Coral reef fishes vary dramatically in the extent of their geographic distributions, and the patterns, causes, and consequences of this extensive variation have long interested coral reef fish ecologists. Although there is still a great deal of uncertainty regarding the drivers of range size variation in coral reef fishes, research over the last several decades has greatly improved our understanding. Ecologists once suspected that larval traits, primarily pelagic larval dispersal potential, had a strong influence on species' ranges, but recent synthetic work has shown that larval dispersal only impacts range sizes when ranges cross the greatest dispersal barriers in the Pacific. Emerging work suggests that adult traits, such as body size, habitat preferences, and even nocturnal activity are correlated with range size, likely by increasing persistence of newly established populations. Processes that operate over evolutionary scales are also likely important, but it has been challenging to empirically examine these factors. However, the quantity and availability of biological information are increasing rapidly, providing reef fish ecologists with richer datasets with which to evaluate a broader range of hypotheses.

One of the most fundamental traits of a species is the size of its geographic range. Coral reef fishes show dramatic variation in range size, and even closely related species can have vastly different geographic extents. For example, the surge wrasse Thalassoma purpureum extends from South Africa to Central and South America, a span of nearly 28 000 km, while the Clipperton wrasse Thalassoma robertsoni is endemic to Clipperton Atoll in the tropical eastern Pacific, an island approximately 6 km long (Figure 10.1). Therefore, it is not surprising that this variation has interested coral reef biologists and biogeographers for decades.

The size of a species' range is likely the product of a wide variety of biotic and abiotic forces and as such can provide great insight into biogeographic and evolutionary patterns. Geographic range size can also have a large impact on a species' extinction risk, since widespread species are thought to be much less vulnerable to extinction than geographically restricted species [357, 915].