World Congress of Malacology Antwerp ... - Unitas Malacologica

same sub-genus. While position within the tidal column does produce variation in shell morphology,
the magnitude of this effect is small enough that, within this genus, it is unlikely obscure taxonomic
designations. This research partially funded by NSF-PEET DEB-9978119.
Judging a mangrove oyster by its cover: Differentiating extremely similar allopatric Isognomon
species (Pterioidea: Bivalvia) using geometric morphometric analysis of conchological features
Wilk, John
University of Illinois at Chicago, 845 W. Taylor St., Chicago, Illinois, USA,
Email: jwilk@uic.edu
Field Museum of Natural History, 1400 S. Lake Shore Dr, Chicago, Illinois, USA,
Email: jwilk@fieldmuseum.org
Dry shell specimens of Isognomon alatus (Gmelin, 1791) and I. ephippium (Linnaeus, 1758) are
nearly indistinguishable. These two members of the subgenus Melina Philipsson, 1788 share a flat,
circular, and highly variable shell type with no distinct conchological characters that differentiate the
two taxa. Strikingly similar and allopatrically situated, I. alatus in the western Atlantic and I.
ephippium in the Western Pacific, these species are generally considered sister taxa. The Isognomon
molecular database I am currently developing suggests that these two species are not as closely
related as their morphological similarity may imply. To understand the nature of this conchological
similarity I examined the shape and development of these bivalves using geometric morphometrics.
A dataset consisting of both traditional and sliding semi-landmarks from 114 left valves were
examined using pairwise, regression, and vector analyses carried out in the TPS and IMP software
suites. Pairwise tests found significant differences in shape between Isognomon alatus and I.
ephippium at all sizes classes with the most pronounced differences seen in early and late ontogeny.
Regression analysis found that significant differences in shape develop relatively early shell
development, at shell sizes below those included in the study. These differences then reduce in
magnitude as the shells approach mean sampled size but then increase in magnitude as the shells
continue growing. It was also found that these two species travel along significantly different
ontogenetic vectors during shell development. Together these analyses indicate that while these
species begin their development at different points in morphospace, the ontogenetic vectors of these
bivalves almost intersect, leading to intermediate size classes that are very conchologically similar
despite different developmental processes leading to those morphologies. This research is partially
supported by NSF-PEET DEB-9978119.
What molluscs can tell us about the genesis of biodiversity in ancient Lake Ohrid
Wilke, Thomas; Albrecht, Christian
Department of Animal Ecology and Systematics, Justus Liebig University Giessen, Heinrich-Buff-
Ring 26-32 (IFZ), D-35392 Giessen, Germany,
Email: Tom.Wilke@allzool.bio.uni-giessen.de; Christian.Albrecht@allzool.bio.uni-giessen.de
With an estimated age of 2-5 million years, the oligotrophic and karstic Balkan Lake Ohrid is the
oldest and most outstanding European ancient lake. Together with its sister lake, Lake Prespa, it
harbors a remarkable degree of invertebrate biodiversity. In terms of number of endemic species per
area, Lake Ohrid is the most species-rich lake in the world. Whereas some workers suggest a high
number of relic species occurring in Lake Ohrid, other workers favor intralacustrine speciation as key
evolutionary process responsible for the high degree of biodiversity seen today.
Based on a genetic survey of nearly all extant molluscan taxa in Lake Ohrid, we are addressing
questions of the genesis of its biodiversity in space and time. Our data indicate:
A) The presence of several ancient lakes species flocks in the Gastropoda
They include radiations in the subfamily Pyrgulinae (Caenogastropoda), and in the genera/subgenera
Ancylus, Acroloxus, and Carinogyraulus (Basommatophora). Our molecular studies indicate that
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