MEPS 181:131-139 (1999)  -  doi:10.3354/meps181131

Cues in salt marshes stimulate molting of fiddler crab Uca pugnax megalopae: more evidence from field experiments

Nancy J. O'Connor1,*, Michael L. Judge2

1Department of Biology, University of Massachusetts Dartmouth, N. Dartmouth, Massachusetts 02747-2300, USA
2Department of Biology, Manhattan College, Riverdale, New York 10471, USA

ABSTRACT: Most marine benthic invertebrates possess complex life histories that involve a planktonic larval stage that must seek the proper adult habitat. For practical reasons, the search for cues that stimulate settlement and metamorphosis to the benthic stage have traditionally utilized either laboratory studies or field correlations between larval and adult distributions. In contrast, we have developed an in situ technique that employs lab-reared fiddler crab Uca pugnax larvae placed in mesh cages in realistic estuarine settings to address the roles of potential adult habitat cues for molting. We extended our prior work to examine the influence of sediment type on molting of megalopae. In addition, we manipulated the duration of field exposure of megalopae to cues. In 2 series of experiments (1996 and 1997), we examined the importance of the nature of cues (waterborne vs sediment), habitat (sandflat vs salt marsh), duration of exposure to field cues (1 vs 3 d), and field test site (different marshes within an estuary). Freshly collected marsh sediment stimulated molting to the benthic first crab stage, whereas fresh sand did not. Contact with natural seawater overlying a marsh also stimulated molting compared to control megalopae in filtered seawater in the lab or field. Moreover, the response appeared additive: the highest molting rates were observed when megalopae were exposed to both marsh sediment and seawater. Cages on the sandflat remained intact but filled with sediment, and almost all megalopae died before molting. Three days of field exposure to marsh sediments and seawater had a greater effect on molting than 1 d of exposure, although megalopae were stimulated to molt following only the 1 d exposure period. In addition, results were consistent in marshes that differed in sediment particle size. Consistency in response among years and between marsh sites suggests a strong response of fiddler crab megalopae to natural marsh ecosystems.

KEY WORDS: Fiddler crab · Uca pugnax · Larvae · Molting · Metamorphosis · Chemical cues

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