Site selection by a small drilling predator: Why does the gastropod Haustrum baileyanum drill over muscle tissue of the abalone Haliotis rubra?

Abstract

The muricid gastropod Haustrum baileyanum bores holes through the shell of the abalone Haliotis rubra to feed on the underlying tissue. Analysis of the borehole positions on shells of H. rubra showed that they were significantly concentrated in the area where the abalone muscle is attached. This is in sharp contrast to other drilling predators of molluscs. The whelks did not change their choice of drilling site in response to experimental removal of the shell surface, or to changes in shell thickness. They may use shell ridges and possibly the morphology of the abalone shell to determine where to drill. The behaviour of the prey appears unlikely to affect the drilling position, and shell repair by the abalone is too slow to act as an effective defence in any area. The area over the muscle is the thickest part of the shell covering abalone tissue, and requires a much longer time to penetrate, as drilling is slow. The cost of drilling through the thicker part of the shell is presumably outweighed by the benefits gained from feeding on the muscle tissue. The muscle tissue has a higher energy content than the visceral tissue found elsewhere, especially when measured per unit volume.