Feeding Behavior Reveals the Adaptive Nature of Plasticity in Barnacle Feeding Limbs

Abstract

Barnacle feeding limbs are extremely plastic in response to spatial and temporal differences in wave exposure: individuals have long, thin feeding limbs in habitats with little wave action, and short, thick feeding limbs in wave-exposed habitats. This difference in feeding limb morphology is assumed to be adaptive. Individuals with shorter limbs may have the ability to feed in breaking waves because their limbs are better suited to withstanding drag forces than are those of individuals with longer feeding limbs. I tested this hypothesis by observing the feeding behavior of two populations of Balanus glandula (one from a protected shore and one from a wave-exposed shore) subjected to five different water velocities. Differences between populations in the ability to feed with the cirral net fully extended were highly significant. The wave-exposed population fed with the cirral net fully extended at all velocities tested (up to 49 cm/s), whereas full cirral extension ceased between 7.25 and 21.4 cm/s in all individuals from the protected-shore population. Clearly, barnacles possessing long, thin feeding limbs experience a strong disadvantage when feeding in faster flowing water, confirming earlier hypotheses that differences in feeding limb length between protected and wave-exposed shores are adaptive. Acorn barnacles are suspension feeders that extend three pairs of modified thoracic limbs (cirri) into ambient flow to capture plankton and smaller food particles (1). The feeding apparatus of many barnacle species is remarkably plastic with respect to wave exposure (2‐4). In the northeastern Pacific, the common intertidal barnacle Balanus glandula Darwin, 1854, shows the greatest degree of plasticity in feeding leg form. Individuals from wave-protected shores possess feeding limbs nearly twice as long and 25% thinner at the base than conspecifics living on wave-exposed shores (3), and experiments show that this species can alter the length and width of its feeding limbs to suit local flow conditions in one or two molts (2). Because of the functional importance of barnacle feeding limbs, their phenotypic plasticity is assumed to be adaptive—shorter, stouter legs may allow barnacles to feed in high-velocity flow under breaking waves. However, the consequences of possessing differently shaped feeding limbs have not been investigated, and thus the adaptive nature (5) of differences in feeding limb length remains speculative.