How costly is molluscan shell erosion? A comparison of two patellid limpets with contrasting shell structures

The energetic cost of replacing shell loss due to erosion has been overlook ed in previous research on molluscan energy budgets, although several studi es have shown that shell breakage incurs a significant short-term cost and may reduce growth. We: measured the rate of shell erosion for two species o f patellid limpets: Patella granatina, which has a shell predominated by ca lcite, and P. argenvillei, in which aragonite is dominant. Highest rates of erosion of P. granatina shells (36% p.a.) were recorded when the shells we re grazed by a co-occurring limpet, P. granularis; in their absence, erosio n was 15.2%. P. argenvillei experienced a slightly lower rate of erosion (3 2.4% p.a.) when its shells were grazed by juvenile conspecifics, whereas un grazed shells lost 12.5%. P. argenvillei shells are often coated with an en crusting coralline, and infested by a boring lichen, but neither had a sign ificant effect on the rate of erosion. By modelling the energetic costs of shell erosion, we showed that the shell mass produced by P. argenvillei each year is substantially greater than th at produced by P. granatina, as is the loss of mass due to erosion. However , the energetic costs of shell production, and the losses due erosion, are higher for P. granatina, reflecting its faster growth and the higher energy content of its shells. For P. argenvillei, the cost of erosion increases w ith age from about 8-20% of the total energy devoted to "production" (somat ic growth, shell production and gonadial output). For P. granatina it is 8- 12%. Experimental elimination of erosion failed to yield higher somatic, viscera l or gonadial masses. This suggests that shell erosion does not carry a sho rt-term cost equivalent to that experienced when shells are broken. Rather, it seems that compensation for shell erosion is an ongoing process involvi ng a long-term cost. As a more explicit test of this hypothesis, we coated the shells of some individuals to protect them against erosion, left others unprotected, and coated one side of the shells of a third series of animal s. Protected shells or sides of shells became thicker than those that were unprotected. Deposition of shell continued whether the shells (or portions of shells) were being eroded or not. Thus, compensation for shell erosion i s an process of continual maintenance that is not simply initiated where an d when erosion occurs, and will carry long-term costs. (C) 2000 Elsevier Sc ience B.V. All rights reserved.