In the autumn of 2001, ships employing compressed air guns were prospecting for gas and oil off the northwest coast of Spain. In the process of mapping the sea floor, they employed pulses of air that created explosively loud, high-intensity, low-frequency waves of sound. During September and October, five giant squid, cephalopods which can grow to be over 40 feet long, were found stranded along the coast, a much higher number than usual; and in 2003, amidst continued oil and gas exploration, four giant squid were found, dead or dying, on the coast or near the shore.
Examination of the animals and further scientific investigation suggested that the squid died as a result of physical damage to their statocysts, the structures they use to detect sound and orient their bodies in the water. (I read about these events here [pdf], here, and here.)
New research conducted on another cephalopod, the common cuttlefish, suggests that, in addition to being damaging at high intensities, lower intensity sound may be an important way that cephalopods sense the environment around them.
A team of scientists monitored the behavior of cuttlefish exposed to a series of sounds of different frequencies and intensities, each lasting for three seconds. The most extreme behaviors the researchers observed were “escape responses” – jetting quickly away, or inking. (Cuttlefish, like most cephalopods, can produce a cloud of ink that obscures them from a predator as they escape.) These behaviors were elicited by low-frequency, high-intensity tones, while lower intensity tones were associated with less extreme behaviors like body movements and changes in body coloring. (Cuttlefish are capable of altering both the color and the pattern of their body, in order to deter predators, attract mates, or blend into their surroundings.)
The researchers also assessed the extent to which cuttlefish become habituated to certain sounds – in other words, whether or not they will stop responding to a repeated tone. They found that “[a]fter several exposures and no imminent threat, the number of escape responses decreased, suggesting the cuttlefish were able to filter out the ‘irrelevant’ acoustic stimuli,” though the cuttlefish never completely stopped responding to the repeated sounds.
Sound may be an important indicator of their surroundings for cuttlefish – the scientists write that the “evasion responses suggest that the cuttlefish initially reacted to the [sound] stimulus as they would react to a predator or other form of danger, and that sound detection could be a mechanism for predator detection” in cuttlefish.
Sound can have disastrous effects on cephalopods – but in other circumstances, or at different intensities or durations, it may serve ecologically important functions like announcing the presence of a predator.
Common cuttlefish (Sepia officinalis) can grow to be up to two feet long.
(Image by David Sim via Wikimedia Commons)