Feeding behaviour and selection of bivalve prey by Octopus vulgaris Cuvier

Abstract

Octopus vulgaris in South Africa preys primarily on mussels. Small mussels are simply pulled open, while large ones are drilled first and then pulled open. Medium mussels fall into an overlap size range and may be opened by either method. Actively pulling a mussel open was estimated to have an energetic cost 4.5 times that of resting metabolism, but active pulling lasts for only seconds and forms a very small proportion of handling time. The energetic cost per minute of handling prey while using force is only 1.29 times greater than when they are drilled. Curves of rate of energetic return ( E/ H) against mussel size were derived from laboratory measurements. Curves varied for individual octopuses and the technique used to open mussels. They were monotonie with sharp discontinuities between the two techniques in the overlap size range. Drilling takes much longer than using force alone and produces lower E/ H values. Mussels just smaller than the length at which 50% are drilled (L drill50) are “optimal” in terms of maximum E/ H values. L drill50 increases with octopus weight. In the laboratory octopuses selectively took mussels with maximum E/ H. The force required to remove mussels from rocks in the laboratoryis less than that needed to open them, but increases with mussel size. Previously published data on prey selection in the field indicate that octopuses of > 500 g take the largest mussel size classes. Below 500 g, mean prey size in the field is related to octopus weight. Small octopuses may take smaller mussels because of they are unable to remove large ones from the rocks.