The complex structure of biogenic habitats including coral reefs, hemlock forests, and seagrass meadows are widely recognized for the diverse communities they shelter. As human impacts accelerate the loss of such foundation species, we need to identify the general characteristics of previously unrecognized species that can also fulfill the role of habitat provider. We conducted surveys and experiments to test whether the slate-pencil urchin (Eucidaris galapagensis) acts as a foundation species on subtidal walls in the Galapagos archipelago. The spines of slate-pencil urchins are more than 90% encrusted with a diverse epifauna, and a single urchin can host over 20 species (e.g., sponges, ascidians, bryozoans, corals, molluscs, worms, and crustaceans). Like many other foundation species, urchins can provide substrate and a refuge from predators. Urchins are consistently abundant throughout the Galapagos subtidal, and the total surface area of urchin spines can rival that of the primary rock substrate, which is significant since substrate availability can limit small-scale species richness in this system. Our experimental manipulation of spine configuration and exclusion of predators revealed that urchins also enhance epifaunal diversity by providing a predation refuge. Unlike previously recognized foundation species, however, the urchin habitat is modular and mobile, and has the potential to redistribute associated epifauna. Characterizing how previously overlooked foundation species can act as reservoirs of biodiversity in ecosystems, such as the Galapagos where other foundation species such as coral have declined, has important implications for how we identify foundation species, predict ecosystem stability, and prioritize conservation efforts.

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