Nudibranch predation boosts sponge silicon cycling

Sponges are singular players in the marine silicon cycle. They accumulate vast stocks of biogenic silica within their bodies and in the sediments beneath them over long periods. These silica stocks are recycled at slow rates, much slower than that of other silicon users such as diatoms. The observation of an abrupt change in sponge biomass in a temperate coastal ecosystem led us to study the effect of nudibranch (Doris verrucosa) predation on the silicon budget of a sponge (Hymeniacidon perlevis) population on an annual scale. Predation rates and the associated sponge silicon fluxes were determined. After 5 months of predation, the abundance of sponge individuals did not change but their biomass decreased by 95%, of which 48% can be explained by nudibranch predation. About 97% of sponge spicules ingested by nudibranchs while feeding was excreted, most of them unbroken, implying a high rate of sponge silica deposition in the surrounding sediments. After predation, sponges partially recovered their biomass stocks within 7 months. This involved a rapid growth rate and large consumption of dissolved silicon, with the highest rates ever recorded unexpectedly occurring when the dissolved silicon concentration was minimal in seawater (< 1.5 M). These findings reveal that the annual sponge predation-recovery cycle triggers unprecedented intra-annual changes in sponge silicon stocks and boosts nutrient cycling. They also highlight the need for intra-annual data collection to understand the dynamics and resilience of sponge ecosystem functioning.