Adaptive radiation of thalassoid gastropods in Lake Tanganyika, East Africa: morphology and systematization of a paludomid species flock in an ancient lake

Abstract

The paludomid gastropods of Lake Tanganyika are one of the most spectacular examples of species flocks among molluscs. However, whether they provide an example of intralacustrine speciation and truly adaptive radiation still remains unresolved, with their monophyly and phylogenetic relationships within freshwater Cerithioidea still to be rigorously evaluated. In the course of providing the data for a future comprehensive phylogenetic analysis, this paper using SEM technique describes and documents the morphology of the adult and particularly the embryonic shell (ornamentation and size) as well as the radular anatomy (including cusp pattern), with remarks on the ontogeny and ecology, for 19 species belonging to 10 genera, mainly from a collection from Kigoma Bay, East shore of Lake Tanganyika, Tanzania. The species studied live either on soft sandy and silty bottoms or on primary or secondary hard substrata, representing two ecological guilds, with rocky substrata supporting a richer species assemblage (including five taxa of Lavigeria) than soft substrata. In addition, a systematic framework for these gastropods is suggested, with protoconch and radular morphology found to be most useful to characterize and differentiate genera and species, while teleoconch and opercular features correspond to three distinct lineages corroborated also by molecular data, viz. the Paramelaniinae with paludomid‐like concentric operculum with spiral nucleus, the viviparous Nassopsinae Kesteven, 1903 (= Lavigeriinae Thiele, 1925) with subspiral to paucispiral operculum, and the minute sand‐dwelling Syrnolopsinae with paucispiral operculum. Oviparity was found in 70% of the 32 thalassoid species from 17 genera and represents the most common and presumably ancestral reproductive mode within Tanganyikan gastropods, while two different viviparous strategies can be differentiated in 20% of the species (in four other species the reproductive mode is unknown). Finally, some standard hypotheses as to the question of relationships with Thiaridae, monophyly, in situ radiation and viviparity as a key innovation for this radiation are briefly reviewed and discussed. It is concluded that all studied species of the thalassoid species flock are actually members of the Paludomidae instead of the true Thiaridae, representing an evolutionary lineage of ancient, and most likely originally riverine Cerithioidea that predated the formation of the East African Rift system. Consequently, neither the broad spectrum of phenotypes nor the unique morphological features should be viewed as adaptations during the course of exclusively intralacustrine speciation and adaptive radiation within Lake Tanganyika. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Lacustrine mollusc radiations in the Lake Malawi Basin: experiments in a natural laboratory for evolution

“…However, while part of the Pliocene malacofauna survived in that lake, there are no indications that the intra-lacustrine molluscan radiations in it are remarkably old. Most Tanganyikan mollusc taxa are paleo-endemics, not neo-endemics, as shown by paleontological Pickford, 2003, 2010), molecular (Wilson et al, 2004) and morphological (Glaubrecht, 2008) evidence. Though significant lake level drops did occur during the late Pliocene and Pleistocene (ca.…”

“…The published literature in which provisional species lists of the Chiwondo fossil molluscs or part of them are provided is quite extensive and widely scattered. It includes Pain (1966), Gautier (1966), Gautier (1970), Van Damme (1984, 1988, Pickford (1999, 2003), Gorthner (1994), Gorthner et al (1992), Schrenk et al (1995), Schultheiß et al (2009) and .…”

The enigmatic Cleopatra broecki Putzeys, 1899 of the Congo River system in Africa - re-transfer from Potadomoides Leloup, 1953 (Caenogastropoda, Cerithioidea, Paludomidae)

“…reviews in Coulter 1991;Rossiter & Kawanabe 2000;Danley & Kocher 2001;Kocher 2004). Providing another instructive model for understanding evolutionary processes, such as speciation and radiation, the gastropod assemblage of Lake Tanganyika has also stirred much interest among malacologist and evolutionary biologists (Glaubrecht 2008; see also review of literature therein). At the same time, though, the knowledge of freshwater molluscs in adjacent regions of Africa is still scarce.…”

“…It was long assumed, according to Brown (1994: 129), that among the living gastropods Potadomoides had the strongest claim for consideration in relation to the ancestry of this Tanganyikan radiation. While addressing the question of the origin of the Tanganyikan gastropod radiation (Glaubrecht 1996(Glaubrecht , 2008 and systematically revising the fluviatile Potadomoides (Glaubrecht & Strong 2007) it was established, however, that this taxon is not ancestral to the entire endemic species flock of thalassoid Tanganyikan gastropods. Instead, it was suggested that a fluvio-lacustrine clade Nassopsinae, uniting two lacustrine taxa (Lavigeria and Vinundu) with the fluviatile Potadomoides, apparently represents an early independent lineage of East African paludomids that until recent times survived in and adjacent to Lake Tanganyika.…”

“…Instead, it was suggested that a fluvio-lacustrine clade Nassopsinae, uniting two lacustrine taxa (Lavigeria and Vinundu) with the fluviatile Potadomoides, apparently represents an early independent lineage of East African paludomids that until recent times survived in and adjacent to Lake Tanganyika. Consequently, while Potadomoides does certainly not serve as a suitable model for the majority of the thalassoid gastropods, it might serve as a model for an African paludomid with the potential to colonize not only fluviatile but also lacustrine habitats (Glaubrecht 2004(Glaubrecht , 2008.…”