Settling barnacle larvae avoid substrata previously occupied by a mobile predator

Abstract

Settlement by the larvae of the barnacle Balanus glandula (Darwin) in the field is decreased on slate tiles that have been previously occupied by a predator of postmetamorphic stages, the whelk Nucella lamellosa (Gmelin). Settlement by another barnacle species, Semibalanus cariosus (Pallas), was not as greatly influenced by the same treatment. The effect on B. glandula also occurs when the substratum is treated with traces of the whelk by either forcing the foot over the plate or rubbing the plate with crushed individuals. This reduction in settlement can persist up to 3 wk after treatment of the substratum. A similar but lesser effect occurs after treatments with traces of a potential biological disturber of settling larvae, the limpet Tectura scutum (Rathke), but also occurs with mucus from the brown alga Fucus distichus (L.), which is not a demonstrated hazard to the barnacles. Mucus is naturally released by all of the above organisms and may be the cause of the reduced settlement. Control manipulations using a nonraucus-producing animal tissue, the pectoral muscle of the chicken Gallus gallus (L.), produced only a slight decrease in barnacle settlement. Mucus could reduce settlement either by acting as a cue for avoidance of high-risk areas, altering other settlement cues, or physically inhibiting attachment. However, not all mucus inhibits settlement: another investigator demonstrated that prior occupation by another predatory whelk induced greater settlement by another barnacle species; and we found that mucus from the dorid nudribranch Archidoris montereyensis (Cooper) increased settlement by B. glandula. Thus, our results suggest that the mucus might possibly act as a cue to the larvae, and we conclude that traces of mobile animals can influence the settlement of sessile animals. Since the threat of whelks to barnacles is delayed until some time after settlement, we hypothesize that the barnacle may be taking into account the long-term future risks (i.e., “the ghost of predation future”) when making the decision to settle. The avoidance response of the barnacle to the traces of limpets and macroalgae offers an alternative explanation (differential settlement) to patterns often attributed to biological disturbance (differential postsettlement mortality).