A complex interaction between a sponge (Halichondria melanadocia) and a seagrass (Thalassia testudinum) in a subtropical coastal ecosystem

Foundation species, such as oysters, corals, and seagrasses, form the basis for entire ecosystems and are characterized by positive interactions with community members. However, many species interactions are context dependent, where the outcome or strength of the interaction depends on the biotic or abiotic conditions. Therefore, a mechanistic knowledge of species interactions, especially those involving foundation species, may allow for a more complete understanding of how anthropogenic changes influence nearshore ecosystems. This study describes the interaction between the seagrass Thalassia testudinum and the sponge Halichondria melanadocia, a species that grows around the base of seagrass shoots. A combination of surveys and experimental manipulations on Abaco Island, The Bahamas, revealed that the interaction between T. testudinum and H. melanadocia is a commensal relationship with the sponge benefiting from the presence of T. testudinum up to medium shoot densities (589-615 shoots per m2). The net neutral effect of H. melanadocia on T. testudinum is likely a balance of the negative effect of the sponge shading the seagrass with the positive effect of nitrogen and phosphorus supplied by the sponge. The mechanisms underlying the interaction between H. melanadocia and T. testudinum suggest that the interaction is likely context dependent. As such, environmental change, namely eutrophication, has the potential to shift the nature of this interaction from commensal to parasitic. A simple simulation showed that if this relationship becomes parasitic, above ground production in seagrass beds could be reduced. This study highlights the importance of a mechanistic understanding of species interactions involving foundation species when predicting human impact on the environment.