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The Caribbean island of Jamaica is home to a lineage of crabs (Crustacea: Decapoda: Brachyura: Sesarmidae) which have colonized the most unusual terrestrial habitats. Today, they can be found nesting in bromeliad leaf axils or empty snail shells, scurrying on the forest floor or among dry rock rubble, climbing on walls of deep cave systems, or digging burrows in the banks of mountain streams. In order to facilitate survival of their offspring in these remarkable and new habitats, complex behavioral adaptations have evolved, including feeding and protecting the offspring, and transport of empty snail shells into the nursery to buffer the pH and provide the necessary calcium carbonate [summarized in Diesel (Proc Roy Soc Lond B 264:1403–1406, 1997), Diesel and Schubart (Evolutionary ecology of social and sexual systems: crustaceans as model organisms. Oxford University Press, New York, pp 365–386, 2007)]. No other American sesarmid crab species shows this degree of terrestriality and elaborate brood care behavior. Instead, the most related species typically are inhabitants of marshes and mangroves and reproduce by releasing their larvae into the ocean, without any additional paternal obligation. Molecular phylogenetic analyses revealed that the Jamaican endemic sesarmids are monophyletic and thus the outcome of a single colonization event, which took place approximately 4.5 million years ago and was followed by a rapid radiation, speciation, and adaptation to ecological niches, which have not been used by crabs in any other parts of the world and required evolution of complex behavioral strategies to render survival in these habitats possible (Schubart et al. Nature 393:363–365, 1998a). This system can be considered a classic example of an adaptive radiation of comparatively young age. It calls for research concerning the mechanisms and the pace of evolution to generate these highly specialized forms. In the present paper, we provide new data on evolutionary processes at the intraspecific level, which may help to understand the potential of diversification in these crabs. Furthermore, published and unpublished data available for these specialized crabs are summarized, giving evidence for the astounding diversity of evolutionary strategies and the dimension of biodiversity beyond the species level.