Consequences of ship noise for camouflage, anti-predation, and movement in crabs.

Abstract

The marine environment is becoming increasingly polluted, with unprecedented levels of anthropogenic noise changing the marine soundscape. Mounting evidence shows that exposure to this noise can cause numerous adverse effects across taxa. However, invertebrates, juveniles, and behaviours not dependent on acoustics have received relatively little attention. Furthermore, research into how individuals may cope with these pressures is lacking. I address these knowledge gaps through a series of laboratory-based playback experiments focussed on juvenile shore crabs (Carcinus maenas), using three noise treatments: ship noise, ambient underwater sounds (control), and ambient underwater sounds played at the same amplitude as the ship treatment (loud control). In chapter 2, I examined the effects of ship noise on brightness change; a strategy employed by juvenile shore crabs to increase their level of camouflage and reduce predation risk. Individuals were repeatedly exposed to one of the aforementioned noise treatments for 8 weeks. Photographs of individuals, taken regularly throughout, were analysed using a predator vision model to determine the level of brightness change and camouflage in an ecologically relevant context. Ship noise reduced the overall brightness change and camouflage, though it did not affect the change in brightness per moult. The level of growth per moult was reduced by ship noise however, and the timing of moulting events was delayed. In chapter 3 I investigated the effects of noise on antipredator behaviour (using the response to a simulated predator) and locomotion, including the frequency of pausing and directionality of movement. By comparing the effects between individuals with varying levels of previous noise exposure, I also tested for signs of acclimatisation. Ship noise reduced the likelihood of individuals responding to a predator and increased their latency of response. Locomotion was not disrupted, but individuals moved away from ship noise, positioning themselves in quieter areas. These findings were consistent for all individuals, regardless of their previous level of noise exposure. The negative consequences of anthropogenic noise in the marine environment are clearly not constrained to species or behaviours reliant on acoustics, as juvenile shore crabs exposed to ship noise suffered decreased levels of camouflage and reduced growth. Individuals also displayed maladaptive behavioural responses to a simulated predator when exposed to ship noise. There is no evidence that acclimatisation occurred, but individuals did attempt to physically avoid noisy areas. Loud natural sounds did not affect any behaviours studied, suggesting the type of noise is important in determining how individuals may be affected. Overall, this thesis shows that juvenile shore crabs suffer multiple negative effects from noise pollution, including the disruption of critical behaviours that are pervasive in the marine environment, with potential implications for survival.