The Species Flocks of the Viviparous Freshwater Gastropod Tylomelania (Mollusca: Cerithioidea: Pachychilidae) in the Ancient Lakes of Sulawesi, Indonesia: The Role of Geography, Trophic Morphology and Color as Driving Forces in Adaptive Radiation

Abstract

The study of species flocks in island settings such as ancient lakes can contribute significantly to our understanding of fundamental evolutionary processes, such as speciation, radiation, adaptation, and coevolution. We here use endemic freshwater gastropods in ancient lakes on the Indonesian island of Sulawesi as models to gain insights into patterns and mechanisms of lacustrine diversification. In two central lake systems on the island, i.e., Lake Poso and the five connected Malili lakes, the pachychilid freshwater snail genus Tylomelania has radiated into a diverse endemic assemblage of c. 53 species of morphologically distinct viviparous gastropods. Molecular phylogenetic data revealed that independent and multiple colonizations of the two lake systems by fluviatile ancestors have led to four morphologically and ecologically diverse adaptive radiations, one in Lake Poso and three in the Malili system. The evolution of habitat preferences and trophic specialization most likely drive diversification in both systems, while geography, i.e., allopatric speciation, is a major factor in the spatially strongly structured Malili system. Highly characteristic lacustrine shell forms have evolved in all lakes through escalation, i.e., coevolution with molluscivorous crabs. The role of the conspicuous body coloration present mainly in Lake Poso species remains to be elucidated, specifically its potential role in diversification processes. Finally, the setting of the Sulawesi snail species flocks with two independent radiations under almost identical extrinsic conditions offers insights into parallel patterns of adaptive evolution. However, a major conservation effort will be required to preserve the lakes’ spectacular species for future research.