I examine the natural barriers to distribution and colonization that have shaped the Galapagos marine invertebrate biota. While diversity is high for some groups, such as hydroids and bryozoans, it is low for many others. Porcelain crabs and molluscs are examples with reduced or unbalanced representation in Galapagos, resulting from their dependency on dispersal of relatively short-lived planktonic larvae by ocean currents and on habitat limitations in Galapagos. Because Galapagos shorelines are predominantly rocky, without the wide expanses of silt typical of much of the Ecuadorian mainland that are favored by infaunal bivalves, gastropod diversity in Galapagos far exceeds that of bivalves. Nearly all hermatypic corals in Galapagos are members of the Panamic province; none is endemic to Galapagos. This suggests that colonization occurred by larval dispersal from there. The ahermatypic (azooxanthellate) coral fauna of Galapagos, with 43 species, is richer and more diverse than the hermatypic corals, with 29% of the shallow-water ahermatypes endemic and the remainder with Panamic, Indo-Pacific, and cosmopolitan affinities. The 73 verified species of Galapagos shallow-water echinoderms are dominated by Panamic species, with additional affinities to the Indo-Pacific and the California province; 8% are cosmopolitan and 8% endemic. With species richness roughly equivalent to that of Pacific Colombia, Galapagos echinoderm representation is not depauperate, but is sufficiently distinctive to characterize it as an isolated, insular biota. Hydroids and bryozoans, two groups with high diversity in Galapagos, accomplish long-distance transport mainly as adults on floating debris and hulls of ships, rather than by the free-swimming reproductive stage. Endemism among marine invertebrates averages 18.3 %, but varies widely among major taxa, from 0% for reef corals to 71% for gorgonians. Unlike the Galapagos terrestrial biota, in which endemic genera are common, the absence of endemic genera among marine invertebrates may be attributed to low isolation arising from greater dispersal and gene flow in the marine environment
