Boating Noise Negatively Affects Marine Species: Sea Hares

Stylocheilus striatus, sea slug.  Photo courtesy of:

Image of a sea slug, Stylocheilus striatus. Photo courtesy:

A study conducted in 2014 by scientists from the Universities of Bristol and Exeter, and the école Pratique des Hautes études (EPHE) CRIOBE in France looked at the affect that acoustic disturbance, in the form of boat noise, had on embryonic development in sea hares. The study specifically focused on the sea slug, Stylocheilus striatus in a coral reef lagoon in French Polynesia.

Sea hare larvae will usually hatch from their eggs and may drift in the water column for 30 days or more before settling on toxic red algae to feed. But when eggs were exposed to playback audio of boating traffic, larvae were more likely to die off, while others failed to develop at all. Co-author, Dr Suzanne Mills, an evolutionary biologist from CRIOBE, Perpignan said, “Our study used controlled field experiments and a split-brood, counterbalanced design to account for any possible site or genetic effects. Nearly 30,000 eggs were placed in plastic tubes. Half the eggs from each mother were near speakers playing boat noise while the other half were near speakers playing coral-reef ambient noise. Both success of embryonic development and post-hatching survival decreased by more than 20% as a consequence of exposure to boat-noise playback.”

Co-author, Dr Andy Radford, a reader in behavioural ecology at the University of Bristol, said: “This is the first indication that noise pollution can affect development and survival during critical early life stages.”

Why Sea Hares Are Important

Sea hares are important ecosystem regulators, consuming toxic alga (cyanobacteria) that can affect recruitment of fish to coral reefs. The cyanobacteria in their diet plays a key role in the potency of their defenses towards predators, i.e. the secretion of ink when threatened. Chemical components from their cyanobacteria diet, called mycosporine-like amino acids (MAAs) act as intraspecific alarm cues for sea hares, resulting in avoidance behaviors in predators. Scientists have established MAAs as molecules of keystone significance, highlighting their influence on the potential to influence distribution of species and playing a key role in structuring an ecosystem. A decrease in any one species could result in a loss of vital ecological functions and lead to alterations in the community structure.

These marine invertebrates are additionally important in the field of neuroecology due to their unusually large nerve cells–fifty times larger than those of mammals. These cells have been discovered to function like biological clocks, producing patterns of electrical pulses that reflect natural cycles in the sea hare’s environment. Certain individuals were observed to produce pulse bursts at the time of transition between day to night, and vice versa. Cyclic variations have also been observed in relation to the variations in bi-monthly ocean tides. These observations have presented new questions in neuroscience involving sea hare’s simple nervous system and its properties similar to human’s. Future research may shed some light on long-term memory loss with the potential for treatments for diseases associated with memory deficits.

Noise Pollution in Our Oceans

Boat noise in our world’s oceans is now recognized as a global pollutant with many of these signals transmitting below the margin of 1000Hz. Large supertankers can omit pulses of 190 decibels or more, while smaller boats may create sound waves of 160-170 decibels. These walls of white noise transmit far and wide, and can be experienced in almost every crevice of the ocean.

Recent studies involving acoustic disturbance have supported the effects on the behavior and physiology on a vast array of marine species. Some fish species experience anti-predator behavior making them more susceptible to predators, while migration patterns and communication between marine mammals are greatly disturbed, negatively impacting feeding and mating grounds.

This study on sea hares is the first of its kind to demonstrate the effects that boat noise pollution has on development and larval survival during critical life stages. Co-author, Dr. Andy Radford suggests that, “consideration should be given to the regulation of human activity in protected areas.” Regulating boating traffic in areas reliant on marine fisheries could help to protect these important marine ecosystems and the communities that depend on them. Some national and international legislation (US National Environment Policy Act and European Commission Marine Strategy Framework Directive) is currently in place to help regulate traffic in the US and EU waters. With a growing number of people with recreational and fisheries demands, its important that these legislations consider better protections for marine ecosystems.

“Undersea noise pollution is like the death of a thousand cuts. Each sound in itself may not be a matter of critical concern, but taken all together, the noise from shipping, seismic surveys, and military activity is creating a totally different environment than existed even 50 years ago. That high level of noise is bound to have a hard, sweeping impact on life in the sea.” — Dr. Sylvia Earle

Boat noise is now recognized as a global pollutant. Image courtesy of Anna Marinenko,

Boat noise is now recognized as a global pollutant. Image courtesy of Anna Marinenko,


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