Dual stable isotope analyses (delta 13C and delta 15N) of fiddler crabs from a forest-fringed, land-locked lagoon in Puerto Rico indicated the differential assim- ilation of material from ingested sediments. Fiddler crabs preferentially selected foi niitrogen-fixing benthic microalgae (cyanobacteria) over vascular plant detritus. These results question the assumption that mangrove detritus is always the prin- cipal source of energy to estuariiie consumers. Previous research fiom this lagoon as well as from Amazonia suggests that the magnitude of lan-d-wvater ecotonal coupling may be low for these particular tropical systems where benthic algal productivity is presumably high.