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

Sponges stand out among sessile marine animals in the great variety of collaborative associations in which they participate. When intimately associated organisms repre-sent taxa with very different characteristics, the ways in which they might benefit each other are readily apparent. For example, non-sponge partners can provide mobility (e.g. decorator crabs, Stachowicz & Hay 2000), solid sta-ble substratum (e.g. clams, Marin & Belluga 2005; man-grove roots, Ellison et al. 1996), skeletal reinforcement (e.g. macroalgae, Rützler 1990; Carballo & Ávila 2004), or food (e.g. zooxanthellae, Rosell & Uriz 1992) for sponges, while the unusual chemistry and flexible morphology of sponges enable them to provide protective coverings or homes (review in Wulff 2006b). How an intimate associa-tion can be mutually beneficial is less clear when both participants are sponges. In particular, associations that involve overgrowth of one sponge by another, blocking water intake surfaces of these efficient filter feeders, see