Changes in the Orientation of Barnacles of Certain Species in Relation to Water Currents

Abstract

Both Balanus improvisus and B. amphitrite are capable of rotation of rostrocarinal axis during growth. Twist in radial canals offers reliable guide to direction and magnitude of this rotation. Rotation may be to right, to left, or may vary in direction during growth. Often no rotation occurs and radial canals are straight. Rotation is related to direction of prevailing water current, and is in such a direction as to cause cirri to beat into current. Reorientation of a barnacle in this way may be of adaptive value under estuarine or coastal conditions where tidal flow may be predominantly in one direction and where wave action is negligible. Primary orientation in daylight is detd. by direction of resultant light intensity, but if there is no light gradient, there is probably a direct reaction to water currents at time of settlement. Latter causes cypris larvae to settle pointing downstream and confers a favorable orientation on adult. It is suggested that rotation follows from twisting of body in relation to shell. This twist brings cirri to beat into current, and predominant feeding position influences a plastic region of growth by setting up mechanical strain. Ability to rotate may vary according to natural strains arising through differences of growth rate between base and its periphery.