The Effects of Predation by the Pacific North West Nudibranch Janolus fuscus (O’Donoghue) on the Bryozoan larvae of Bugula pugeti (Robertson)

Abstract

The sensory organs of larvae of the bryozoan Bugula c.f. californica are very important during the short transition from the swimming phase to the brief settlement phase. These sensory organs detect environmental cues that initiate and regulate settlement within a favorable habitat. However, these organs may also allow the larvae to perceive unfavorable habitat and avoid predation. This study looked at the settlement response of the competent larvae of Bugula c.f. californica when they came in contact with traces of a predatory nudibranch, Janolus fuscus, with a known positive settlement cue (biofilm), and with traces of a non-predatory nudibranch, Archidoris montereyensis. A known number of larvae were kept in small petri dishes filled with filtered seawater at about 12 degrees Celsius with no flow inside the dishes. Prior to adding the newly released larvae, the petri dishes were divided into four groups that were smeared with the three following treatments: mucus from the predatory nudibranch, mucus from the non-predatory nudibranch, and a thin layer of biofilm. Clean (un-filmed) dishes with FSW served as the control. A second study looked at preferential settlement with the known positive- biofilm, the biofilm substratum with predatory water, and 1⁄2 biofilm substratum next to 1⁄2 predatory mucus substratum. We found that the larvae in the biofilm with FSW settled early in their planktonic period. The larvae in the dishes with predator mucus, dishes with biofilm and predator water, in dishes with a choice of biofilm or predator mucus delay their settlement- perhaps in search of optimal habitats. Furthermore, when the biofilm and predator mucus exist in the same dish, the larvae preferentially settle in the side with biofilm, which suggests that they try to avoid potential predation.