Even the crabs in the water get too loud: Frequent ship noise affects their ability to camouflage themselves, researchers report. The animals therefore no longer properly adapt their appearance to the environment and therefore become conspicuous prey. This can become a problem in the fine-tuned ecological balance. In addition to crustaceans, other marine life may also be affected by similarly unexpected effects of man-made underwater noise, the scientists say.
Not even the Pacific Ocean lives up to its name: the realm of Neptune is extremely loud. The underwater noises have many natural causes: In addition to water movements, many living creatures in the ocean make noises and also have appropriate hearing skills. In many places, however, there is a roaring disruptive factor mixed into this natural soundscape: Ship traffic in particular causes deafening noise under water. Studies show that many marine animals can be considerably irritated by this. So far, the focus has been on species that have an acoustic communication or orientation system – such as marine mammals and some fish species.
Beach crabs in the roar of ships
In the current study, researchers led by Emily Carter from the University of Exeter have now investigated whether ship noise also affects marine animals that might initially be thought to be less sensitive to noise: crabs. It was known that they basically react to sound. As part of their study, the researchers have now investigated how ship noise affects crustaceans. The experimental animals were specimens of a species that many people know from vacationing by the sea: the beach crab (Carcinus maenas) is a common and widespread inhabitant of the European coasts and therefore plays an important role in ecosystems.
In the experiments, dark-colored young beach crabs were placed in test pools with a white background. For a period of eight weeks, some pools were exposed to underwater sounds every hour for about two minutes, as would be produced by a cruise or container ship that passed about 200 meters away. In the control pool, on the other hand, the crabs heard natural water noises, which, however, corresponded to the volume of the ship’s noises. The question now was whether there were any differences between the crabs in the ship’s ambience and the test animals in the natural noise environment.
Stress interferes with color matching and behavior
As the scientists report, the difference was literally obvious: as was to be expected, the crabs in the control tanks had adapted the color of their shell to the light background in the course of molting in order to camouflage themselves. They had turned their black color into almost pure white over time. But the crabs, which were exposed to the noise of the ship, had not succeeded: they had only achieved a shade of brown that did not match the light background.
The scientists suspect that the lack of adaptability is a result of the stress caused by ship noise. “Changing the color of crabs is a slow, energetically expensive process that is controlled by hormones that activate specialized pigment cells in their shells,” says Carter. “Stress consumes energy and disrupts the hormonal balance. We therefore believe that the stress caused by ship noise either deprives the crabs of the energy required to change the color properly or disrupts the hormonal balance required for the color change, ”says the scientist.
The researchers were also able to prove by behavioral investigations that ship noise leads to acute irritation or stress: when exposed to crab simulated bird attacks, the animals hid comparatively ineffectively when ship noise was heard at the same time. “About half of the crabs that were exposed to ship noise did not respond to the attack at all, and others hesitated to hide,” says Carter. “Just as people have difficulty concentrating when stressed, the crab’s responsiveness also appears to suffer,” said the scientist.
The study once again illustrates how subtle many man-made disorders can affect nature. Because who would have thought that noise could affect the appearance of crabs. “Apparently, it is not only animals affected by noise pollution that are known to actively use noise, such as whales or some fish species,” sums up senior author Martin Stevens. The team therefore now wants to continue to investigate the effects of man-made stress factors in the marine environment that may have been previously unknown.
Source: Cell Press, technical article: Current Biology, doi: 10.1016 / j.cub.2020.01.014