Background Poecilogony, the presence of two developmental modes in the same animal species, is a rare phenomenon. Few cases of poecilogony have been suggested for marine invertebrates including molluscs and even less stood extensive testing, mostly revealing a species pair with differing developmental modes. We studied a textbook example of poecilogony in the viviparous snail Planaxis sulcatus (Gastropoda: Planaxidae), for the first time throughout its entire distribution range. Results In the Western Indian Ocean and Red Sea this intertidal species is observed to have large, shelled juveniles, whereas in the Indo-West Pacific planktotrophic veliger larvae are released from a subhaemocoelic brood pouch. We uncovered a shift in developmental modes across its range: from west to east successively earlier developmental stages are released. Furthermore, genetic data based on mitochondrial DNA suggests to recognize P. sulcatus as a single species rather than a group of cryptic species. A reconstruction of the ancestral area of P. sulcatus based on molecular data outlines the Western Indian Ocean and the Indo-West Pacific as area of origin. Conclusion The findings supporting Planaxis sulcatus as a single widespread species and the geographical shift from one reproductive mode to another suggest for this species to truly represent a case of geographic poecilogony, i.e. differing developmental modes between populations of the same species. Furthermore, the results of our ancestral range estimation imply the release of planktotrophic larvae as the ancestral developmental mode.

The freshwater thiarid gastropod Tarebiagranifera (Lamarck, 1816), including taxa considered either congeneric or conspecific by earlier authors, is widespread and abundant in various lentic and lotic water bodies in mainland and insular Southeast Asia, with its range extending onto islands in the Indo-West-Pacific. This snail is, as one of the most frequent and major first intermediate host, an important vector for digenic trematodes causing several human diseases. As a typical thiarid T.granifera is viviparous and parthenogenetic, with various embryonic stages up to larger shelled juveniles developing within the female’s subhemocoelic (i.e non-uterine) brood pouch. Despite the known conchological disparity in other thiarids as well as this taxon, in Thailand Tarebia has been reported with the occurrence of one species only. In light of the polytypic variations found in shell morphology of freshwater snails in general and this taxon in particular, the lack of a modern taxonomic-systematic revision, using molecular genetics, has hampered more detailed insights to date, for example, into the locally varying trematode infection rates found in populations of Tarebia from across its range in Thailand as well as neighboring countries and areas. Here, we integrate evidence from phylogeographical analyses based on phenotypic variation (shell morphology, using biometry and geometric morphometrics) with highly informative and heterogeneous mtDNA sequence data (from the gene fragments cytochrome c oxidase subunit 1 and 16 S rRNA). We evaluate both the morphological and molecular genetic variation (using several phylogenetic analyses, including haplotype networks and a dated molecular tree), in correlation with differences in the reproductive biology among populations of Tarebia from various water bodies in the north, northwest, central, and south of Thailand, supplementing our respective analyses of parasite infections of this thiarid by cercaria of 15 trematode species, reported in a parallel study. Based on the comparison of topotypical material from the island of Timor, with specimens from 12 locations as reference, we found significant, albeit not congruent variation of both phenotype and genotype in Tarebiagranifera, based on 1,154 specimens from 95 Thai samples, representing a geographically wide-ranging, river-based cross-section of this country. Our analyses indicate the existence of two genetically distinct clades and hint at possible species differentiation within what has been traditionally considered as T.granifera. These two lineages started to split about 5 mya, possibly related to marine transgressions forming what became known as biogeographical barrier north of the Isthmus of Kra. Grounded on the site-by-site analysis of individual Tarebia populations, our country-wide chorological approach focussing on the conchologically distinct and genetically diverse lineages of Tarebia allows to discuss questions of this either reflecting subspecific forms versus being distinct species within a narrowly delimited species complex. Our results, therefore, provide the ground for new perspectives on the phylogeography, evolution and parasitology of Thai freshwater gastropods, exemplified here by these highly important thiarids.

Mimicry is one of the best examples of coevolution. For a mimetic system to function, the mimic has to equal its model. Due to this close dependence, mimetic systems promise deep insights into modes and means of evolution. Mimicry is known to occur in many taxa across different groups of organisms. However, while a plethora of mimetic systems exist, cross-phyla convergences have only rarely been reported in shelled gastropods. Our literature survey brought to light several mimetic systems including gastropods (as model or mimic), all of them in either a marine or a terrestrial setting. We here report on the first potential case of mimicry involving freshwater snails. We found larval cases of European Helicopsyche caddisfly to closely resemble Valvata gastropod shells in shape and size. In particular, stunning is the detailed similarity of features in these trichopteran cases to those characteristic for snail shells, for example, apex, aperture and umbilicus, hinting at a strong selection pressure to be involved. We discuss this unique case of mimicry that might hold unparalleled insight in mimetic relationships, taking into account alternative environmental factors and potential predatory dupes, in particular birds that might have successively caused the evolution of coiled cases in helicopsychid trichopterans. K E Y W O R D Scross-phyla convergence, freshwater gastropods, Helicopsyche, mimicry, Valvata

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While Africa’s lacustrine gastropod fauna, in particular of Lake Tanganyika, has received much attention, the continent’s riverine malacofauna has long been neglected. Pseudocleopatra is a relatively poorly known paludomid gastropod genus with species found throughout the lower reaches of the West African Volta and Congo rivers. In the course of ongoing systematic revisions of African paludomids, we present here a morphometric analysis and revision of the recent species assigned to the genus, i.e., P. togoensis, P. voltana, P. dartevellei and P. bennikei, to improve taxonomic acuity for this group. We use available museum material for geometric morphometrics, multivariate ratio analysis and comparisons of radular and opercular characters. Our results demonstrate that the four recent species of Pseudocleopatra are clearly distinguishable on the basis of ratios of shell measurements as well as radular and opercular characters. Pseudocleopatra has generic-level synapomorphies including: concentric opercula with relatively large paucispiral nuclei, and rachidian radular teeth with usually 13–20 cusps. On the basis of this characterisation, the nominal species Cleopatra broecki is transferred to Pseudocleopatra. Additionally, the nominal taxon P. bennikei is synonymized with P. broecki n. comb. The phylogenetic relationships of Pseudocleopatra are currently unknown, but the observed tendency of some fossil taxa assigned to Pseudocleopatra towards thalassoidism, i.e., the resemblance to marine gastropods, has led to the hypothesis that some of the thalassoid endemics in Lake Tanganyika may have originated from or be related to Pseudocleopatra. Should this hypothesis be correct, which is in need of testing by molecular genetic methods when suitable samples become available, Pseudocleopatra may play a crucial role in understanding of the evolution of thalassoidism in African Paludomidae.