland has deployed a hydrophone (an underwater microphone) to record sounds much higher-pitched than boat noise. Bland has also designed an algorithm that can hunt out snapping shrimp pulses within the sound files we capture while ignoring similar sounds produced by boat propellers. We’re also planning to listen to snapping shrimp pulses at Pier 15 in San Francisco at the Exploratorium museum.
We chose snapping shrimp for this project because their acoustic behavior has been studied vigorously in many different habitats, and snapping shrimp are detectable year-round in at least two sites within the Bay. But surprisingly, very little is known about our local snapping shrimp. This summer, when the shrimp are loudest, I plan to capture snapping shrimp to determine which species are present in the Bay. It may be that our resident shrimp are the same species present near Bodega Bay or in Cordell Bank just north of us. Or it might be that some of the SF Bay’s shrimp hitched a boat ride from other parts of the Pacific.
And there are many possibilities for future research with the snapping shrimp. Measuring their pulse year-to-year could help us track how underwater habitats are affected by sea level rise and climate change, or simply by year-to-year changes in weather like this past wet winter.
Research projects like this one help us to eavesdrop on the interactions of our underwater neighbors and to appreciate the richness of our natural soundscapes, which for the most part are hidden or drowned out by human noise that we introduce to the Bay. If you’d like to help us by providing your own underwater recordings of the Bay, you can email me at firstname.lastname@example.org.
George Czeck is a student researcher and National Science Foundation Research Traineeship fellow in Dr. Piero Mazzini’s Coastal Oceanography Lab at San Francisco State Univeristy’s EOS Center. This column is one of an occasional series about the bay and sea around us through the eyes of researchers at SFState’s Estuary and Ocean Science Center.