Ancestry to an endemic radiation in Lake Tanganyika? Evolution of the viviparous gastropod Potadomoides Leloup, 1953 in the Congo River system (Caenogastropoda, Cerithioidea, Paludomidae)

Abstract

Providing another spectacular model for understanding speciation and radiation, the origin of the gastropod species flock in Lake Tanganyika (with an estimated age of approximately 12 Myr) remained enigmatic to date. Although, for a long time, an in situ radiation was assumed, Lake Tanganyika could have functioned as a reservoir for ancient African lineages, implying that the now lacustrine taxa originiated elsewhere. However, the fluviatile gastropod fauna of adjacent river systems in Central and East Africa is only poorly known. Here, we provide conchological, anatomical, phylogenetical, and biogeographical data on the fluviatile genus Potadomoides Leloup, 1953, which was hitherto regarded as ancestral to the entire Tanganyika gastropod radiation. The type species Potadomoides pelseneeri is restricted to the delta region of the Malagarasi River east of Lake Tanganyika, whereas three congeneric species (Potadomoides bequaerti, Potadomoides hirta, and Potadomoides schoutedeni) inhabit the Congo River with its tributaries Lualaba and Luvua, west of the Tanganyikan Rift. We describe and document, with scanning electron microscopy, the ontogenetic development of embryos of this uterine brooder as well as the detailed reproductive anatomy. Phylogenetic analysis of 44 morphological characters (including adult and embryonic shell, operculum, radula, reproductive tract) for 15 paludomid taxa could not support monophyly of the Tanganyika species flock. Instead, we found two major lineages that colonized Lake Tanganyika independently, one comprising the Nassopsinae Kesteven, 1903 (= Lavigeriinae Thiele, 1925 with the riverine Potadomoides plus the lacustrine Lavigeria and Vinundu, the second comprising the riverine Cleopatra together with the rest of the lacustrine species (except for Tiphobia horei). The analysis identifies Potadomoides as paraphyletic, with the uterine brooder P. pelseneeri being the sister taxon to the uterine brooder Lavigeria plus the oviparous Vinundu, but not to the entire Tanganyika species flock. We reconstruct the independent evolution of an fluviolacustrine taxon Nassopsinae for which we evaluate the synapomorphic characters, in particular those of reproductive biology, and discuss systematic and evolutionary implications of repeated origin of (ovo-)viviparity in these limnic Cerithioidea. Finally, we outline a hypothesis on the evolutionary history of Potadomoides in the context of the gastropod radiation in Lake Tanganyika.

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

“…5). The radulae found in shells attributed here to Cleopatra broecki (MCZ 171520, from Kibunzi) is clearly distinct, in particular with respect to the rachidian and laterals, from that known in all radulae of Potadomoides; for comparison and description see Glaubrecht & Strong (2007). While all known four species of the latter genus exhibit a narrow rachidian tooth, slightly more than 1.5-fold larger than wide, tapering to a V-shaped base, and with only few, mostly three cusps of the 1/1/1 pattern only, the central tooth in Cleopatra, here shown for Cleopatra bulimoides (ZMB Moll.…”

“…This study resulted from a revision of the constituent species of Potadomoides from the Congo River system that is described in detail elsewhere (Glaubrecht & Strong 2007). For Cleoptara broecki the type material in the Muse Royal L'Afrique Centrale, Tervuren, Belgium (MRAC) was re-studied ( Fig.…”

“…However, all three of them actually exhibit radula features very similar to Leloup's P. pelseneeri. Mandahl-Barth (1967) examined (and in one case depicted) the radula of these three new Potadomoides species, which were recently re-discovered and studied by Glaubrecht & Strong (2007). However, in case of broecki, MandahlBarth neither figured nor described the radula.…”

“…Kocher 2005;Joyce et al 2005) but also of Asian and African cerithioidean gastropods (e.g. Glaubrecht 2004;Glaubrecht & Kaehler 2004;Glaubrecht & Strong 2007;, lacustrine species flocks are not necessarily to be seen as evolutionary dead ends. Quite to the contrary, it can be expected that the potentially repeated switch of limnic taxa between riverine and lacustrine habitats during the course of their evolution guaranteed their survival, and at the same time provided ample evolutionary opportunities for speciation and radiation under various conditions, as Glaubrecht (2004) and Glaubrecht & Strong (2007) have suggested for African paludomid gastropods.…”

“…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.…”