Population density, habitat dynamic and aerial survival of relict cave bivalves from genus Congeria in the Dinaric karst

Caves are some of the least-known ecosystems on Earth and long-term ecological studies and population size estimates are very rare. Genus Congeria is a Tertiary relict that comprises three species from Dinaric karst area; C. kusceri, C. jalzici and C. mulaomerovici, each with very limited distribution. They are the only known cave bivalves and in contrast to many other cave species, they form populations with high densities. We estimated that the population of C. kusceri in Jama u Predolcu is between 72,454 and 72,906 individuals. The highest density occurred between one and three meters depth, and reached maximum of 1,625 individuals per square meter. Here we also present the results of a two-year water temperature and water level survey in several C. kusceri and C. jalzici localities. Data loggers showed that C. jalzici occurs in colder caves and higher water level oscillations then C. kusceri. From our data, it is obvious that the natural hydrological conditions in Congeria habitats were seriously altered. This and a series of other threats led to Congeria species being highly endangered. During low water levels, a part of the bivalve population becomes exposed to air in most localities. Uniquely, they are active during that period and are able to survive those conditions for more than 2 months.