Comparative Biogeography and Evolutionary Systematics of an amphidromous snail genus (Stenomelania, Thiaridae, Gastropoda)

Species distribution and biodiversity patterns are usually considered separately for terrestrial freshwater and marine habitats. Traditionally, in marine species a dispersion-driven distribution is assumed. By contrast, it is common to assume a vicariance driven distribution pattern for freshwater species. So far, in-depth studies combining and comparing these processes in relation to each other are lacking. The freshwater snails genus Stenomelania (Fischer, 1885) (Thiaridae, Cerithioidea, Gastropoda, Mollusca) provides model organisms, where these issues can be examined and evaluated in direct connection. Adults of these viviparous snails inhabit freshwater habitats of Southeast Asia and Australasia. During their larval stage, juveniles hatch from their mother’s brood pouch and disperse through marine systems in a manner similar to other marine organisms. Studying this taxon in direct comparison to other marine and limnic taxa enables a comparative study of the both intrinsic and extrinsic factors and their effect on speciation and species distribution. The foundation of this study forms a contemporary taxonomic-systematic review, including modern methods. Based on this revision a study of biogeography and speciation processes in Stenomelania can be conducted. A major focus in this study is the reproductive biology of Stenomelania. Therefore, our Project is formed by three fields of research.

I - Taxonomic revision: delimitation of species and taxonomic redundancy

After our first survey, at least 135 first descriptions Stenomelania species in the Indo-West Pacific region are available. According to a previous purely morphological assessment of the regional presence by Starmühlner (1976, 1984, 1993) only about 13 morphospecies appear valid. This results in a preliminary synonymy rate of 1: 10.4. This is by far above the expected rate of 1:4-1:5 found in gastropods (Glaubrecht 2009). This unusually high level of redundancy indicates a high phenotypic plasticity within this genus. Therefore, the exact accounts of species diversity within this genus can only be accessed after in-depth studies using molecular and morphological methods alike.

II - Reproductive Biology: Reproduction diversity and dispersal ability

Using anatomical-histological examination methods features of the genital tract and details of the brood pouch morphology of Stenomelania are documented and contextualized with other Cerithioidea groups. Additionally, data on the ontogenetic development of veliger larva and reproductive modes of adults is collected by brood pouch content analysis. By conducting on-site examinations of living specimens, the larval ontogenesis with pelagic and benthic phase as well as the metamorphosis of viviparous freshwater gastropods will be documented for the first time. Thereby, our major interest lies in differences between species of different regions. For gastropods in general, the largely unexplored dispersal ability of amphidromous larvae is of special interest. Through assessment of these findings, we hope to unravel the Biogeography of Stenomelania.

III - Biogeography: distribution, dispersal and history of the Indo-West Pacific

Based on the classification and the reproductive biology of Stenomelania species, their distribution areas can be reconstructed. In conjunction with the data on the reproductive biology, we will study the potential of a trans-oceanic dispersal and colonialization of geographic habitats by veliger larvae. Based on our findings, we aim to evaluate if the distribution of Stenomelania species is mainly a result of dispersal or vicariance. As a result of this study, using the calculated genealogies of Stenomelania species hypotheses accounting for the historical biogeography of Australasia can be tested, so that conclusions on the earth history of this region can be drawn.

In the project as a research assistant and doctoral candidate employed: Benedikt Wiggering (2015-2018).

Publications

• Talk: Wiggering, B. & Glaubrecht, M. 2016. Spotting specifics in nature’s look-alike contest: Evolutionary systematics of Thai marsupial snails. in Book of abstracts DZG meeting 2016.

• Talk: Wiggering, B. & Glaubrecht, M. 2016. Escaping from Adultoicentrism: On the Biogeography and Lifehistroy of Cerithioidean Gastropods from a Freshwater Thiarid Perspective. in Hwai, A. T. S.; Yasin, Z.; Peng, C. T. C.; & Razalli, N. M. (Editors) World Congress of Malacology 2016, Programme & Abstract Book.
  
  
• Talk: Wiggering, B. & Glaubrecht, M. 2016. Reproduktionsbiologie der Cerithioidea mit besonderer Berücksichtigung der Süßwasser-Thiariden in Hinblick auf ihre Biogeographie. – Abstract not jet printed.

Literature cited

Glaubrecht, M., Brinkmann, N., Pöppe, J. 2009. Diversity and disparity „down under“: Systematics, biogeography and reproduction modes of the „marsupial“ freshwater Thiaridae (Caenogastropoda, Cerithioidea) in Australia. Zoosystematics and Evolution, 85(2): 199-275.

Starmühlner F. 1976. Ergebnisse der Österreichischen Indopazifik-Expedi­tion 1971 des 1. Zoo­logischen Insitutes der Universität Wien: Beiträge zur Kenntnis der Süßwassergastropoden pazifischer Inseln. Annalen des Naturhistorischen Museums in Wien, 80/B: 473-656.

Starmühlner, F. 1984. Results of the Austrian-Indian hydrobio­logi­cal mis­­sion 1976 to the Andaman-Islands; Part IV: the freshwater gastropods of the Andaman-Islands. Annalen des Naturhistorischen Museums Wien, 86/B: 145-204.

Starmühlner, F. 1993. Ergebnisse der österreichischen Tonga-Samoa Expe­di­ti­on 1985 des Instituts für Zoologie der Universität Wien: Beiträge zur Kennt­nis der Süß- und Brackwasser-Gastropoden der Tonga- und Samoa-Inseln (SW-Pazifik). Annalen des Naturhistorischen Museums Wien, 94/95 B: 217-306.