Aspects of the ecology of three species of Cellana in the Wellington area

Abstract

Wellington Harbour is lined by rocky shores ranging from sheltered inner harbour to extremely exposed outer coasts. Within these extremes lies a wide variety of exposure regimes. The community structures of fifteen uniform shores of differing degrees of wave exposure have been assessed, and the change in species composition over the exposure gradient has allowed a biologically defined wave exposure scale, or 'Ballantine' scale, to be constructed. This scale has been used to grade 69 hard shore sites in and around the harbour giving an exposure map of the harbour. On an eight point scale the inner harbour and approaches encompass six exposure categories. The scale was also used to assess the relative exposure to wave action of five sites chosen for the interspecific and intraspecific comparisons of some morphometric characteristics within the genus Cellana. Three littoral species belonging to this genus (C. radians, C. ornata and C. denticulata) are common on Wellington rocky shores and are often the dominant grazers among the molluscan fauna. Comparisons of density, shell length, shell height, radula length and orientation were made between and within each species along two major environmental gradients of increasing exposure to wave action and increasing height on the shore. In general it was found that the two low shore species, C. radians and C. denticulata, showed a decrease in shell length and height with increasing littoral height and exposure to wave action. All three species showed a preference for a head-down orientation regardless of exposure regime or height on the shore. C. ornata showed no clear response (positive or negative) in either shell length or height along the two gradients investigated, suggesting the presence of additional factors overriding the influence of the two gradients studied. No relationship was found between the radula ratio and either gradient investigated, although C. radians was shown to have the smallest radula length to shell length.