Sponge Coating Decreases Predation On The Bivalve Arca Noae

Abstract

Mediterranean populations of the bivalve Arca noae were found to be specifically fouled by the demosponge Crambe crambe. The surface of the shells forms spines which favour settlement of the sponge larvae. The reproductive strategy of this symbiotic sponge contrasts with other Mediterranean sponges, which produce numerous small larvae, while Crambe crambe produces relatively small numbers of quite large larvae containing secondary metabolites. The selective shell fouling of Arca noae suggests that this is behavioural farming of a chemically defended sponge by a bivalve which does not sequester or synthesize de novo metabolites. Some of the observed benefits of this symbiosis are the enhanced survival of the bivalve, and the sponge being able to live in a site free of competitors. This mutual symbiosis is optional for the sponge but essential for the bivalve, since the fouled bivalves can use the secondary metabolites produced by the sponge to deter predators. We tested the effects of sponge epibiosis on predator preferences. Feeding trials were carried out on the starfish Marthasterias glacialis and the snail Hexaplex trunculus in Hornillo Bay, SE Spain. When A. noae fouled by the sponge Crambe crambe were offered to starfish and snails, they survived significantly more than unfouled bivalves. Field studies in the Mar Menor lagoon indicate that the aposymbiotic populations of A. noae have disappeared from this hypersaline lagoon due to the biological invasion by the non-native snail Hexaplex trunculus. This symbiotic relationship was also effective against octopus. Aposymbiotic bivalves were rapidly eaten when presented to octopuses, while fouled bivalves were consistently rejected.