Geographic trends in mangrove crab abundance in East Africa

Abstract

The aim of this work was to determine the abundance of crabs in mangrove communities along a latitudinal gradient along the eastern coast of Africa from 4°S to 32°S. Surveys were made at Mombasa (Kenya), Zanzibar (Tanzania), Maputo (Mozambique) and in the Transkei (South Africa). Crabs were estimated at three designated levels in the mangroves by visual census using a common protocol, and numbers were converted to biomass. Even after standardising the selection of sites and methods of census there was still extensive variability in the data, emphasising the complex heterogeneity of mangrove ecosystems. Lunar phase (full versus new moon springs) did not have a consistent effect on results, but shore height had several effects. Total crab biomass was similar in the two lower shore strata examined, but about twice as high at the top-Avicennia level. The ratio of grapsid biomass:ocypodid biomass also changed with height: from near unity in the lower mangrove, to 0.14 in the middle strata, but to 15 at the top. There was no consistent latitudinal trend in total crab numbers, but total crab biomass increased from north to south. In addition there was a consistent and marked change in the grapsid biomass:ocypodid biomass ratio: this swung from 0.65 at Mombasa to 6.8 in the Transkei. This has implications for the transfer of primary production through the food chain. Grapsids are important macrophagous feeders on the leaves and other parts of mangroves, whereas ocypodids are microphagous deposit feeders.