Nature © Macmillan Publishers Ltd 1998 8 letters to nature NATURE | VOL 393 | 28 MAY 1998 363 Rapid evolution to terrestrial life in Jamaican crabs Christoph D. Schubart*†, Rudolf Diesel*† & S. Blair Hedges‡ * Fakulta ¨t fu ¨r Biologie I, VHF, Universita ¨t Bielefeld, Postfach 100131, 33501 Bielefeld, Germany ‡ Department of Biology and Institute of Molecular Evolutionary Genetics, Pennsylvania State University, University Park, Pennsylvania 16802, USA † Present addresses: Department of Biology and Laboratory for Crustacean Research, University of Southwestern Louisiana, Lafayette, Louisiana 70504-2451, USA (C.D.S.); Max-Planck-Instit fu ¨r Verhaltensphysiologie, Postfach 1564, 82305 Starnberg, Germany (R.D.) ......................................................................................................................... Crabs of the family Grapsidae are abundant organisms in most intertidal communities. However, relatively few species live in complete independence of the sea 1 . Of those species that do, Jamaica’s nine endemic species of land crabs are unique in their exceptional adaptations to terrestrial life, which include the only active brood-care for larvae and juveniles known in crabs 2–6 . These adaptations, and the morphological similarity to a group of southeast Asian land-dwelling crabs, have raised the question of the number and age of land invasions of the Jamaican species. Here we present molecular evidence that Jamaican land crabs represent a single adaptive radiation from a marine ancestor that invaded terrestrial habitats only 4 million years (Myr) ago. A Late- Tertiary origin has also been found for lizards and frogs of Jamaica 7–9 and probably reﬂects the Mid-Tertiary inundation of that island 10 . To allow survival in the freshwater environment, all Jamaican crabs of the endemic genera Sesarma and Metopaulias undergo an abbreviated non-feeding larval development phase 2,6,11 . Offspring survival is further enhanced by highly complex brood-care strategies in locally conﬁned breeding habitats 2–6 . For example, the bromeliad crab, M. depressus, raises its young in water-ﬁlled bromeliad leaf axils. The mother crab manipulates water quality by removing detritus, circulating the water to oxygenate it, and carrying empty snail shells into leaf axils as both a calcium source and a pH buffer 2–4 . She also protects the leaf axil against potential predators, including damselﬂy nymphs and spiders 2,5 . The snail-shell crab, S. jarvisi, breeds in empty shells of the snail Pleurodonte; the crab either turns the snail shells upside down to collect rainwater or carries water into the shell 6 . Both species feed their offspring, which remain in the nursery habitat for several months 2,6 . In the bromeliad crab, successive broods may coexist on the same host plant, resulting in the formation of family groups 2 . Species of the endemic Jamaican land crabs occupy various terrestrial and freshwater habitats and show different degrees of dependence on water. Morphological differences between these species are often pronounced and can be related to their habitats 11,12 . For example, the ﬂattened body of the bromoliad crab allows it to squeeze into the leaf axils of bromeliads. Such major changes in the body plan have made it difﬁcult to study the phylogenetic history of these crabs by looking at morphological characters. Initially it was suggested that the radiation of land crabs arose from the only marine Sesarma species of Jamaica, S. curacaoense, or from a related stock 11,13 . However, other morphological studies 1,14–16 have sug- gested that at least some of the Jamaican land crabs are more closely related to freshwater species (genus Sesarmoides) from southeast Asia. We obtained sequences from two mitochondrial genes, the large subunit ribosomal RNA (16S rRNA) and cytochrome oxidase I (COI), from 22 crab species of the family Grapsidae, including Metopaulias and all the American representatives of the genus Sesarma. We constructed phylogenetic trees using parsimony 17 and distance methods 18 (Fig. 1); these trees indicate that the Jamaican endemic terrestrial crabs are monophyletic, the result of a single colonization event. The closest relatives of the Jamaican terrestrial crabs seem to be marine intertidal American representa- tives of the genus Sesarma and not southeast Asian Sesarmoides. These results indicate that the bromeliad crab M. depressus should be placed in the same genus as the other Jamaican endemic crabs, of the genus Sesarma. The well constrained dating of the geological closure of the Panama landbridge (3.1 Myr ago) 19,20 allowed us to calibrate a molecular clock for the evolution of Sesarma using genetic distances between recognized trans-isthmian sister species groups (see Methods). We used this molecular clock to estimate a separation of the Jamaican endemic Sesarma and Metopaulias from their Figure 1 Molecular phylogeny of grapsid crabs, subfamily Sesarminae, inferred from an NJ analysis of DNA sequences of two mitochondrial genes, the large subunit (16S) rRNA and the cytochrome oxidase I genes. Pachygrapsus transversus (subfamily Grapsinae) was used to root the tree. Numbers are conﬁdence values from an NJ analysis using the interior-branch method 18 (above lines) and an MP analysis using the bootstrap method (below lines). Interior branches of d  0:001 and conﬁdence values of 50% not shown. The reticulatum and sulcatum groups contain the trans-isthmian species used in calibration of the molecular clock; node A represents the origin of the Jamaican lineage (see Methods). Scale bar, genetic distance.