Bacterial Chemical Defenses of Marine Animal Hosts
In the intense predation environment of the oceans-especially tropical oceans-many animals have obvious, visible means of self-defense. Molluscs are usually shelled; corals are encased in calcium carbonate and are capable of stinging attackers, and so on. Standing out as glaring exceptions to the defensive rule, the soft-bodied marine animals such as sponges and sea slugs manage to survive and thrive often as major contributors to biomass in a given environment. It has long been postulated that marine animals lacking physical defenses would instead be chemically defended, relying on small molecules to deter predation via numerous possible allelochemical pathways. Based upon this hypothesis, well over 10,000 marine natural products have been isolated from invertebrate animals. Many of these compounds are potent toxins, leading to clinical use in cancer therapy (Miljanich, 2004; Newman and Cragg, 2004). Many more have been shown to provide chemical defenses by various mechanisms, including feeding deterrence, toxicity, warning signaling, and prevention of surface colonization (fouling) (McClintock and Baker, 2001). The striking thing about these chemicals is that many, if not most, are actually synthesized by bacteria living within their animal hosts. Thus, bacterial symbionts allow many of the soft-bodied animals to exist in predation-intense environments, such as coral reefs.