The horizontal zonation of two species of intertidal barnacle in South Africa

Abstract

Along the west and south coasts of South Africa, the two most abundant intertidal barnacles Octomeris angulosa Sowerby and Tetraclita serrata Darwin have similar vertical ranges, but tend to be segregated along a horizontal gradient of wave exposure. This horizontal pattern is described and the mechanisms producing the pattern at Glencairn in False Bay are examined. Survival rates of Tetraclita were significantly higher in Tetraclita-dominated areas than in Octomeris-dominated areas, whereas survival rates of Octomeris were higher in Octomeris-dominated areas. Five hypotheses to explain differences in survival were examined. There was no support for three of the hypotheses, i.e. the Structure of Cirri, Competition and Predation hypotheses. There was support for the Wave Tolerance and Temperature Stress hypotheses. The results suggest that: (1) physical factors vary along the horizontal gradient: wave height and wave strength are greatest at the exposed end, whereas temperatures and desiccation rates are highest at the calm end; (2) Octomeris is better adapted to withstand strong wave forces because it forms a strong, single-layered matrix; and (3) Tetraclita is better adapted to hot and dry conditions because its shell appears to have many characteristics that reduce heat-loading. Tetraclita and Octomeris therefore occur alone at either end of the gradient and overlap in the middle, where conditions are intermediate.