The determinants of species richness of a relatively young coral-reef ichthyofauna

Aim Unique topographic features left the Red Sea and its north-eastern extension into the Gulf of Aqaba practically devoid of coral-reef-based organisms during the last glacial maximum. The current ichthyofauna in these two 'regions' thus represents the product of relatively recent colonization by species found in the Arabian Sea, which adjoins the Red Sea at its southern tip. We used this system to test why some marine species seemingly fail to extend their geographic range, thereby generating spatial heterogeneity in biodiversity. Location The Arabian Sea, Red Sea, and the Gulf of Aqaba. Methods A list of coral-reef-associated fish species, belonging to the 10 most speciose families, was compiled for each region using published sources. The data were analysed (major axis regression, randomization tests) for taxonomic and body-size-dependent biases in colonization probabilities. A simple probabilistic model was used to examine the potential contribution of local (within-region) extinctions to determining species composition in the Red Sea. Results Of the 462 reef-associated species that inhabit the Arabian Sea, 69% have crossed successfully into the Red Sea; of these, 55% have crossed into the Gulf of Aqaba. A species' probability of being found in either 'target' was independent of presumed innate differences, i.e. ecological correlates of taxonomic affiliation and body size. Similarly, local extinctions were found unlikely to have been of consequence over the past several thousand years. Main conclusion Present-day differences in the species richness of reef-associated fish species among the Arabian Sea, Red Sea and Gulf of Aqaba appear to be the product of external, non-selective constraints on colonization. The random nature of the colonization process is suggestive of ecological redundancy among coral-reef fish species. Importantly, the study places a time frame on the processes that determine spatial patterns of biodiversity in reef fish.