Identifying the Environmental Cues that Induce the Invasive Bryozoan Watersipora spp. to Preferentially Settle on Red Algae in the Genus Prionitis

Invertebrate larval choice of settlement sites are commonly influenced by environmental cues. Recognizing these cues, which can be physical, chemical, biological or any combination of the three, are critical to survival, reproduction and growth for sessile organisms. We conducted laboratory and field experiments to identify settlement cues for the invasive bryozoan Watersipora spp. on red algae Prionitis spp. In the field and in the laboratory, we introduced larvae to algae subjected to one of three treatments: (1) live (untreated), (2) ethanol-killed and (3) plastic algae, and found that the live algae strongly induced larval settlement. Settlement on the ethanol-treated algae was significantly lower than on the live algae although the two treatments were morphologically the same. We also examined whether one of the cues might be chemical. Watersipora spp. larvae were induced to settle on crude extracts of crushed Prionitis spp. However, they were not induced to settle by seawater in which Prionoitis spp. had been previously immersed. Our results indicate that while one of the environmental cues inducing larval settlement is likely a chemical compound, that compound is probably not water soluble, and that morphology is not one of the cues inducing larval settlement. We have not yet identified whether biological cues such as bacteria or diatoms may be at work as well. This information may be important for predicting invasions of this bryozoan and discovering methods to limit such invasions.