Sponges as agents of biological disturbance

Sponges have diverse roles in marine ecosystems and here I describe a newly identified functional role for sponges as agents of biological disturbance. Sponges are thought to be one of the top spatial competitors in benthic environments, and in temperate regions periods of increase(] space acquisition and successful Spatial Competition (overgrowth) during positive sponge growth periods are often followed by seasonally driven periods of space loss. I investigated the potential role of 2 intertidal sponges (Halichondria panicea and Hymeniacidon perlevis) as agents of biological disturbance. The population dynamics and competitive interactions of these 2 sponge species were measured at 2 sites on the coast of west Wales over a 2 yr period. Both species were common on the lower shore at the 2 study sites covering Lip to 18%, of the available rocky Substrate (both species combine(]) depending on the sampling date. Both sponges effectively overgrew most organisms they encountered during growth periods, but the new space they occupied was mostly released as tissue retraction occurred during the autumn and winter months. Since sponge re-growth in the spring and summer did not necessarily occupy the same area of rock as the previous year, weaker competitors were able to recruit to the newly available space (ca. 3 to 4% of the lower shore area per year) and persist over time. Given the widespread abundance of sponges in intertidal and subtidal environments, and the increasing evidence for only short-term temporal stability in many sponge assemblages, this example demonstrates that many sponges may be important biological disturbance agents in marine communities.