118 E). After being cleaned of epiphytes, the brood stock was brought to the laboratory and immediately placed in trays with running seawater (salinity 32 ¬± 1%). Balanus improvisus, when regularly fed with nauplii of Artemia salina, will spawn throughout the year. For larval rearing, we used the method of Berntsson and co-workers. The experiment for evaluating the effect on settlement and mortality was conducted using polystyrene Petri dishes ( 48 mm) to which was added 10 mL of the peptide dissolved to different concentrations in filtered seawater (0.2 m, 32 ¬± 1%). Competent cyprids (20 ¬± 2 individuals) were added to each dish in four replicates, with dishes containing filtered seawater serving as controls. Dishes were maintained for 3-4 days at room temperature, after which they were examined under a stereomicroscope for attached and metamorphosed individuals, and also for dead cyprids. Reversibility Test. For each Petri dish incubated at the highest tested concentration (2.5 M), half the cyprids were moved to a Petri dish containing 10 mL of fresh seawater. These Petri dishes, as well as the original ones incubated at this concentration, were examined (as described above) after 5 days of incubation. Statistical Method. Results from a 1-factor analysis of variance (ANOVA) are reported as means ¬± standard error.