Tenacity and shell shape in six Patella species: Adaptive features

Abstract

Measurements were made of te forces necessary to detach various South African Patella spp. These ranged from 5.18 to 1.95 kg/cm 2, with significant differences between the species. P. cochlear Born. had the highest value, followed in sequence by P. argenvillei Kr., longicosta Lam., P. granularis L., P. granatina L., and P. oculus Born. Adhesion is the only mechanism capable of providing forces of this magnitude. Differences between the species are related to differences in their morphology, high tenacity being associated with low mucus secretion, small number of mucocytes, and inflexibility of the foot. This is associated with a large area of muscle attachment on the shell and the size of basal haemocoelic spaces. Features favouring high tenacity conflict with those favouring mobility, and the limpets fall into two groups. P. granularis, P. oculus, and P. granatina occur on the upper shore and the last two usually occur in areas which are sheltered from strong wave action. They a rapid growth and high gonadal output, demanding extensive foraging and hence mobility at the cost of tenacity. P. cochlear and P. longicosta are territorial and together with P. argenvillei, remain low on the shore, grow slowly, and have a low reproductive output. Feeding is localized and mobility sacrificed for high tenacity which is essential in P. cochlear and P. argenvillei as they occur in areas of strong wave action. Shell height is not correlated with tenacity nor with the intensity of wave action normally experienced by each species, but P. cochlear, P. argenvillei and to a lesser extent P. granularis, are subjected to strong wave action and have proportionally narrower shells (increasing streamlining) and low coefficients of drag. The latter are low due to the rough but regular texture of the shells creating a turbulent boundary layer and hence reducing drag.