Rossi, T., Connell, S. D., & Nagelkerken, I. (2016). Silent oceans : ocean acidification impoverishes natural soundscapes by altering sound production of the world’s noisiest marine invertebrate. Proc. R. Soc. B 283: 20153046. http://dx.doi.org/10.1098/rspb.2015.3046
Noisy Life Histories and Impending Stress
The ocean is hardly a silent place; from the anthropogenic clamor of motorboats to biological sources like whale songs, coastal waters resonate with many different noises. One of the ocean’s most “vocal” inhabitants is the percussive snapping shrimp. These invertebrates are known to emit pops that can be deafening at close range and are able to be heard far offshore. These pops, along with other natural noises, comprise the soundscape that some fish and invertebrate larvae rely on to find suitable habitats as they transform into their adult stages.
Because soundscapes play such an important role in local biodiversity, scientists have turned their attention towards determining how manipulated soundscapes will affect ecosystems. One team in particular, led by Tullio Rossi, focused its attention on the snapping shrimp’s response to ocean acidification (OA)—an aspect of climate change predicted to greatly impact coastal areas. OA results from increased levels of dissolved carbon dioxide (CO2) in seawater, which stresses and overwhelms the ocean’s ability to resist pH changes. Ultimately, organisms that incorporate calcium carbonate into their bodies (think shellfish or coral) are left with fewer available minerals, and subsequently struggle to create their skeletons.
It turns out, snapping shrimp also use calcium carbonate in their eponymous claw. So a decreasing ocean pH, thus an increasing acidity, could physically alter the snapping shrimp’s sound producing capability. And in the event the physical mechanism isn’t impacted, the overall behavior of the shrimp could change in response to other OA-related environmental changes.
Rossi and his colleagues therefore set out to examine how OA will affect sound produced by the snapping shrimp using a two-pronged approach: 1.) finding natural CO2 vents in situ—that is, out in the ocean—to measure for soundscape differences, and 2.) collecting snapping shrimp and exposing them to artificially elevated CO2 levels in a controlled laboratory.
Three natural CO2 vents in the Mediterranean and near New Zealand were selected for pH monitoring along a gradient; probing stations closer in proximity to the vent measured CO2 levels at and beyond end-of-the-century predicted values, while stations further away measured levels matching our current atmosphere. To give you a better reference, Earth’s atmosphere currently has ~380 parts of CO2 per million molecules (ppm) and is predicted to have upwards of 1000 ppm by the year 2100 if emissions are not curbed. Once the probing stations had been established, Rossi’s team deployed hydrophones in each area, recording during the day and night over the course of a few days in September and November in 2014. Abundances of kelp and algae, as well as sea urchins, were noted during dives as they provide suitable habitat and contribute to the natural soundscape, respectively.
Snapping shrimp were collected from areas unaffected by elevated CO2 levels, acclimated to laboratory conditions, and then assigned to control or elevated CO2 treatments for up to three months. During treatment, shrimp activity levels, molting time, and claw and carapace (shell) length were measured; acoustic activity was also monitored when shrimp were undisturbed or provoked.
Audio recordings made at the various CO2 vents showed a decrease in snapping shrimp sound intensity at stations closer to the vent openings. This decrease was not noticed in the control areas, further from the vent. Not only did the overall sound intensity decrease, but the average number of snaps and pops also declined significantly.
The laboratory studies on the individual shrimp agreed that elevated CO2 did indeed reduce the frequency and amplitude of their snaps, however provoked shrimp were still capable of producing snaps of normal intensities. This indicated the physical claw mechanism was not affected by OA, rather, the acoustic behavior of the shrimp changed.
The Big, Quiet Picture
But wait, you might say—the in situ observations revealed fewer pops at lower sound intensities. If the shrimp are still capable of producing snaps, why the change in the soundscape near the vents? While not directly addressed in this study, other works have shown that snapping shrimp populations decline closer to CO2 vents in both temperate and tropical waters. The explanation for the change in soundscape could be as simple as: there are fewer shrimp in the area.
The reason for this could be an indirect effect of OA, where the acidity begins affecting other organisms—like kelp and algae. These plants provide shelter for snapping shrimp and if they can’t survive in areas of higher acidity, fewer shrimp will set up shop, which would lead to an alteration of the soundscape. For other fish and invertebrate larvae relying on snapping shrimp pops for orientation and finding settlement sites, this could be detrimental. Entire coastal communities could see shifts in biodiversity as different fish and invertebrates less reliant on sound cues move in. Scientists are unsure what the ultimate outcome will be, but just as human activity has created this problem through OA, our influence on coastal soundscapes might solve it. Coastal waters are rich in biological and anthropogenic noise, as previously mentioned. Should the biological noises decline, human generated sounds will dominate the scene. So maybe the fish and invertebrates will just have to retune their ears to find homes near coasts.