Noise pollution in the ocean can confuse and even injure marine species such as dolphins and fish. Now, a new study finds that the same is true of squid and other cephalopods.
"If the relatively low intensity, short exposure used in our study can cause such severe acoustic trauma, then the impact of continuous, high-intensity noise pollution in the oceans could be considerable," study researcher Michel Andre of the Technical University of Catalonia in Barcelona said in a statement.
Most research on noise pollution has focused on dolphins and whales, which have been found to shout over the racket of noisy waters. But in separate incidents in 2001 and 2003, strandings of giant squidshot up along the west coast of Spain. The strandings coincided with nearby ocean seismic surveys, which use air guns to send high-intensity, low-frequency bursts of sound through the ocean in order to image the subsurface of the ocean floor, usually for petroleum prospecting.
The stranded squid had various injuries, but all shared one common feature: damage to their statocysts. These organs are small, balloonlike sacs lined with sensitive hair cells. Much like the human vestibular system, the statocysts are responsible for detecting the squid's position and maintaining its balance in the water.
The statocyst damage raised the possibility that noise played a role in the squid deaths, but no one had tested whether low-intensity sound can cause that kind of damage. So Andre and his colleagues collected 87 wild cephalopods belonging to four species (the common cuttlefish, the common octopus and two species of squid). They then exposed the animals to short sweeps of low-intensity, low-frequency sound for two hours. Next, they dissected the animals to examine their statocysts and compare them with the statocysts of unexposed cephalopods.
What they found was alarming: Every squid, octopus and cuttlefish exposed to sound had damaged statocysts. Hair cells were ruptured and sometimes missing altogether. The nerve fibers that carry signals from the hair cells were swollen. In some cases, there were lesions and holes in the sensory surfaces of the statocysts.
Damage to hair cells caused by low-frequency sound. (Photo: Laboratori d'Aplicacions Bioacústiques, Universitat Politènica de Catalunya)
The damage could explain the dead squid on the Spanish beaches, Andre said.
"Since the statocyst is responsible for balance and spatial orientation, noise-induced damage to this structure would likely affect the cephalopod's ability to hunt, evade predators and even reproduce," he said. "In other words, this would not be compatible with life."
The researchers aren't yet sure why low-frequency sounds are so damaging to cephalopods, but they suspect that the noise could trigger an excessive release of glutamate, a neurotransmitter, which poisons the sensory and nerve cells. The damage seemed to grow worse the longer the researchers waited to kill and dissect the cephalopods, consistent with the toxic neurotransmitter hypothesis. (The researchers decapitated the study subjects in order to kill and then dissect them.) Those dissected after 12 hours showed the least damage, with injury increasing up to the last dissection time of 96 hours.
The findings open up a new area of concern for the safety of marine species, Andre said.
"This is the first study indicating a severe impact on invertebrates, an extended group of marine species that are not known to rely on sound for living," Andre said. "It left us with several questions: Is noise pollution capable of impacting the entire web of ocean life? What other effects is noise having on marine life, beyond damage to auditory reception systems? And just how widespread and invasive is sound pollution in the marine environment?"