WATER‐BORNE CHEMICAL CUES AS ELICITORS OF ALGAL DEFENSES

Toth, G. & Pavia, H. Göteborg University, Tjärnö Marine Biological Laboratory, SE‐452 96 Strömstad, Sweden It is well established that water‐borne chemical cues from predators or predator‐wounded conspecifics can induce defensive changes of aquatic prey animals, but few examples of such inducible defenses have been reported for aquatic algae. We have found that water‐borne cues from actively feeding gastropods (Littorina obtusata) can induce increased concentrations of phlorotannins in the intertidal brown seaweed Ascophyllum nodosum. Elicited A. nodosum plants are also less susceptible to further grazing bygastropods compared to undamaged plants. Since seaweeds have poorly developed internal transport systems and may not be able to elicit systemic induced chemical defenses through conveyance of internal signals, induction through water‐borne cues ensures that seaweeds can anticipate future periwinkle attacks without receiving direct damage by herbivores. We have also found that water‐borne cues from a parasite (Parvilucifera infectans), can serve as signals to induce morphological defence in a toxic dinoflagellate (Alexandrium ostenfeldii). The dinoflagellates formed temporary cysts when exposed to filtrates from cultures with infected conspecifics. The effect of the filtrate disappeared after 8 h, indicating that the chemical cues are short‐lived. Furthermore, the cysts were resistant to parasite infections for several weeks after exposure to direct parasite contact, indicating that they are an effective defense. We suggest that induction of defenses in marine algae through water‐borne cues may be a common phenomenon and that more examples will be revealed as this field of science progresses.