LEAF-LITTER REMOVAL BY THE SNAIL TEREBRALIA-PALUSTRIS (LINNAEUS) AND SESARMID CRABS IN AN EAST-AFRICAN MANGROVE FOREST (GAZI BAY, KENYA)

Quantitative data on leaf litter removal activity of macrozoobenthic o rganisms in the mangrove forests of East Africa are virtually non-exis tent. In the present study, litter removal activity was determined in two contrasting types of mangrove stands in Gazi Bay (Kenya). In the r elatively elevated Ceriops tagal vegetation, which is only flooded dur ing spring tides, the detritivorous snail Terebralia palustris (Linnae us) was the major macrobenthic organism responsible for litter removal . Analysis of the delta(13)C value of the foot tissue of the snail ind icated a segregation in the food consumed by individuals below and abo ve a size of 50 mm, in agreement with the observation that only larger individuals were feeding on the leaf litter. In the low lying Rhizoph ora mucronata stand, which is flooded by each high tide, the crab Sesa rma guttatum (H. Milne Edwards) was responsible for most of the litter removal (consumption and burial). The availability of water in the C. tagal stand, caused by tidal inundation or by rainfall, was a determi ning factor in the amount of litter being removed. When the stand rema ined dry around neap tides, the median litter removal, as a percentage of the litter fail, was only 0.8%. Under wet conditions around spring tide this percentage was much higher: 41.6% by night and 25.2% by day , respectively. These figures reflect the behaviour of T. palustris, w hich is inactive under dry conditions in order to avoid desiccation. M edian litter removal in the R. mucronata vegetation, expressed as a pe rcentage of the litter fall, was 40.3% by day and 21.7% by night. No r elation was observed between lunar cycle and activity of the litter pr ocessing crabs. Taking into consideration differences in inundation fr equency and duration, and in litter removal activity by benthic animal s as related to tidal height and day/night cycles, we estimate that in this East African mangrove, on average. 11.2% and 18.6% of the fallen litter is processed by macrobenthic animals in the C. tagal and in th e R. mucronata vegetation, respectively. Our results indicate that rem oval of fallen leaf litter in mangrove forests is not effected by bent hic communities dominated by crabs only, but that activities of litter feeding snails may also be significant. (C) 1997 Elsevier Science B.V .