PATTERNS OF ENDEMISM FOR NEARSHORE GASTROPOD MOLLUSCS: A NEW VIEW OF THE GULF OF CALIFORNIA

Abstract

The Gulf of California, Mexico, is often viewed as an insular environment for marine faunas. Moreover, its geographical history, geomorphology, latitudinal orientation and a convergence of three different water masses at the mouth of the Gulf make it an ideal setting for isolation, and probably contribute to the expectations of high endemism. However, rigorous analyses of biogeographic patterns are an essential prerequisite to an understanding of evolutionary processes in the Gulf and other biogeographic realms. A new 'sliding window' technique is presented here to test whether the Gulf of California is an isolated habitat. Endemism in the Gulf of California is evaluated by comparing the number of endemic gastropods for the Gulf of California with the number of endemic gastropods found in neighbouring coastlines of the same coastal length by using 'sliding window' analysis. Range sizes and diversity within the Gulf are compared with regions outside of the Gulf using variations of'sliding window' analysis. In addition, examining range endpoints and the percentage of ranges that lie across the mouth of the Gulf of California tests the hypothesis that the mouth acts as a barrier to migration into and out of the Gulf. The results of these analyses do not support the hypothesis that the nearshore Gulf of California faunas are structured by this supposed isolated body of water.