Flexibility in timing of molting of fiddler crab megalopae:evidence from in situ manipulation of cues

Rather than relying on chance delivery of larvae to experimental field substrata, we developed a field caging method to expose lab-reared crab megalopae to natural cues that could affect timing of molting to the first crab stage. Uca pugnax megalopae were placed in plastic mesh cages containing freshly collected sediments from a marsh occupied by U. pugnax, or similar sediments that had been combusted to remove organic material. Cages of each sediment type were placed at identical tidal elevations in a Spartina alterniflora marsh for 1 or 3 d of exposure to natural seawater. Significantly more megalopae molted in cages containing fresh than combusted marsh sediments (90% vs 32%. ANOVA F,,,= 53.95, p < 0.001) after a 3 d penod of exposure. Three days after the 1 d exposure period. nearly all megalopae from cages of both sediment types had molted. Furthermore, all sibling megalopae remaining In the lab during the field experiment fa~led to molt. High rates of molting of megalopae caged in the marsh w ~t h natural and combusted sediments ind~cate that chemical substances both in the water overlying the marsh and associated with marsh sediments stimulated molting of megalopae.