CHEMICAL INTERACTIONS IN INVERTEBRATE LARVAL SETTLEMENT

Michael G. Hadfield
Kewalo Marine Laboratory, University of Hawaii, 41 Ahui St., Honolulu,HI 96813, USA.

Larvae of most marine invertebrates settle and metamorphose in responseto chemical cues associated with an appropriate habitat for survival, growthand reproduction. Among other attributes, such habitats provide attachmentsites (most critical for sessile organisms), more-or-less specific food,and potential mates. Thus the specificity of cues and their specific detectionby larvae are critical to species survival. The hypothesis that larvaedetect chemical cues by specific receptor molecules located on the surfacesof excitable sensory cells is supported by growing evidence from a varietyof phyla. Pharmacological data demonstrate that in many instances wellknown signal-transduction systems transduce external chemical cues intoalterations in cell chemistry and subsequent nervous signals. Specificityof chemical cues varies with habitat: species dependent on very restrictedprey typically employ prey-specific metabolites as metamorphic inducers,while larvae of sessile members of fouling communities respond to productsof common biofilms. Data from a coral eating nudidranch and a common biofoulingmicro-serpulid worm illustrate these trends. Larvae of Phestilla sibogaesettle in response to a small polar metabolite from corals of the genusPorites. Larvae of Hydroides elegans respond to cues arisingfrom marine biofilms. Evidence indicates that anteriorly placed sensorycells bear receptor molecules that lead to signal transduction by mechanismsdifferent from those found in most olfactory systems. Work continues onthe structural chemistry of inducer molecules and on the signal transductionsystem of larvae.