Lethal and sub-lethal effects of phototrophic endoliths attacking the shell of the intertidal mussel Perna perna

Animals that bore into calcareous material can cause considerable damage to molluscan shells. In contrast, smaller microbial phototrophic endoliths ha ve until recently been thought of as relatively benign. Phototrophic endoli ths (primarily cyanobacteria) infest the shells of 50 to 80% of midshore po pulations of the mussel Perna perna (L.) in South Africa. This infestation causes clearly visible shell degradation, and we record here ecologically i mportant lethal and sub-lethal effects (e.g. changes in growth and reproduc tive output) of the endoliths on their mussel hosts. Endolith infestation r educed the strength of shells significantly and also affected shell growth. Insitu marking of shells, using the fluorochrome calcein, showed that infe sted and non-infested mussels increased in shell length at the same rate. H owever, the rate of increase in shell thickness (associated with shell repa ir) was significantly faster in infested than in uninfested individuals. Th is increase in the rate of shell thickening was not sufficient to compensat e for rapid endolith-induced shell degradation and, around the site of addu ctor muscle attachment, infested shells were thinner than their uninfested counterparts. The shells of 18% of recently dead mussels had holes induced by endolith erosion. This effect was highly size dependent, and the proport ion of mortality due to endoliths rose to almost 50% for the largest mussel s. The re-routing of energy due to shell repair had important sub-lethal ef fects on the reproductive rates of mussels. During the reproductive period, mean dried flesh mass for large (>70 mm), non-infested P. perna was substa ntially higher than for infested individuals. This difference was almost en tirely due to differences in gonad mass, which was approximately 100% highe r for non-infested mussels. We conclude that, by attacking the shell, photo trophic endoliths reduce both the longevity and reproductive output of larg e mussels on the midshore.