Collaboration among sponge species increases sponge diversity and abundance in a seagrass meadow

Caribbean sponge species typical of coral reefs are generally inhibited from living in seagrass meadows by their vulnerability to predation by the large starfish Oreaster reticulatus (Linnaeus 1758). Although readily consumed by Oreaster, the conspicuous coral reef sponge species Lissodendoryx colombiensis Zea & van Soest, 1986 has expanded its habitat distribution to include a seagrass (Thalassia testudinum Banks ex Konig, 1805) meadow in Belize, where individuals grow to volumes of nearly 7 l. By simple observation, L. colombiensis appears to be an inferior competitor in this system, because portions of many individuals are overgrown by seagrass sponge species. However, experimentally clustering seagrass sponges around L. colombiensis individuals deterred starfish from feeding on them, suggesting an advantage to being overgrown. Sizes of individual L. colombiensis can fluctuate widely over short time intervals, reflecting both a relatively fast growth rate and the high rate at which starfish consume this species. At the population level these fluctuations are not evident, as losses of L. colombiensis due to Oreaster are balanced by a combination of efficient recruitment, rapid regeneration and growth, and protection of portions of many individuals by the overgrowth of seagrass sponge species that are unpalatable to Oreaster. In turn, the seagrass sponges acquire stable perches on L. colombiensis individuals in this sediment-dominated habitat. Community ecology theory relating to diversity patterns in sessile organisms has focused on competition between space-requiring neighbors as the underlying process that inevitably decreases diversity unless curtailed. Sponges, with their propensity for engaging in beneficial interactions with neighbors, demand expansion of the theory to acknowledge how collaboration can increase abundance and species diversity within a community.