Density- versus trait-mediated responses of a symbiont (Betaeus jucundus) to burrows of a burrowing crustacean (Callichirus kraussi)

Ecosystem engineering by burrowers is one of the most important determinants of spatio-temporal community patterns in marine soft-sediments. Current understanding of broad mechanisms by which this occurs is based on research on free-living assemblages, while equivalent knowledge for burrow-dwelling symbiotic assemblages is scarce. In this paper, we aim to differentiate the relative strength of density-versus trait-mediated responses of a symbiont (Betaeus jucundus; Crustacea: Alpheidae) to burrows created by a southern African axiidean sandprawn (Callichirus kraussi; Crustacea: Axiidea) in a marine dominated lagoon on the west coast of South Africa. Results of multiple linear regression analyses indicate that spatio-temporal variability in B. jucundus abundance was not related to densities of burrows produced by C. kraussi, but was better predicted by burrow traits, particularly the concentrations of potential trophic resources and fine sediment fractions along burrow walls. Chl-a and carbohydrate concentrations on burrow walls as well as abundance of B. jucundus peaked during spring upwelling, particularly at sites with greater sediment mud content. Quantities of fine sediment fractions along burrow walls were positively linked to concentrations of chl-a and carbohydrates and to abundances of B. jucundus. Our findings suggest that extrinsic processes (seasonal pulses of nutrients) and intrinsic burrow features (mud content) may be important in determining concentrations of trophic resources in burrows potentially available to symbionts. Our results also indicate that accurate prediction of spatio-temporal abundance patterns of certain symbionts may require knowledge of these intrinsic and extrinsic processes that influence the amount of potential trophic resources available in burrows to symbionts.