Radiating in a river: systematics, molecular genetics and morphological differentiation of viviparous freshwater gastropods endemic to the Kaek River, central Thailand (Cerithioidea, Pachychilidae)

Abstract

Speciation in the context of adaptive radiation is regarded as a key process in the creation of biodiversity. While several lacustrine species flocks provide ideal models for elucidating the underlying evolutionary mechanisms, riverine radiations are both rarely known and studied. The Kaek River, a third-order tributary of the Nan River and Chao Praya drainage in central Thailand, harbours an exceptional endemic species assemblage of morphologically distinct, viviparous pachychilid gastropods. Our systematic revision, combining a morphological and molecular genetics approach, reveals the sympatric existence of at least seven species of the genus Brotia that is widespread in rivers of South-east Asia where usually only two species at the most coexist. At eight locations along a 100-kilometre stretch of the Kaek River, we found the syntopic occurrence of two to three species that are separated by specific habitat preferences and exhibit trophic specialization in their radula morphology. Phylogenetic analyses (using MP, NJ, ML and Bayesian inference statistics) of partial COI and 16S sequence data of 17 samples from six species occurring sympatrically and parapatrically, respectively, in the Kaek River drainage (plus the type species B. pagodula as outgroup) indicate monophyly of all these endemic species. Brotia solemiana, which also occurs in the Loei River, a tributary of the Mekong drainage system, was found to be sister to all other Kaek River pachychilids. The distinctive morphotypes, proposed here to represent biospecies, do not show high levels of genetic variation consistent with long periods of reproductive isolation. This suggests a relatively recent origin of this intrariverine radiation and rapid morphological divergence in the Kaek River Brotia. Recent diversification combined with ecological separation and trophic specialization parallels conditions found, albeit on a more specious level, in the lacustrine species flock of the closely related pachychilid genus Tylomelania, which is endemic to ancient lakes on the Indonesian island of Sulawesi. We discuss and compare the allopatric and ecological aspects of speciation in this unique riverine radiation and outline a putative historical biogeography of the Kaek River species, employing the most recent geological and palaeohydrological data for Thailand.