Spekia

Abstract: The shells of most lacustrine gastropods are typically small, weakly calcified, and mod- estly ornamented to unornamented. Similarly, most lacustrine crabs are usually small detritivores with weak chelae. A number of invertebrate taxa in Lake Tanganyika, however, deviate from these generalities. This study explores a predator-prey coevolution model as an explanation for the large, heavily calcified, and ornate gastropods and the robust, durophagous crabs of Lake Tanganyika. The endemic thiarid and viviparid gastropods from Lake Tanganyika have significantly thicker shells and higher frequencies of terminal apertural lip thickening than closely related cosmopolitan taxa from outside the lake. Tanganyikan gastropods also display considerably higher incidence of shell repair, following nonlethal shell damage, than cosmopolitan taxa of the same families. There is a strong positive correlation between gastropod apertural lip thickness and shell repair frequency among all the gastropod species analyzed. The endemic Tanganyikan potamonautid crab Platy- telphusa armata (a molluscivore) possesses larger, more robust crushing chelae than other African potamonautid or potamonid crabs. In contrast with the cosmopolitan African crabs, the Tangan- yikan crabs display molariform, rather than serrate dentition on their crushing chelipeds. In shell-crushing experiments, the Tanganyikan gastropod shells were an order of magnitude stronger than typical lacustrine gastropod shells, many well within the range of tropical marine gastropod shell strengths. Predation experiments with the endemic gastropods Spekia, Neothauma, Lavigeria spp., Para- melania spp. and the crab Platytelphusa armata showed that increased size, apertural lip thickness or shell sculpture reduced the successful predation rate of P. armata. Crabs with large chelae have a greater ratio of successful: unsuccessful attacks than crabs with small chelae. Among cases of successful predation, crabs with large chelae employed predation methods that required less time and energy (such as crushing the shell in the cheliped) than the methods employed by crabs with small chelae (such as peeling the shell from the aperture or the spire). The morphological, shell-crushing, and aquarium experiment data, considered in concert, pro- vide strong support for the idea that the endemic gastropods and crabs of Lake Tanganyika have coevolved over the past 7 million years.

Abstract: The endemic gastropod fauna of Lake Tanganyika is remarkable not only for its great species richness, but also for its unusually ornate and heavily calcified shell morphologies that are convergent with diverse marine forms. The origin and intralacustrine radiation of these thiarid gastropods have been debated since the late nineteenth century, as they are perhaps the most dramatic lacustrine radiation of gastropods in the world. They parallel the endemic cichlid fish fauna of the African Great Lakes in their potential for providing information about the mechanisms of evolution. This chapter presents the first molecular phylogenetic treatment of 12 of the 18 endemic gastropod genera of Lake Tanganyika, based on a mitochondrial gene fragment of cytochrome oxidase I ( COI ). The endemic thiarid fauna of Lake Tanganyika was found to be paraphyletic, but a larger clade including Cleopatra , a thiarid genus widely distributed throughout East Africa, is monophyletic. The data reveal five robust clades within this larger monophyletic group: (1) (( Reymondia, Cleopatra ) Spekia ), (2) ( Stanleya, Tanganyicia ) as sister group to group 1, (3) the trochiform genera (( Bathanalia, Chytra ) Limnotrochus ) as a clade and (4) sistem-taxon pairings for ( Lavigeria , Nov. gen.) and (5) ( Anceya, Paramelania ). Analyses using parsimony, neighbour-joining and maximum likelihood analyses agreed on sister-taxon relationships at terminal nodes, but were unable to resolve relationships among these Tanganyikan clades. This may be interpreted as an indication of rapid, burst-like radiation at the time of origin of this fauna. The term “superflock” ( sensu Ribbink) may be used to describe the generic level radiation of Tanganyikan gastropods, as it preserves the information that this is a group of closely related endemics that have radiated in situ , but does not imply complete monophyly.