Symbiont-mediated shifts in sandprawn behaviour: Implications for ecosystem functioning in marine soft-sediment ecosystems

Ecosystem engineering by burrowing species is one of the most influential determinants of community structure in marine sedimentary ecosystems. However, per capita ecosystem engineering rates displayed by burrowers are variable, and thereby contribute to enhancing local biotic and abiotic variability. Lacking in current understanding of processes influencing variability in ecosystem engineering rates by burrowers and consequent feedbacks to assemblages are interactions occurring between burrowers and their burrow-symbionts. In this paper, we quantify behavioural responses of burrowing sandprawns (Callichirus kraussi: Crustacea: Axiidea) to burrow symbionts (Betaeus jucundus: Crustacea: Alpheidae) using controlled laboratory experiments. We demonstrate that sandprawns display discrete behaviours in response to B. jucundus with distinct levels of tolerance. Such variability in tolerances of sandprawns to B. jucundus is consistent with the idea that the outcomes of symbiotic relationships are variable, depending on processes that shift the costs and benefits involved for partners. Importantly, these discrete tolerances were associated with significant changes to time spent by sandprawns on ecosystem engineering activities (sediment turnover, burrow irrigation) with intolerant sandprawns increasing sediment turnover rate by 50% and decreasing irrigation by 30%. In a second experiment seeking to determine if variability in sandprawn responses to B. jucundus could be due to differences in sandprawn sexes, we demonstrate that male and female sandprawns respond differently to the presence of B. jucundus, with differences in time spent on ecosystem engineering in the presence of B. jucundus. Taken collectively, our results highlight the complex behaviours occurring within burrows in marine sediments and their potential to influence ecosystem activities of burrowers.