Causation of a riverine radiation: assessment and evaluation of the morphological and genetic differentiation in a species flock of viviparous gastropods (Cerithioidea: Pachychilidae)

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Abstract

The Kaek River in central Thailand is unique in harbouring a diverse
species assemblage of viviparous gastropods of the genus Brotia. A stretch of this
river less than 100 km long is inhabited by seven, mostly endemic species that are
essentially differentiated by their shellmorphology. Earlier, it has been suggested that
this species flock fulfils some basic requirements of a radiation (monophyly and
phenotype–habitat correlation). However, the present study has shown that there is no
strict correlation between radula and shell morphology and the utilisation of substrates,
such as rock or sand, thereby refuting the hypothesis that ecological speciation
may have played a significant role. Phylogenetic analyses based on mtDNA show that
haplotypes cluster together in drainage-specific clades rather than according to the
taxonomy. There are also strong indications that introgressive hybridisation has
occurred, which may have resulted from secondary contact of previously isolated
species due to dispersal or river captures during the Cenozoic. It is assumed that the
high species diversity in the Kaek River results from two phenomena that interdigitate.
Firstly, the Kaek River fauna may have originated from multiple species
invasions from different source areas, while traces of these events may have been
obscured by introgression of Kaek River-specific haplotypes. Secondly, waterfalls in
the Kaek River seem to affect the directionality and amount of gene flow between
local populations within the river and several smaller tributaries. Together with
temporally changing water regimes, this highly structured environment may have
conserved local genetic differentiation and triggered diversification and speciation in
peripheral isolates within relatively short periods of time.