Molting of fiddler crab Uca minax megalopae: Stimulatory cues are specific to salt marshes

Abstract

Waterborne chemicals can be habitat cues for pelagic larvae of benthic invertebrates by stimulating settlement and metamorphosis. Less clear is whether cue effectiveness declines with increasing distance from the source. We examined whether the ability of seawater to stimulate metamorphosis (molting) of fiddler crab Uca minax megalopae to the first-crab stage is restricted to water overlying marshes. Laboratory-reared megalopae were enclosed in situ for 3 d within nylon mesh cages in both a marsh and a river site 15 m away from the marsh. Even in the absence of contact with sediment, megalopae in the marsh had high rates of molting, whereas those in the river molted at very low rates similar to control megalopae enclosed in glass jars containing filtered seawater and deployed in the marsh habitat. In a second experiment, we tested for a dose-dependent molting response from the presumed source of the cue. We hypothesized a negative linear relationship in molting frequency with increasing distance from the marsh. Megalopae were enclosed for 3 d in cages within the marsh and at distances of 2, 5, and 15 m away from the marsh edge, and in jars within the marsh. Molting response was greatest in the marsh, intermediate 2 and 5 m away from the marsh, and lowest 15 m from the marsh and in the control jars (test of regression slope, F1,25 = 13.290, p = 0.001, r2 = 0.321). Therefore, chemical cues for molting of fiddler crab megalopae originate in marshes and decline in effectiveness within a short (<15 m) distance of the marsh habitat.