. The Biological bulletin. Biology; Zoology; Biology; Marine Biology. 340 J. E. WILLIAMSON ET AL. Time ihi Figure 7. Percent metamorphosis over lime ol competent lar\;ie ot Hulopnfusles pnrpurascenx (one larva per replicate; 10 replicates per treatment) when exposed lo seawater surrounding plants of Delisea pul- chra and Ecklonia rculiala, and from seawater distant 2 m) from any macroalgae. Sterile, tillered seawater was used as a control. Discussion Environmental cues—physical, biological, or chemical- have long been thought to play an important role in the settlement and metamorphosis of mar
. The Biological bulletin. Biology; Zoology; Biology; Marine Biology. 340 J. E. WILLIAMSON ET AL. Time ihi Figure 7. Percent metamorphosis over lime ol competent lar\;ie ot Hulopnfusles pnrpurascenx (one larva per replicate; 10 replicates per treatment) when exposed lo seawater surrounding plants of Delisea pul- chra and Ecklonia rculiala, and from seawater distant 2 m) from any macroalgae. Sterile, tillered seawater was used as a control. Discussion Environmental cues—physical, biological, or chemical- have long been thought to play an important role in the settlement and metamorphosis of marine invertebrate larvae (Svane and Young. 1989; Pawlik. 1992: McEdwards. 1995), and chemical cues in particular have received considerable attention in the last 20 years. Such chemical cues ma> be waterborne, associated with the surfaces of substrata, or derived from conspecifics (Pawlik. 1992). and there is now a substantial list of metabolites known to ha\e strong effects on the settlement or metamorphosis of invertebrate larvae (reviewed in Pawlik. 1992; Slattery. I997i. Few if any of these metabolites, however, have been characterized, quantified in situ, and shown to affect settle- ment or metamorphosis of larvae in a way that is consistent with the demography of the target organisms in the field. Many studies in this area have focused on what Pawlik (1992) termed "artificial" inducers; that is. the metabolites have not been shown to be present (cither at all. or in active levels) in the environment of the settling organism, hut rather are thought to mimic the action of naturally occurring inducers. Kxamples include the neurotninsmittcrs -y-ami- nobutyric acid (GABA) (Morse ct 1979). i -/3-3,4-dihv droxyphenylalanine (i-IX)PA) (Coon and Bonar. I')X7>. and epinephrine i' Hun <•/ til., 19X6). Such compounds. many of which are widely distributed throughout the animal phyla important internal roles as mediators of settle! [iliosis (see review in Col
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