Settlement Cues and Determination of the Vertical Limit of an Intertidal Barnacle

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Abstract
The settlement behavior of the barnacle Chthamalus anisopoma near its upper intertidal limit was investigated using field surveys and experiments. The upper limit of Chthamalus was found to be extremely variable, ranging from 1.5 to 2.6 m above MLW. Chthamalus showed increased settlement in response to chemical cues produced by several species whose vertical distributions overlapped with its vertical distribution. Within a single species there was significantly more settlement in response to cues associated with Nertia funiculata that occurred with Chthamalus than to N. funiculata that occurred only above the vertical limit of Chthamalus. Another species that induced settlement, Acanthina angelica, is the major predator of adult Chthamalus. Acanthina may be a good settlement cue because it is a reliable indicator of the vertical distribution of Chthamalus and because (1) the clusters of Acanthina that induced settlement may move away before the new barnacles become edible; and (2) Chthamalus can develop a predator—resistant morphology in responses to Acanthina. I was also able to induce Chthamalus recruitment into areas well above their normal upper limits using a conspecific extract, indicating that conspecifics function as a strong settlement cue. The evolution of larval attraction to settlement cues should be dependent on the reliability of the cues and the consequences of not using them. Reliable cues and harsh consequences should lead to the development of "strong" (i.e., attractive) cues. However, reliable cues may be conservative; their absence may not indicate a "bad" settlement site. Therefore, a variable distribution of strong settlement cues could lead to temporal and spatial variability in a species regardless of other factors such as pre— and postsettlement physical conditions or biological interactions.