Subterranean habitats are often colonised, either actively or passively, by unusual and highly distinctive animals, which in many cases are remnants of the surface fauna that once lived above them. These animals are often referred to as living fossils, or relict species. Congeria kusceri Bole, 1962, the only known troglobiotic bivalve, is a good example of this.
During the Tertiary, most of Europe was covered by a vast aquatic ecosystem of swamps and lakes spreading from the Swiss molasse Basin to Lake Aral in Central Asia. Within this system, known as the Paratethys, a spectacular radiation of many molluscs and other animal taxa occurred. Here, the Dreissenidae Gray, 1840, a family of freshwater bivalves, flourished and diversified. All of the five dreissenid genera evolved in the Neogene lake systems of the Paratethys but only three have survived until the present day: Mytilopsis Conrad, 1857, Dreissena van Beneden, 1835 and Congeria Partsch, 1835. Many different Congeria species inhabited the Paratethys. Harzhauser & Mandic identified 16 species and 11 subspecies, while Kochansky-Devide & Sliskovic identified ~30 species from Miocene deposits in Croatia and Bosnia and Herzegovina alone. By the end of the Miocene, however, all but one had become extinct. Congeria kusceri, the only species known to have survived this dramatic period, is restricted today to but a few caves in the Dinaric Karst.
The Dinaric Karst extends for about 56,000 km2 along an 800 km arc from Trieste, Italy in the north, throughout most of Slovenia (SI), Croatia (HR) and Bosnia and Herzegovina (BA) to Albania in the south and is intersected by a network of caves, pits, underground lakes, rivers and streams containing one of the most complex and diverse subterranean faunas in the world[6, 7]. It has been argued that the causes of such high subterranean biodiversity in the Dinarides lie in its complex geological history and intensive karstification that enabled multiple entries into the subterranean realm. Congeria species, unlike other cave animals, have a rich fossil record, and can provide new insights into the timeframe, sources and causes leading to the biodiversity hotspot within the Dinaric Karst.
Congeria kusceri was first discovered in the 1930’s in deposits of empty shells, but a living population was not found until 20 years later in Žira cave in Popovo polje, southern Herzegovina, allowing J. Bole to describe the species in 1962. Later, additional living populations were found in distant areas of the Dinarides[9, 10]. Recent extensive field researches have resulted in the discovery of a total of 15 known Congeria populations (Jalžić & Bilandžija, unpublished). Because of such a small number of sites, habitat destruction, and declines in population numbers, the species is listed as vulnerable (VU) in the Red List of European freshwater molluscs and, in Croatia, C. kusceri is assessed as critically endangered (CR).
It is currently assumed that there is only one species of stygobiotic bivalve - Congeria kusceri - that has a wide, holodinaric, distribution. Subterranean habitats are, however, subjected to fragmentation, leading potentially to lineage isolation and speciation. Conversely, the extreme character of the subterranean karst environment drives convergent adaptations in its inhabitants, resulting in cryptic morphologies and possibly masking real diversities[13, 14]. Accordingly, widely-distributed cave animals have split into a number of lineages with small fragmented ranges, as demonstrated by molecular studies of several groundwater Dinaric taxa – the olm, Proteus anguinus Lorenti, 1768, the cave shrimp, Troglocaris[17, 18] and the water louse Asellus aquaticus (Linnaeus, 1758).
In this study we have gathered biogeographical and paleontological data and used both molecular and morphological analyses to address several questions. We deal with contentious issues regarding the phylogenetic position and affinities of Congeria within the Dreissenidae. We have examined the question of Congeria lineage diversifications in separate parts of the Dinaric Karst, and explored the evolutionary history that ultimately caused a shift, uniquely amongst bivalves, towards a subterranean way of life. Finally, we have reported the effects of the underground environment on Congeria shell morphology. For the first time, therefore, this study combines several approaches to provide a new understanding of the evolutionary biology of Congeria and uncovers speciation events leading to the description of new Congeria species.