Jalžić’s Congeria cave shell – mysterious creature of the Semič underworld
The Bela krajina karst area in the south-eastern part of Slovenia, at the junction between the high Dinaric karst and the Bela krajina karst plain, was formed over the course of 200 million years. This area, with several endemic cave species, contributes significantly to the underground biodiversity of Slovenia, which is one of the richest countries in the world in this respect. The groundwater has dug numerous channels, crevices, and caves in the limestone, where life has developed independently of the sunlit surface, making Slovenia one of the areas with the greatest underground biodiversity in the world. The vast majority of underground species are endemic, meaning they live only in a certain area of the world.
The Bela krajina caves are home to numerous cave animals such as the small-necked crab, the cave crab, the white and black proteus, cave pipefish and snails, and new species are constantly being discovered. Along the Krupa river, a living fossil was discovered, namely Jalžić's Congeria. It is a new species of congeria – small freshwater bivalve molluscs – because it is morphologically different from its previously discovered relatives in the Dinaric Karst.
This tiny, light brown cave shell, measuring just over 13 mm in size, is a relic of the Tertiary geological era. It has survived the extinction of the dinosaurs over 50 million years, as well as the pollution of the Krupa river in the 1980s. Unfortunately, its underground habitat in the Krupa hinterland is not accessible for research because of the impact of PCB pollution, which accumulates in the organisms' tissues. This hardy shell teaches us that it is possible to survive by adapting.
Congeria is unisexual. Favourable conditions for reproduction prevail when the water level is low, when fertilisation typical of aquatic organisms takes place from the outside. The larvae hatch for a few weeks before being transported to other locations by higher water levels. The larva finds its place on the rock of the water cave, where it settles for life. It can live up to 40 years and feeds by filtering organic matter from the water. Due to its narrow habitat and endangered status, the species is protected in Slovenia and a Natura 2000 site has been designated along the Krupa River to conserve it.
In the Semič Museum House, in the natural history collection of the Bela krajina Nature Centre, you can get to know the congeria shell. See it life-size with a magnifying glass and take a peek into its home environment. On the karst Krupa trail from Lebica to Krupa, walking just one kilometre from the source of the Krupa to its confluence with the Lahinja, you will pass the source of the cave shell. Its home in a water cave is only accessible to experienced divers, who are credited with the discovery of this endemic shell. In 2010, cave diver Martin Ilenič, a member of the Bela krajina Diving Club, discovered a longer water cave, which has so far been explored to a length of 60m: “I dove into a shallow siphon, and by pushing rocks under the overhanging wall, I widened the entrance. On my second attempt, I opened the entrance to the underwater tunnel. About 5m from the entrance I discovered the first air pocket, perpendicular to the water flow, about 7m wide, a good metre high and just as wide. /… /Further on, a slightly wider siphon tunnel led into a bricked-up hall about 15m from the entrance. /… / On Sunday, 14 November 2010, the Institute of the Republic of Slovenia for the Conservation of Natural Heritage organised a biospeleological survey of the cave, which was attended by eminent researchers. The dive into the cave by Martin Ilenič and the experienced clam expert Branko Jalžić (after whom the species found at Krupa is named) was commented on by Andrej Hudoklin (ZRSVN): After less than an hour, the two divers returned from underground with the first specimens of live cave shells. It was in the canyon of the Bela krajina Krupa River, and it was the first time that such a find had been made in Slovenia."