The strategy of orientation in the barnacle Balanus trigonus

Abstract

The orientation of the opportunist, sublittoral barnacle Balanus trigonus is investigated. Water movement is the primary orientation stimulus for this species, but this effect may be modified by the influence of light acting in opposition to it. The barnacles orientate at right angles to the axis of wave-surge movement, and observations of feeding behaviour showed that in this position the cirral net could be swivelled 90° each way to make best use of both the advancing, and the reversing, water flow. In a steady unidirectional tidal current the barnacles aligned themselves so as to beat with the water flow, and observation suggests that stroking with the flow is more efficient than forcing the cirral net against the motion of the water. It is suggested that, for an opportunist short-lived species such as B. trigonus which is subjected to high predation rates, there is considerable selection pressure for rapid growth and early sexual maturity. To achieve this in a filterfeeding organism for which food is limiting, food collection must be as efficient as possible. B. trigonus has adopted a system of orientation to a variety of water movement regimes that allows efficient food collection over a wide range of conditions.