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Behavior

Listening for Symptoms: A new use for hydrophones in the face of harmful algal blooms

Coquereau, L., Jolivet, A., Hégaret, H., & Chauvaud, L. (2016). Short-Term Behavioural Responses of the Great Scallop Pecten maximus Exposed to the Toxic Alga Alexandrium minutum Measured by Accelerometry and Passive Acoustics. Plos One, 11(8), e0160935. doi:10.1371/journal.pone.0160935

Marisc_Pecten_maximus

Pecten maximus for sale at the market. Photo from https://commons.wikimedia.org/

Scallops and Algae

When scientists talk about the use of hydrophones (underwater microphones), a common thought is that they’re out to record whale song. But there are many organisms besides whales that contribute to the soundscape of the ocean. In fact, if you listen very closely, it can be possible to determine if an organism is stressed, just by the sounds it’s making.

A new study conducted by Laura Coquereau and her colleagues presents the first use of passive acoustics to monitor changes in the behavior of scallops—yes, scallops—in response to high levels of phytoplankton in the water. Scallops are an integral part of the economy of coastal France; the seasonal algal blooms that have been noted in the region for decades can impact their health. You can think about algae blooms like the ocean’s equivalent of air pollution, where the algae make up the particulates in this underwater smog. Depending on the species of algae, a bloom can be relatively harmless; however, some species produce harmful toxins that can accumulate in the tissues of filter feeders like scallops.

Because harmful algal blooms (HABs) aren’t always close to shore, remotely determining if scallop populations offshore are being impacted is an important step forward, so Coquereau set out to passively record scallops in a controlled laboratory setting while also monitoring the animals’ movements using accelerometers.

Methods

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Fig. 1: A) the audiogram of scallop sounds; B) the accompanying movements recorded by the accelerometer. From Coquereau et al. (2016) Fig. 3.

Coquereau and her team collected scallops from local waters near Brest, France. The animals were brought into the lab, cleaned of epibionts (like attached barnacles), and acclimated to their new tanks. The scientists attached small accelerometers to one of the scallops’ shells that could measure how often the animals opened and closed their housing. These movements could indicate if the animal was respiring, forcibly expelling water, closing itself off entirely, or actively swimming.

Scallops were tested in waters containing various concentrations of either harmless or harmful algae in trials lasting 2 hours a piece. In addition to monitoring the motion of the scallops’ shells, the animals were recorded via video and hydrophone to see if there were corresponding sounds to the movement. (See Fig. 1)

Results

Under the laboratory conditions—admittedly very unnatural, considering the tanks were set up within a sound-proof room—the scallops showed modified behavior to toxic levels of algae, and these alterations in behavior could be discerned using passive acoustic recording. Data from the accelerometers showed that the scallops significantly increased their total movements, the number of forcible water/waste expulsions, and the number of complete closure of the shells to the outside environment. The hydrophone recordings showed the same significant differences in sounds indicating swimming movements and water/waste expulsion. (See Fig. 2)

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Fig. 2: Showing the recorded behaviors (A=accelerometer, B=passive acoustic monitoring) of scallops in response to increasing levels of both harmless (light columns) and harmful (dark columns) algal species. Taken from Coquereau et al. (2016) Fig. 4.

Big Picture
This study has repurposed passive acoustic monitoring to remotely observe the health of an ecologically and economically important species, and although the methodology has only been tested for short periods in a laboratory, the technology itself requires little modification. If it were properly adapted, scientists and fishermen alike would benefit from such remote listening.

These behavioral changes were witnessed at the highest concentrations of the harmful algae, indicating the scallops rapidly modify their responses. However, the highest level tested was fifty times greater than the sanitary threshold established by the French HAB monitoring network. The study’s “intermediate” level (10,000 cell/liter) is the set sanitary threshold, but no observable differences in sound production or movement were detected in those trials. As a result, the first question Coquereau and her colleagues want to pursue is if noticeable differences arise after longer exposure at these lower HAB concentrations. If such a future study were to show promising results, passive acoustic monitoring could become a new, normal practice in fisheries and conservation alike.

So now that you know we can detect when scallops cough, what other kinds of creatures do you want to listen for in the ocean? And what do you think their sounds will tell us?

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