The abundance and trophic classification of molluscs upon coral reefs in the Sudanese Red Sea

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Abstract
A quantitative survey was made of the abundance and habitat distribution of mollusks along transects on 2 fringing reefs and 2 offshore patch reefs in the Sudanese Red Sea. Each of the reefs supports a diverse and abundant gastropod fauna, with bivalves relatively uncommon. Around 70% of species are uncommon. The highest diversity and abundance of gastropods occurs on the seaward edge of each reef. The composition of the faunas from the seaward parts of the fringing reefs is similar to those of the offshore reefs, but the vegetated inshore habitats support a suite of species tolerant of terrigenous sedimentation and fluctuating environmental conditions. Gastropods occupy a number of trophic levels in reef communities and using gut content analysis and previously published work, the gastropods were classified into trophic categories. On the most exposed of the offshore reefs, coral feeding browsers are the most abundant feeding category, with predators and algal grazers less important. The other, less exposed, offshore reef is trophically complex with suspension feeders and algal grazers abundant on the reef edge; deposit feeders and browsing herbivores on the reef-top and on the sheltered western edge, coral browsers and herbivores are more common. The fringing reef are trophically more uniform, being dominated by herbivorous browsers and predators. Although molluscan diversity is lower in the Red Sea than elsewhere, limited evidence suggests that the diversity on the transects is as high as some other sites in the Indo-Pacific Province. Most of the molluscan assemblages are similar to those occupying similar habitats elsewhere in the Province. The remarkably high abundance of the large mucus-net feeding vermetid gastropod Dendropoma maxima in the Red Sea is unusual. Similar high abundances of this species were reported only from the extreme southeastern Pacific.
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