Molting of megalopae of the Asian shore crab Hemigrapsus sanguineus in intertidal habitats of Northeastern North America

Planktonic larvae of many benthic taxa settle and metamorphose in response to certain habitat cues. Although laboratory investigations of the importance of potential cues are numerous, field experiments are less common. We modified a field caging technique first developed for analysis of molting specificity of fiddler crab megalopae to test potential habitat cues affecting molting of megalopae of the non-native Asian shore crab Hemigrapsus sanguineus (de Haan). Potential cues consisted of waterborne substances from 2 potential habitats (cobble beach and salt marsh), and the material used to construct experimental cages. Experiments were conducted in 2002, 2003, and 2004 in Massachusetts and New York (USA). Megalopae (4 or 5 d post molt) were placed in either nylon mesh (500 μm) cages open to flooding seawater or transparent glass jars containing filtered seawater. Cages and jars were placed at identical intertidal heights at the field sites and retrieved 3 or 4 d later. The proportion of megalopae molting to the first crab stage was determined after the period of field exposure and for 2 d thereafter. More megalopae molted in cages than jars placed in the adult habitat (cobble beach) and in a marsh without adults. The cage mesh had little effect on molting response. We conclude that waterborne cues stimulate molting of H. sanguineus megalopae, and that cues are present in both the adult habitat and marsh. Molting of H. sanguineus megalopae is substantially less habitat-specific than molting of native fiddler crab megalopae.