Trophic dependence of the emerald neritid Smaragdia viridis (Linnaeus, 1758) on two seagrasses from European coasts

Abstract

separately in plastic containers with natural sea water. Using a binocular microscope, shoots of both seagrasses were examined and it was found that 10 – 15% of them displayed characteristic scraping marks along the central part of the leaves. In other shoots, the typical marks of the herbivorous fish Salpa salpa were also seen on the edges of the leaves. Shoots free of any type of marks were selected as food for S. viridis individuals in order to test if it was able to feed on these seagrasses. In each experimental set-up, one shoot of Z. marina or C. nodosa was placed inside a plastic tray (35 24 cm) with natural seawater together with one individual of S. viridis located in the corner of the tray. Only one species of seagrass was given as diet to each individual. Each experiment was replicated 2 – 3 times for each juvenile and adult individual of S. viridis. A total of 41 experiments were carried out, including 22 experiments with C. nodosa and 19 with Z. marina. Each experiment was run for 24 h. At the end of each experiment, faeces egested by S. viridis were collected using a Pasteur pipette and examined for the presence of digested Z. marina or C. nodosa tissues under the microscope. Shoots of both seagrasses were examined for the presence/absence of radular marks of S. viridis under a binocular microscope after the experiment. Some measurements of radular marks on seagrass leaves and in faeces were made using image analysis and the software Visilog 6. In each experiment, all individuals moved fast to the Z. marina or C. nodosa shoot once placed in the trays. After 2 h, individuals of S. viridis moved slowly along the leaves in both seagrasses and started to leave radular marks while eating the epidermal tissues of the plants. In experiments using Z. marina, radular marks of S.