Marking hard-shelled gastropods: tag loss, impact on life-history traits, and perspectives in biology

Individual marking has long been used in many fields of biology. However, capture-mark-recapture (CMR) studies are not evenly distributed among taxonomic groups, with most studies focusing on vertebrates. For example, a limited number of studies have been conducted in gastropods, in sharp contrast to their important role as biological models or exploited natural resources. The lack of standard, validated marking techniques certainly contributes to the limited use of CMR. In this article, we evaluate two fundamental requirements for a marking technique to be suitable, i.e., tag loss (two experiments) and impact on life-history traits (survival, growth, and fecundity, in three experiments). Five marking techniques for hard-shelled gastropods were tested in the freshwater snail Physa acuta (Pulmonata). The tag-loss rate per month was lower for glued plastic marks (0.01) than for paint marks (0.03-0.07), and the tag-loss rate varied among colors (gouache paint). Under laboratory conditions, the life-history traits were not significantly affected by marking. We thus recommend using glued plastic marks for long-term studies, and paint marks for mass marking, with double marking to account for tag-loss rate. Based on an extensive literature survey, we also review current CM R practices in gastropod studies and suggest general improvements that would prove useful in both this group and invertebrates in general.