Bilharzia Snails and Water Level Fluctuations in a Tropical Swamp

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Abstract
The snail, Bulinus globosus (Morelet), intermediate host of the [human] biharizia parasite, Schistosoma haematobium, is common in the marginal swamps of Lake Chilwa, Malawi. Most snails live in the marsh on the periphery of the Typha swamp; an unstable habitat which is seasonally inundated during each rainy season when the shallow lake expands. Water level fluctuations are important in affecting the distribution and abundance of B. globosus. When the lake rises, snails move actively with the advancing margins along canoe channels cut by fishermen and disperse into the marsh to lay eggs on lily leaves (Nymphaea sp.). Later in the year, the lake recedes and the high juvenile mortality is related to mating of marsh plants and low levels of dissolved O2 in the water. Eventually, snails are confined to the deeper canoe channels which act as reservoirs from which colonization of the marsh occurs during the next rainy season. Heavy rains delay the recession of the lake and improved juvenile survival leads to the large increases in snail numbers observed periodically.