Biological structures exert a major influence on species diversity at both local and
regional scales on deep continental margins. Some organisms use other species as
substrates for attachment, shelter, feeding or parasitism, but there may also be
mutual benefits from the association. Here, we highlight the structural attributes
and biotic effects of the habitats that corals, sea pens, sponges and xenophyophores
offer other organisms. The environmental setting of the biological structures
influences their species composition. The importance of benthic species as
substrates seems to increase with depth as the complexity of the surrounding
geological substrate and food supply decline. There are marked differences in the
degree of mutualistic relationships between habitat-forming taxa. This is especially
evident for scleractinian corals, which have high numbers of facultative
associates (commensals) and few obligate associates (mutualists), and gorgonians,
with their few commensals and many obligate associates. Size, flexibility and
architectural complexity of the habitat-forming organism are positively related to
species diversity for both sessile and mobile species. This is mainly evident for
commensal species sharing a facultative relationship with their host. Habitat
complexity is enhanced by the architecture of biological structures, as well as by
biological interactions. Colony morphology has a great influence on feeding efficiency
for suspension feeders. Suspension feeding, habitat-forming organisms
modify the environment to optimize their food uptake. This environmental
advantage is also passed on to associated filter-feeding species. These effects are
poorly understood but represent key points for understanding ecosystems and
biodiversity on continental margins. In this paper we explore the contributions of
organisms and the biotic structures they create (rather than physical modifications)
to habitat heterogeneity and diversity on the deep continental margins.