The impact of physical stress and molluscan grazing on the settlement and recruitment of Tetraclita species (Cirripedia: Balanomorpha) on a tropical shore

Abstract

Tetraclita japonica [Bull. U. S. Natl. Mus. 93 (1916) 241] and Tetraclita squamosa [Encycl. Method.: Hist. Natl. Vers 1 (1789) 158] are common barnacles on exposed, oceanic Hong Kong coasts, occupying the mid and lower shore, respectively. The percentage cover of the two species is moderate (<60%) and the remaining rock space is bare or occupied by sparse patches of encrusting algae and grazing molluscs. Settlement of the two Tetraclita species occurs during the physically stressful summer over a wide vertical range, exceeding their adult upper limit. A manipulative experiment including roofs (to reduce heat stress) and fences (to exclude grazers) at two tidal levels (one in the adult T. japonica zone and the other above this zone) showed that recruits, which settled above the adult upper limit (except those under roofs), were killed by heat stress. Recruitment of the two species was also higher where grazers were excluded and where the rock surface was shaded as compared to unmanipulated areas in the adult T. japonica zone. Heat stress appears the dominant factor limiting recruit density above the barnacles' upper limit, whilst removal by grazers, combined with heat stress, controlled density within the adult zone. The vertical distribution of each species is therefore limited by post-settlement and post-recruitment mortality induced primarily by physical stress, whereas abundance within their normal heights is mediated by an interaction between heat stress and the impact of molluscan grazers.