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The early life cycle of brachyuran crabs has a planktonic dispersal stage consisting of a variable number of zoeal larvae followed by the molt to the megalopa stage. Megalopae undergo horizontal transport to the settlement site where they settle out of the water column and metamorphose to the first crab (juvenile) stage. This review provides an overview of recent laboratory studies of cues that shorten or lengthen the time to metamorphosis (TTM) of the megalopa stage. Megalopae cannot delay metamorphosis indefinitely and have a temporal threshold beyond which metamorphosis occurs without habitat cues. The TTM can be shortened about 15–25% upon exposure to acceleration cues, which include chemical cues and odors from adult substrate, aquatic vegetation, biofilms, conspecifics, estuarine water, humic acids, related crab species, and potential prey. Cues shown to delay metamorphosis include ammonium, hypoxia, predator odor and extreme temperature and salinity conditions. There is no evidence that structural mimics of natural substrate affect TTM.
Forward et al. (2001) studied this question.