Reference: Biol Bull 181:442-452. (December, 1991) Modes of Feeding in Aggregations of Barnacles and the Shape of Aggregations JULIE PULLEN' 2 AND MICHAEL LABARBERA 2 * [ Macalester College. 1600 Grand Avenue, St. Paul, Minnesota 55105, and 2 Department of Organ ismal Biology and Anatomy. The University of Chicago, 1025 East 57th Street. Chicago. Illinois 60637 Abstract. The interactions between the form of a bar-nacle aggregation, its flow environment, and the feeding behavior of each individual was determined in unidirec-tional flows, both models of barnacle aggregations and live barnacles were used. Hill-shaped aggregations of model barnacles captured significantly more particles than flat aggregations. In general, rows upstream of, and at the peak of, all hill-shaped profiles captured significantly more particles than downstream rows. Living barnacles located at, or upstream of, the peak of natural clusters captured significantly more food particles than did barnacles located downstream. Living barnacles located at, or upstream of, the highest point in a natural cluster fed passively, whereas barnacles downstream of the peak actively swept their cirral net against the flow. Flow was laminar up to the highest point in natural clusters, whereas flow was both reduced and turbulent over the downstream portions. In-dividual barnacles within a cluster differ in their feeding rates and net energy gains, and therefore differ in their growth such that, in unidirectional flow, the peak of a cluster will shift upstream over time; in oscillating flows, the clusters will develop a symmetrical profile. Introduction Acorn barnacles are sessile suspension-feeding animals. The feeding apparatus, the cirral basket, is a sieve-like net (LaBarbera. 1 984) that typically is oriented perpendicular to the direction of ambient flow (Crisp and Bourget, 1985). Barnacles can draw upon a repertoire of feeding behaviors (Hazlett, 1988; Okamura, 1990). Changes in food con-centration or flow rates elicit different feeding behaviors. Received 5 April 1991; accepted 9 September 1991. * To whom all correspondence should he addressed. largely expressed in the motion of the cirral basket. A feeding barnacle may use any of three patterns of cirral basket movement: normal beat, fast beat, and extension (Crisp and Southward. 196 1 ). When flow velocity exceeds some threshold value, barnacles shift from fast beat to extension feeding (Crisp and Southward, 1961; Trager el ai, 1990), switching from sweeping their cirral baskets through the water, to simply holding their baskets against the flow. These two feeding behaviors are also known as active and passive, respectively (Jorgensen, 1966). Both Crisp and Southward (1961) and Trager et at. (1990) focused on single barnacles in analyzing this flow-induced behavioral change. However, barnacle cyprids are gregarious settlers (Knight-Jones, 1953; Wethey. 1984). a behavior that often leads to aggregations of in-dividuals (clusters) that generally display a hill-shaped contour. A common form consists "of a cluster of indi-viduals enormously elongated at the centre of the hum-mock, and decreasing symmetrically towards the periph-ery" (Barnes and Powell, 1950). Several explanations have been put forward for the or-igin of hill-shaped aggregations. Barnes and Powell (1950) suggested that pressure from neighbors caused growth of the central barnacles to be constrained to the upward di-rection; "barnacle shell growth is very plastic and . . . whenever forces are applied to the shell, its shape becomes modified" (Bourget and Crisp, 1975; also see Crisp, 1960). Crisp and Bourget (1985) suggested that food capture played a pivotal role in the development of hill-shaped aggregations. If some barnacles in a heavily settled area captured more food than their neighbors, they could only display their good feeding fortune by upward growth. Be-cause taller barnacles feed higher in the flow boundary layer, they sample a higher flow velocity than their shorter neighbors (Crisp and Bourget, 1985). Faster flow implies 442
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