Grazing by the intertidal gastropod Melampus coffeus greatly increases mangrove leaf litter degradation rates

Abstract

Melampus coffeus, a pulmonate gastropod, forages for mangrove leaf litter at low tide and climbs tree trunks to avoid inundation during high tide. Unlike many grazers, these snails can assimilate mangrove leaf material. At Boca Ceiga Bay, Florida, densities of adult snails were high (> 100 snails m -2) throughout a 130 m wide intertidal zone. A mark-recapture study indicated that over the course of 1 mo 48.6 ± 6.1% of snails returned to their initial tree during high tides, while those that did relocate moved 6.66 ± 0.60 m and were recorded from 33 new trees at high tide. A field experiment comparing tethered leaves with small and large litter bags showed that snail grazing greatly increased the rate of red (Rhizophora mangle) and black (Avicennia germinans) leaf litter breakdown. Grazing by M. coffeus resulted in 90 % weight loss in < 4 wk (A. germinans) and 7 wk (R. mangle), compared to slower breakdown in litter bags (12 to 26 wk, A. germinans; > 26 wk, R. mangle). Another experiment showed greater leaf litter accumulation on the forest floor in plots where M. coffeus was excluded. The decomposition coefficient (k) was 10x greater when M. coffeus grazing was allowed than k for any studies that enclosed leaves in litterbags. During our experiment, M. coffeus consumed an estimated 40.5 % of mangrove leaf fall, and 19.8 % of leaf litter was exported as particulate or dissolved material. This M. coffeus population can produce an estimated 3 × 106 larvae m -2 yr-1, Which suggests that the larval pathway is an important conduit of mangrove leaf resources to the estuarine food web. © Inter-Research 2005.