Molluscs in Mangrove Swamps: Physiognomy, Diversity, and Regional Differences

Vermeij, G. J. (Department of Zoology, University of Maryland, College Park, Maryland 20742) 1974. Molluscs in Mangrove Swamps: Physiognomy, Diversity, and Regional Differences. Syst. Zool. 22:609–624.—This paper on mangrove-associated molluscs is designed to provide comparative data on physiognomy, diversity, geographic distribution of species, and similarities and differences between parallel assemblages in various geographic regions. In Singapore, both the absolute range and the upper limit of the range of maximum adult size and three other parameters of shell shape are smaller in the littoral-fringe tree-associated gastropod assemblage than in either the intertidal tree-associated or sediment-associated groups. Shells in three of the four families in the littoral-fringe exhibit either internal whorl resorption or apical decollation, possibly resulting from acidic conditions during anaerobic respiration while the animals are exposed to air. Intertidal tree-associated gastropods are morphologically diverse, but there is a tendency toward squatness related to the need for maintaining gravitational stability on hard substrata. In soft sediments, this requirement is obviated, and the diversity of form is even greater. Aside from a number of taxonomic differences between the mangrove assemblages in different provinces, most within-habitat differences in physiognomy and width of geographic distribution are related to species richness. The absolute range, upper limit of the range, and number of categories for all parameters are greater in assemblages of many species than in those with few species. Maximum adult size exhibits a similar correlation with species richness. Physiognomic diversity within a habitat or region is largely independent of taxonomic composition. The high proportion of narrowly distributed species among Singapore littoral-fringe molluscs and the rapid decrease in maximum adult size and number of species in this habitat from Singapore to more peripheral parts of the Indo-West-Pacific suggest that regional differences among littoral-fringe assemblages are greater than those of other mangrove habitats.