Driven to distraction? Behavioural impacts of anthropogenic noise on the European hermit crab (Pagurus bernhardus) from individual to group level

Abstract

Human activities are altering the planet at an unprecedented scale and pace, ranging from effects on global systems such as climate and carbon cycles to localised but globally wide-spread exposure to anthropogenic pollution such as noise. Anthropogenic noise is an example of human-induced rapid environmental change (HIREC) which can mask, distract and disrupt natural stimuli and sensory-cognitive processes. Since HIREC can alter the sensory environment of animals, and how they detect and process information from their biotic and abiotic environment to make accurate decisions, this process has been termed sensory pollution. While growing evidence shows detrimental effects on across taxa, behavioural contexts and situations, invertebrates are understudied despite contributing to global faunal biodiversity to a vastly greater extent than vertebrate animals.

In this thesis, I study how anthropogenic noise as a form of HIREC affects a marine crustacean using the European hermit crab Pagurus bernhardus as a model organism. For hermit crabs, empty gastropod shells are a crucial resource affecting growth, reproduction and survival. Crabs are known to have a preferred, optimal shell weight (% PSW) relating the occupied shell weight to the crab’s own body weight but the shell size they occupy in nature can diverge from the optimal shell size. First, I exposed hermit crabs over 10 days to low-intensity ship noise playbacks (chapter 2). The sound treatment had no effect on assessment behaviour until the last day of the experiment whereby individuals under noise showed longer latency to assess the new, optimal shell. Crabs in small shells under the noise treatment accepted the new shell more frequently than crabs under ambient sound. This pattern was reversed for crabs in larger shells. This experiment suggests that properties of anthropogenic noise beyond the intensity affect animals. Besides the noise effects, I show that shell assessment is a repeatable behaviour.

Next, I demonstrate that the effects of noise are modulated by natural factors (chapter 3). I exposed hermit crabs not only to noise and different sized shells but also to a visual predator cue of the common shore crab Carcinus maenas. Overall, the interaction between noise, predator presence and shell size influenced the mean duration for the final decision to accept or reject the optimal shell. Hermit crabs in shells of 50% optimal size took less time for their final decision when exposed to both ship noise and predator cue while crabs in shells of 80% optimal size showed shorter decision time only when the predator cue was absent. Moreover, crabs are less likely to accept an optimal shell in the presence of ship noise, suggesting that exposure to ship noise disrupted the information gathering ability of the crabs.

In addition to the noise effects on solitary animals, I examined its effects on intraspecific behavioural interactions (chapter 4 and 5). Under ambient sound, crabs in optimal shells spent most of their time close to a single crab and crabs in suboptimal shells showed no clear preference. Under ship noise, however, this pattern was reversed (chapter 3). Furthermore, noise reduced the aggregated benefit of the arrival of a new shell resource unit to a group of crabs exposed to noise for 24 h (chapter 5) showing that noise effects can accumulate over time. After crabs have been exposed to noise for 24 h I measured the direct effects on their oxygen consumption. In addition, I accounted for the influence of the % PSW of the occupied shell on the oxygen consumption of crabs. Since crabs obtained those shells during the 24 h group process under ship noise (chapter 5), this measure allows to quantify the indirect physiological costs of decisions made under noise (chapter 6). While there was no direct effect of the sound treatment on oxygen consumption, crabs in shells that were too small in relation to their body size had a higher oxygen consumption than hermit crabs in shells closer to the optimal size. Finally, in a field experiment, I found that the mean startle response duration increased with observation number and that the mean startle response duration was repeatable over the observation period. There was no effect of ship noise, presumably because other natural factors such as wind and water turbulence overrode the effects of noise exposure.

My results indicate that noise affects shell assessment decisions and that the effects can be modulated by natural factors such as predation threat, resource quality and potentially abiotic variables. This suggests that noise can disrupt across multiple sensory channels. In addition, noise can alter not only individual behaviour but the disruption of individual decisions go beyond a single exposure and scaled up to population levels. I discuss the implications of my findings and suggest avenues for future research to gain a more complete picture of the effects of anthropogenic noise on animals.