Analysis of echinoderm datasets for the Arabian Gulf and Red Sea region reveals lower species richness in the Gulf at 4 different spatial scales: Point, Sample, Large area and Biogeographical province. Species richness correlates positively with spatial scale in both regions, as does endemism, although levels of the latter are comparable for both regions.

Values of taxonomic distinctness (Δ*), a relatedness measure based on average properties, are very similar for the Gulf and the Red Sea over all spatial scales. This might seem surprising, given that (i) the Gulf's environment is highly stressful, and (ii) the Red Sea is one the world's top 10 coral reef high diversity areas, and the habitat associated with most echinoderm records for this region, but not for the Gulf. These findings may partly reflect the behaviour of Δ* to ‘saturate’ (reach maximal values as sampling/spatial scale increases) much sooner than species richness, endemism and other ‘species’ measures.

Our results suggest the Gulf may not be the lowspot of biodiversity generally acclaimed, especially if a broad set of measures is utilised. Biodiversity is an important constituent of ecosystem health and criterion for determining where conservation attention should be targeted. Until the functional/ecological significance of Δ* is better understood, management implications of our study's findings remain unclear. Further attention should also be given to β-diversity, the change in species composition along a gradient. We also stress that any conclusions drawn from the study are tentative, pending compilation and analysis of datasets for other groups.

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