Microalgal availability and its consumption by Siphonaria pectinata (L., 1758) on a rocky shore

Siphonaria pectinata (L., 1758) is a pulmonate gastropod of the order Basommatophora, which has adopted a lifestyle superficially similar to that of patellogastropod limpets, and is thus often referred to as a «false limpet». In this paper, we present the results of a study investigating microalgal availability and resource utilization by this species in its intertidal habitat, on a rocky shore off Gibraltar (southern Iberian Peninsula). A number of different experiments are described, which serve to provide an overview of the main food types and their relative availability to this species. These include the sampling of the surface microbial algal film, scanning electron microscopy (SEM) and light microscopy of rock chips and slurry, radular structure, gut contents and faecal pellets. The results of this study suggest that S. pectinata does not feed on encrusting «hard» algae, but rather uses its fine-toothed radula to graze superficial, soft algae, including spores and emerging germlings. We suggest that such a feeding strategy would allow it to reduce competition with other patellogastropod limpets, which tend to scrape deeper into the rocky substrate while foraging. Possible avenues for further research are discussed.