Disparity pattern of unionoid bivalves from Lake Malawi (East-Africa): a case study for adaptive strategies to heterogeneous environment

Abstract

The shell shape of many unionoid bivalves is known to be highly variable in order to cope with the heterogeneity and instability of freshwater environments. Along environmental gradients shell shape of many species is changing fairly consistently. In this case study the disparity pattern of three species of unionoid bivalves from Lake Malawi (Nyassunio nyassaensis, Chambardia nyassaensis, Mutela alata) is studied along one environmental transect in order to better understand potential adaptive strategies of the species to heterogeneous environment. Using Fourier shape analysis, the shell morphology of about 2,500 unionoids is analysed. The morphometric data are processed by multivariate statistical analyses. The disparity of Nyassunio nyassaensis is high and shell shape is changing consistently along the environmental gradient: in offshore habitats, the shells of this species are more swollen and triangular than in nearshore habitats. Disparity and shell shape differences are lower in Chambardia nyassaensis and Mutela alata. It is suggested, that the three species developed different adaptations to cope with environmental fluctuations: (1) a high level of phenotypic plasticity allows bivalves to adapt the shell shape to variable environmental conditions, as it is mainly the case for Nyassunio nyassaensis; (2) other key features such as fused mantle margins help Chambardia nyassaensis and Mutela alata to survive environmental fluctuations. More transects need to be sampled and more data in particular on growth pattern and migration behaviour of the bivalves need to be collected to test the hypotheses. (© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)