Ecological aspects of soft bottom meiofauna in Eastern Africa

Abstract: The meiobenthos is numerically the most abundant metazoan group in shallow water habitats. While the meiobenthos characteristically shows variable distribution, the causal factors controlling this variation remain poorly understood. It has previously been shown that the distribution of the meiobenthos in tropical benthic systems is influenced by several factors. In this thesis the spatial and temporal distribution patterns of the major meiobenthic taxa in Zanzibar and the meio-macrofauna interactions were studied in both field surveys and laboratory experiments. In this study it was found that the distribution of major meiobenthic taxa varied considerably between the different benthic habitats. Nematoda was found to be the dominant meiobenthic group in all studied habitats, with over 100 genera identified. Sediment granulometry was found to be the most important factor influencing meiobenthic communities, although the distribution of some faunal groups were also shown to be correlated to chlorophyll a and organic matter content. However, in some habitats no correlation could be found between meiofaunal distribution and physico-chemical factors. Among different habitat-types the nematode assemblage was found to be very distinct. The major meiofaunal groups showed distinct seasonality in abundance, indicative of the seasonal change in invironmental factors. This was vividly shown by the most abundant nematode genera and feeding-types. However there was lack of consistency and uniformity between years and among habitats. The causes of such observations were attributed to seasonality in environmental factors, annual variations and shore-level differences which lead to differences in biotic and abiotic pressures on the meiobenthos. Differences in meiobenthos density and nematode assemblage were noted between farmed and unfarmed areas of the lagoon. This was found to be due to predation by fish and/or the sediment surface brushing effect of the algal thalli. The intertidal fish Gerres oyena of size < 5 cm was found to feed exclusively on harpacticoid copepods but fish of size < 3.5 cm took also large numbers of larval polychaetes. However prey selectivity was shown to vary depending on food availability. Furthermore, a laboratory experiment on the effect of mangrove crabs on meiofauna indicated that their feeding habits did not significantly alter the structure of the nematode assemblage found in benthic sediments. However the density of harpacticoids was influenced. It was concluded that the ocypodid crabs do not regulate resident assemlages of nematodes, but may inhibit the settlement of colonisers that have not adapted to the intense surface disturbance or predation by these crabs.