Water-soluble compounds from the breadcrumb sponge Halichondria panicea deter attachment of the barnacle Balanus improvisus

Sessile invertebrates and algae are ubiquitous as hosts for a range of fouling organisms in marine benthic habitats. Fouling can sometimes impose negative fitness effects, and hosts may decrease the amount of fouling through production of chemical defenses. Waterborne chemical defenses may rapidly become diluted and should be released continuously in order to inhibit fouling; this process may be costly for the host. Therefore, it has been hypothesized that natural selection should favor the evolution of defense metabolites that stay on the host surface in aquatic environ- ments. We investigated chemical deterrence of fouling in the marine sessile invertebrate Halichon- dria panicea (breadcrumb sponge), through both field observations and a series of laboratory bio- assays. Natural populations of H. panicea had very few macrofouling organisms. Cyprid larvae of the barnacle Balanus improvisus were deterred from attaching to the substratum in the presence of water-soluble compounds that were rapidly exuded from the sponges both under laboratory condi- tions and in the field. Artificial substrata placed near H. panicea in the field received less fouling by barnacles than control substrata placed farther away from sponges. These results clearly show that H. panicea releases water-soluble compounds that deter attachment of B. improvisus. However, these compounds may not have evolved as a chemical defense against fouling in the sponges. H. panicea can release epidermal cell layers, and therefore the barnacles may have evolved an ability to recog- nize water-soluble compounds from the sponges in order to avoid attaching to unsuitable substrata.