Relatively complex three-dimensional structures are formed by slow-growing, filter-feeders like sponges, corals and bivalves. Without light, they rely on currents and fallout from upper ocean layers for energy and also nutrients. However, their structural complexity provides habitats for a great diversity of dependent species including symbionts, microbial biofilms and associated grazers, and filter-feeding epifauna. Mobile predators like crabs and benthic demersal fish contribute to diverse communities.