First report of extraordinary corkscrew gastropods of the genus Bulinus in Lake Malawi

Open-coiled shells are rarely observed in gastropods, but they may occur at a relative frequency ranging from a small percentage of specimens in a population up to being specific to populations or even species. The origin of such corkscrew-like shells are poorly understood, but may include developmental anomalies, ecophenotypic plasticity or genetic inheritance. Here, we report the first case of gastropods from Lake Malawi with corkscrew-like shells, and we use comparative molecular studies with two mitochondrial gene fragments (COI and 16S rRNA) to examine its origin. The specimens belong to the genus Bulinus, more specifically to one of five clades of Bulinus recovered from the Malawi Basin. Beyond being seemingly specific to one of these Bulinus clades, network analyses indicate low genetic differentiation from the other, regularly shaped specimens in this clade. Although our results do not fully discard the possibility of corkscrew-like shells being genetically inherited or being caused by growth disturbance, their restrictive occurrence in a few geographically isolated populations in sympatry with specimens displaying a regular shell shape supports the hypothesis that the corkscrew-like shell shape in Bulinus is caused by ecophenotypic plasticity. For now, it is unclear whether aspects of the physical environment, or biotic interactions, such as a release from predation, caused the corkscrew-like morphotype. The phenomenon may have been caused by the multiple environmental changes that Lake Malawi has been undergoing over recent decades. (C) 2020 International Association for Great Lakes Research. Published by Elsevier B.V. All rights reserved.