Littoraria flava is a common snail on rocky shores and stems of mangrove trees along the southeastern coast of Brazil. However, studies on this species are absent. Our previous observation indicated a distance distribution pattern among shell size classes of this snail, depending on distance from the shore and occurrence in aggregation. We hypothesized that (1) snails with large shells would occur on tree stems far from the shore and (2) snails occurring in aggregation on rocky shores would be smaller than those occurring in isolation. We evaluated the effect of desiccation on the mortality of different size classes of snails experimentally submitted to a temperature of 40°C. We measured snail shells on 14 tree stems occurring in two mangrove strips (near and far from shore). Snails on tree stems of mangrove strips far from shore presented larger shells than snails near the shore. Snails in aggregations on rocky shores were smaller than isolated ones. The desiccation experiment showed that small snails (< 1 cm) lost water at a faster rate than larger ones (> 1.55 cm) and the mortality of small snails was above 70%. The distribution pattern of L. flava is probably related to differential shell size class tolerance to prolonged periods of emersion. Also, aggregation may be a behavioral mechanism to avoid water loss, as small snails are frequently found in aggregation and lose water at a faster rate.433277281Boulding, E.G., Hay, T.K., Quantitative genetics of shell form of an intertidal snail: Constraints on short-term response to selection (1993) Evolution, 47, pp. 576-592Britton, J.C., Evaporative water loss, behaviour during emersion and upper thermal tolerance limits in seven species of eulittoral-fringe Littorinidae (Mollusca: Gastropoda) from Jamaica (1992) Proceedings of the Third International Symposium on Littorinid Biology, pp. 69-83. , J. Grahame, P. J. Mill & D. G. Reid (eds.)Butler, A.J., Relationships between height on the shore and size distributions of Thais spp. (Gastropoda: Muricidae) (1979) Journal of Experimental Marine Biology and Ecology, 41, pp. 163-194Chapman, M.G., Small-scale patterns of distributions and size-structure of the intertidal littorinid Littorina unifasciata (Gastropoda: Litorinidae) in New South Wales (1994) Australian Journal of Marine and Freshwater Research, 45, pp. 635-652Chapman, M.G., Spatial patterns of shell shape of three species of co-existing littorinid snails in New South Wales, Australia (1995) Journal of Molluscan Studies, 61, pp. 141-162Chapman, M.G., Relationships between shell shape, water reserves, survival and growth of highshore littorinids under experimental conditions in New South Wales, Australia (1997) Journal of Molluscan Studies, 63, pp. 511-529Chapman, M.G., Underwood, A.J., Influences of tidal conditions, temperature and desiccation on patterns of aggregation of the high-shore periwinkle, Littorina unifasciata, in New South Wales, Australia (1996) Journal of Experimental Marine Biology and Ecology, 196, pp. 213-237Feare, C.J., The adaptive significance of aggregation behavior in the dog whelk Nucella lapillus (L.) (1971) Oecologia, 7, pp. 117-126Garrity, S.D., Some adaptations of gastropods to physical stress on a tropical rocky shore (1984) Ecology, 65, pp. 559-574Garrity, S.D., Levings, S.C., Aggregation in a tropical neritid (1984) The Veliger, 27, pp. 1-6Levings, S.C., Garrity, S.D., Diel and tidal movement of two co-occurring neritid snails: Difference in grazing patterns on a tropical rocky shore (1983) Journal of Experimental Marine Biology and Ecology, 67, pp. 261-278Magalhaes, C.A., Density and shell-size variation of Nodilittorina lineolata (Orbigny, 1840) in the intertidal region in south-eastern Brazil Proceedings of the Fifth International Symposium on Littorinid Biology, , Developments in HydrobiologyMarchetti, K.E., Geller, J.B., The effects of aggregation and microhabitat on desiccation and body temperature of the black turban snail, Tegula, funebralis (A. Adams, 1855) (1987) The Veliger, 30, pp. 127-133McCormack, S.M.D., The maintenance of shore-level size gradients in an intertidal snail (Littorina silkana) (1982) Oecologia., 54, pp. 177-183Mcmahon, R.F., Thermal tolerance, evaporative water loss, air-water oxygen consumption and zonation of intertidal prosobranchs: A new synthesis (1990) Hydrobiologia, 193, pp. 241-260Mcquaid, C.D., The establishment and maintenance of vertical size gradient in population of Littorina africana knysnaensis (Philippi) on an exposed rocky shore (1981) Journal of Experimental Marine Biology and Ecology, 54, pp. 77-90Underwood, A.J., The ecology of intertidal gastropods (1979) Advances in Marine Biology, 16, pp. 111-210Vermeij, G.J., Intraspecific shore-level size-gradient in intertidal molluscs (1973) Ecology, 53, pp. 693-700Zar, J.H., (1984) Biostatistical Analysis, , Prentice Hall: Englewood Cliffs: New Jersey XIV + 718 pp