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Sponges, like other sessile organisms, have developed chemical defences against predators; yet, certain organisms such as opisthobranchs have specialized to overcome these defences in order to feed on sponges and, commonly, live on them. Although numerous cases of specialized grazer-sponge interactions are known, there is limited research on grazing pressure on natural populations. Grazing may add an additional stress to abiotic factors like warming and acidification, which are already detrimental for marine life. We are running a citizen science project, “Bite a sponge” (www.biteasponge.com), aiming at (i) explore predator prevalence on sponge populations over time, and (ii) investigate whether there is a correlation between the life cycle, the aggregation patterns and the seasonality of these predators and the sponge health. This citizen science project has as well an educational component, using opisthobranchs as flagship species to increase awareness on sponge diversity and their ecological role in the ecosystem. Our study case is the predator-sponge interaction between Aplysina aerophoba and the heterobranch Tylodina perversa. The project is supported by a group of 8 experienced divers from the “Club d’Immersió de Biologia”, the diving club based at University of Barcelona. Together with these volunteers, we are running a survey of the sponge population in l’Escala (Girona). We combine two monitoring approaches, seasonal transects in different depths (0-5, 5-10, 10-15m) and picture tracking of 20 sponges over 14 months, with and without grazer, distributed in the three different depth ranges, and including 3 different …