The Asian shore crab, Hemigrapsus sanguineus, is an invasive species along the Atlantic coast of North America. It has spread rapidly since its discovery in the USA in 1988 and has become the dominant crab species in rocky intertidal habitats at many locations in the Middle Atlantic Bight and in the southern Gulf of Maine. The reproductive biology of H. sanguineus has received only cursory study in native east-Asian waters. Available information consists mainly of descriptions of the larval stages and general characterizations of the natural history of the species. This study examined the periodicity of larval release in an invasive population of H. sanguineus near its original site of introduction at the mouth of Delaware Bay. We investigated larval release at diel and tidal frequencies. Analysis of laboratory data showed that larval release always occurs near the time of nocturnal high tide. Field sampling confirmed the presence of stage I zoeae in waters near adult habitat at the expected time of larval release. This pattern of release minimizes the impact of visual predators on newly hatched zoeae and takes advantage of circulation at tidal frequency to transport zoeae away from intertidal habitats where conditions may not be favorable for larval development. However, H. sanguineus is not unique among intertidal crabs in releasing larvae near the time of nocturnal high tide. This pattern has been reported for a large number of species with wide representation among families. Thus, the pattern of release observed in H. sanguineus does not provide any special competitive advantages over co-occurring native species, and the remarkable success of the species in North American habitats may be attributable to competitive interactions occurring after settlement and metamorphosis.
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