Cave clam or Congeria kusceri it is the only living subterranean bivalve on the planet and is believed to be a specimen of a bivalve that lived in the Tertiary period and survived the Ice Age by descending deep underground. Until it was discovered, it was considered an entire genus Congeria became extinct during major changes in weather conditions.
Cave clams – small but mighty
The Dinaric cave clam is very small, up to 20 mm in size and oval in shape. It consists of two shells that are elongated in the shape of a triangle. They are also convex and rounded and completely depigmented, giving them a very unusual appearance. The receptors of the organs of vision are reduced, which is an adaptation to life underground where there is no light.
Specimens of this species live either singly or in groups, clinging to underground rocks by their byssal threads. These bivalves feed like most other bivalves – by filtering water. This is how they get to small organic particles, which they use as food. There are female and male bivalves and their fertilization is internal. The female is responsible for guarding the eggs in her gills until the small clams break free from her embrace.
The Dinaric clam is long-lived and can live up to fifty years, which is common among many cave organisms. They move slowly and rarely change their location, so they can be found in one place.
One of the adaptations of this mantle is resistance to dry periods that occur when the water level in the ground is very low. Then it breathes air, which contains a lot of moisture, and can go without food for some time. However, if there is a long period with a reduced amount of water underground, this bivalve will die.
A relic on the brink of extinction
Although this cave bivalve is an inhabitant of the Dinaride, it is not found in all its parts, so only one locality is known in the Slovenian part of the Dinaride, while in Bosnia and Herzegovina no less than three localities are known where this rare endemic lives.
In Croatia, this species was found in the most sites, up to fifteen, but out of these fifteen sites, this species was found alive in only five sites, while elsewhere only shells were discovered. Unfortunately, as early as 2000, it was found that these bivalves had disappeared from two locations and that the species is on the verge of extinction if something is not done to protect it. The areas where this subterranean bivalve currently lives are the Lika-Velebit region, the Metkov region and the Vrgora region.
Jamský karst in Predolac near Metković is considered the largest colony of Dinaric bivalves.
At the time, the Dinaric cave bivalve was classified as a critically endangered species at risk of complete extinction. There are many reasons why this endemic is endangered, and a few are the most important. One of the most important is groundwater pollution caused by the widespread use of pesticides and industrial pollution. This species is also very sensitive to hydrological interventions that occur in the karst fields under which it lives and as a result of groundwater pumping. Hydroaccumulation is currently considered the greatest threat to the survival of this species.
It is especially sensitive to changes in the groundwater level, but also to the urbanization of the area, which has intensified in the area where this rare living fossil lives in recent decades.
The Dinaric cave clam is included in the list of strictly protected species not only in the Republic of Croatia, but also in the entire EU.