Ancient lakes as hotspots of diversity: a morphological review of an endemic species flock of Tylomelania (Gastropoda: Cerithioidea: Pachychilidae) in the Malili lake system on Sulawesi, Indonesia

Abstract

The viviparous freshwater gastropod Tylomelania (Caenogastropoda: Cerithioidea: Pachychilidae) endemic to the Indonesian island Sulawesi has radiated extensively in two ancient lake systems. We here present the first systematic species-level review of taxa in the five lakes of the Malili lake system, which contains the most diverse and best studied freshwater fauna on Sulawesi. Our results indicate a significantly higher diversity of Tylomelania in these lakes than previously perceived based on morphological evidence for delimiting the taxa. We describe nine new species, thus increasing the number of taxa known from the Malili lakes to 25. Tylomelania species are inhabiting all available substrates in the lakes, and the diversity of habitats is reflected in an unparalleled range of radula types in this closely related group. Several species show a high intraspecific variability in some characters, and their closer investigation will probably lead to the discovery of more cryptic species. As it is, this species flock on Sulawesi is among the largest freshwater mollusc radiations known. Since the Malili lake system also contains other large endemic species flocks of e.g. crustaceans and fishes, it is a major hotspot of freshwater biodiversity in Asia to become a conservation priority.

Cyclopoid and calanoid copepod biodiversity in Indonesia

“…High levels of endemism have already been evidenced by previous exploration of the phytoplankton, fish, decapods, and gastropods of Sulawesi (Haffner et al, 2001;von Rintelen et al, 2007;Bramburger et al, 2008;Schubart and Ng, 2008;von Rintelen and Cai, 2009). Our study confirms this endemism in the copepod fauna of the island.…”

Hybridisation as potential source of incongruence in the morphological and mitochondrial diversity of a Thai freshwater gastropod (Pachychilidae, Brotia H. Adams, 1866)

“…However, this alternative can be ruled out because we found specimens of both sexes in each of the two morphs (as recognised by a distinct anatomy of their gonads; see Kaehler & Glaubrecht 2003). In addition, marked sexual dimorphism is currently not documented from pachychilids (Kaehler & Glaubrecht 2003Rintelen et al 2007). …”

“…Among pachychilids, hybridisation has been suggested to occur within flocks of closely related species. For instance, the viviparous gastropod genus Tylomelania has radiated extensively in the Central Lakes of Sulawesi where it forms several lake-specific clades (Rintelen et al 2004;Rintelen et al 2007;Glaubrecht & Rintelen 2008). Within these clades similar patterns of incongruence between mtDNA and morphological data were found (Rintelen et al 2007) and also (but not exclusively) attributed to interspecific hybridisation (Rintelen et al 2009).…”

“…Studies of all constituent families have revealed partially rampant conflicts between the branching patterns of mitochondrial gene trees and the delineation of (putative) species by means of morphology (Lydeard et al 2002;Minton & Lydeard 2003;Dillon & Frankis 2004;Glaubrecht & Kaehler 2004;Wilson et al 2004;Kaehler & Glaubrecht 2006;Lee et al 2007;Rintelen et al 2007;Glaubrecht & Rintelen 2008). The taxonomic incongruence was attributed to a number of causes, such as incomplete lineage sorting among evolutionarily young species flocks (Glaubrecht & Kaehler 2004;Kaehler & Glaubrecht 2006;Rintelen et al 2007), incomplete lineage sorting among evolutionarily old lineages (Dillon & Frankis 2004;Dillon & Robinson 2009), the presence of unrecognised morphologically cryptic species (Lydeard et al 1998;Minton & Lydeard 2003), or taxonomic oversplitting of not well differentiated species complexes (Lee et al 2007). Although there are plenty of cases from a wide range of other animal groups that emphasised the significance of introgression of mtDNA through interspecific hybridisation as an explanation for such incongruence, this process has rarely been taken into consideration for cerithioideans (Glaubrecht & Kaehler 2004;Kaehler & Glaubrecht 2006;Glaubrecht & Rintelen 2008;).…”

“…This neglect is remarkable since according to the hybrid swarm theory of Seehausen (2004) hybridisation appears to be a realistic explanation for rampant mismatch between mitochondrial gene trees and morphology-based species delineations at least in evolutionarily young species flocks that result from adaptive radiations. Both the pachychilid species flocks of Tylomelania in the Central Lakes of Sulawesi (Rintelen et al 2007) and Brotia in the Kaek River, Thailand (Glaubrecht & Kaehler 2004; were discussed to form such adaptive radiations. In both cases, hybridisation has been postulated to partly account for the observed mismatch between morphology-based taxonomy and mitochondrial phylogenies next to incomplete lineage sorting of ancestral polymorphisms Rintelen et al 2010).…”