Abstract

In the rocky intertidal, invertebrates living in dense, intraspecific groups may be particularly important community members because they provide structural habitat for other species. Despite the prevalence of conspecific aggregates, there is scant knowledge of the proximate and ultimate causes of their distributions. Phragmatopoma californica is a gregarious, suspension-feeding tubeworm that forms extensive reefs ("honeycombs") along the California coast. Local distributions and sizes of worm aggregations, tube orientations and worm mass were quantified to identify patterns and generate hypotheses regarding potential structuring processes. Field surveys were conducted at five intertidal boulder fields in northern and southern California, spanning much of this species' range. Observational data were obtained at four ecologically meaningful spatial scales. The most striking patterns occurred at the smallest (<1 m, individual rock), pervading throughout the largest (50–500 km, among beaches), scales. Aggregations were significantly more abundant and larger on back (shoreward) than front (seaward) faces of boulders. Worm tube orientations also showed a significant directional bias that differed between opposing rock surfaces. In contrast, worm mass was not significantly different between the two faces, perhaps due to relatively uniform growth conditions on the scale of a rock or because worms had reached a terminal size. We hypothesize that within-rock aggregation distributions are associated with the local boulder-induced flow regime. The recirculation zone that forms in the lee of a flow obstacle (rock) would preferentially retain larvae, and thus, enhance the flux of settlers to the back surface. Potentially a region of relatively low wave disturbance and high fertilization rate, life in the lee may be yet another adaptation for survival in the hostile rocky intertidal.

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