Defensive roles for secondary metabolites from marine sponges and sponge-feeding nudibranchs.

In the marine environment sponges (Porifera) constitute one of the most interesting sources of bioactive natural products. The high frequency of bioactive components in these primitive filter-feeders is interpreted as chemical defence of sponges against environmental stress factors such as predation, overgrowth by fouling organisms or competition for space. The highest incidence of toxic or deterrent sponge metabolites is consequently found in habitats such as coral reefs that are characterized by intense competition and feeding pressure due, for example, to carnivorous fish. Further support for the adaptive significance of sponge constituents is derived from the observation that sponges which are growing exposed are usually more toxic than those growing unexposed. Whereas the chemical defence of sponges seems to be highly effective against most species of fish, a group of shell-less gastropods, the nudibranchs, has specialized on sponges. While feeding on sponges the nudibranchs sequester the effective chemical armoury of their prey, which is subsequently employed for their own protection. Some nudibranchs, however, have become independent of this interspecific flow of natural products and are able to accumulate defensive compounds through de novo synthesis.