WATERBORNE CHEMICAL-COMPOUNDS IN TROPICAL MACROALGAE - POSITIVE AND NEGATIVE CUES FOR LARVAL SETTLEMENT

Settlement sites of marine invertebrate larvae are frequently influenc ed by positive or negative cues, many of which are chemical in nature. Following from the observation that many shallow-water, Hawai'ian mar ine macroalgae are free of fouling by sessile invertebrates, we predic ted that the algae are chemically protected and dependent on either su rface-bound or continuously released soluble compounds to deter settli ng invertebrate larvae. To address the importance of waterborne algal compounds, we experimentally determined whether larvae of two of Hawai 'i's dominant hard-surface fouling organisms, the polychaete tube worm Hydroides elegans and the bryozoan Bugula neritina, would settle in t he presence of Waters conditioned by 12 species of common Hawai'ian ma croalgae (representing the Phaeophyta, Chlorophyta, Rhodophyta and Cya nophyta). The results included a full spectrum of biological responses by each larval species to waterborne algal compounds. Larval response s to conditioned water were consistent for each algal species, but the outcomes were not predictable based on the taxonomic relationships of the algae. For example, among the species of Phaeophyta examined, dif ferent conditioned waters were: (1) toxic, (2) inhibited settlement, ( 3) simulated settlement, or (4) had no effect, compared to larvae in c ontrol dishes containing filtered seawater. Additionally, larval respo nses to aged (24 h) conditioned waters could not be predicted from the results of assays run with conditioned waters utilized immediately af ter preparation. Finally, settlement by larvae of one species did not predict outcomes of tests for the other species. Four of 12 shallow-re ef Hawai'ian macroalgae tested released compounds into surrounding wat ers that immediately killed or inhibited settlement by both H. elegans and B. neritina (toxic: Dictyota sandvicensis; inhibitory: Halimeda d iscoidea, Sphacelaria tribuloides, Ulva reticulata); the remaining 8 a lgal species prevented settlement by one of these fouling organisms bu t for the other had no effect or, in some cases, even stimulated settl ement