SURFACING AND WATER LEAVING BEHAVIOUR OF THE FRESHWATER PULMONATE SNAILS LYMNAEA PEREGRA (MULLER), BIOMPHALARIA GLABRATA (SAY) AND BULINUS JOUSSEAUMEI (DAUTZENBERG)

Surfacing and water-leaving behaviour (quitting) of aquaticpulmonate snails was investigated in the laboratory. Regressioncoefficients of surfacing rate per hour for 16 snails in 200cm³ of water were 1.28 for L. peregra, 1.35 for B. glabrata(albino form), 0.47 for B. glabrata (pigmented form) and 0.17for B. jousseaumei. Disturbance, snail size and conditioningof the water were relatively unimportant factors; water volumeadjusted by depth was also unimportant up to 50 mm depth. Importantfactors controlling surfacing appeared to be population density,time of year, temperature, water volume adjusted by perimeterand dissolved oxygen concentration. The response to the latterwas not clear and both B. glabrata and L. peregra orientatedalmost as well to a nitrogen/water interface as to an air/waterinterface. Water-quitting regressed on volume, which had beenadjusted by perimeter variation, revealed a curvilinear relationshipfor B. glabrata, with maximum quitting at 1 snail per 7cm³ (1snail per 14cm of climbable perimeter). L. peregra showed anegative exponential curve, with maximum quitting at 1 snailper 3 cm³ of water (1 snail per 8 cm of climbable perimeter).These data could be relevant to design and management of irrigationchannels where bilharzia might occur. (Received 28 November 1990; accepted 22 October 1991)