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

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Three microhabitat types (dead coral fragments, coral gravel and coral sand) were distinguished and sampled at two locations (Matemwe and Makunduchi) in a tropical lagoon (Zanzibar Island, Tanzania), and the community structure, habitat preferences and biodiversity of the associated harpacticoid copepod fauna was investigated. The harpacticoid fauna is affected by sediment granulometry and by the structural differences between coral and both gravel and sediment. The coral fragments contained a specific assemblage composed of typical 'phytal' taxa (Tisbe, Paradactylopodia and Dactylopusia) along with other eurytopic and sediment-dwelling forms (Ameira, Ectinosoma and Amphiascus), which may be attracted by the sediment retained between the coral branches. The assemblages of coral gravel and upper sediment layer did not differ significantly from each other and had mostly the same dominant genera. The sediment from Matemwe was dominated by the interstitial Paramesochridae and the sediment from Makunduchi by Tetragonicipitidae. The coral fragments from Makunduchi sustained a more diverse assemblage than gravel and the different sediment layers. It was assumed that coral form and complexity, with implications for habitable space, nutritional resources and level of predation, are important in structuring diversity of the associated assemblage.

Copepods are known as important consumers of primary production and are eaten by larger animals. They therefore form a main link to higher trophic levels. While feeding pathways and specificity of planktonic copepods have been well studied, the selectivity of the benthic harpacticoid copepods is far less documented. A better knowledge of the functional ecology of harpacticoids as important grazers on primary producers may have consequences for the re-evaluation of basic energy flow in benthic ecosystems.We tested whether size selectivity for diatoms exists in harpacticoid copepods. We hypothesized that size selectivity of harpacticoid copepod species is strongly related to body size. Because of morphological constraints, we expected smaller copepods to prefer smaller diatoms while larger copepods should be able to consume both small and large diatoms. We tested this hypothesis in four harpacticoid copepod species of varied body size: Tigriopus brevicornis, Harpacticus obscurus, Amphiascus minutus and Paramphiascella fulvofasciata. As food source we used two 13 C labelled strains of the benthic diatom Seminavis robusta with a four-fold difference in cell biovolume.Three out of four harpacticoid species showed size selectivity: H. obscurus and A. minutus preferred the larger Seminavis cells, while P. fulvofasciata selected the smaller Seminavis cells. Based on monoclonal treatments, there was no clear preference found for T. brevicornis although there was a small preference for large cells in the mixed treatments. Except for P. fulvofasciata, all species showed a lower uptake when offered the mixed diet (both small and large cells). Although most species showed a size selectivity, our results suggest that this selectivity was not related to their body size. However, the only species that ate significantly more of small diatoms was characterised by comparatively small mouthparts in relation to its body size. D