Snails (Gastropoda) are members of the phylum Mollusca and, together with insects, represent the most numerous order of animals with over 70.000 described species. The primordial habitat of snails is the sea bed; over time, some taxa have adapted for moving in a column of water and later inhabited terrestrial and freshwater habitats. In Croatia, there are 300 species of terrestrial and 200 species of freshwater snails. Marine habitats are still where we find the biggest number of species ,which is also true for this area – over 600 species of snails can be found in the Mediterranean Sea.
A shared characteristic of snails is called torsion. It’s a phenomenon where their visceral mass and mantle twist during the embryonal development, a unique process in the animal kingdom that results in the characteristic twisted look of their shells. When moving, snails use contractions of their foot, which is the only outer visible part of the snail except the head; they also use numerous cilia and mucosal glands incorporated in the epidermis of the foot.
Subterranean snails feed by scraping the biofilm using the radula, an organ which is made from numerous rows of hitin teeth. A visible characteristic which connects them to other subterranean animals is their small size and depigmented shells. Because of theirs small size, subterranean snails belong to a group called microgastropoda, a group that consist of taxa not bigger than few millimeters. This makes their handling and species determination difficult, and they are often misrepresented in lists of malaco-fauna.
The origin of most subterranean snail species from the Dinarids can be found in the Paratethys sea, a body of water that existed 34 Mya. With drying and fragmentation of the sea, freshwater lakes came into existence. Due to their new ecological conditions, these lakes represented fertile land for the evolution of new species. Further fragmentation and speciation was brought on by the emergence of the Dinaric massif. The result of that geological process were numerous endemic species and not so rare cases of species which can be found in just one cave.
According to current knowledge, family Hydrobiidae is the most represented group of subterranean snails in Croatia. Hydrobiidae are freshwater snails living mostly in groundwater, which feed by scratching biofilm and breathe using their mantle and gills. Stenoendemic species from genera like Hadziella, Hauffenia and Lanzaia can be found living only in one or two caves.
The other group of remarkable subterranean species is genera Zospeum. Members of this genera show complete troglomorphic traits – depigmented body, loss of eyes and depigmented, often translucent shells that show the snail’s inner body structure. These are terrestrial snails from the group Pulmonata who feed with detritus and have small movability. Their dispersion depends on the forces of water flow and other animals. In Croatia, there are 9 species of subterranean snails from the genus Zospeum. Noteworthy is the species Zospeum tholossum, which was found in the depths of Lukina jama pit and is one of “Top 10 new species of 2014” on a list made by the International Institute for Species Exploration.
There are numerous reasons why subterranean snails and other groups are endangered: from the fact that these are uncommon species and thus a target for illegal collecting, to the devastation of caves, pollution of water and lack of public knowledge about the hidden and fascinating underground world. A good example of this hidden world is the spring of the Ombla river near Dubrovnik, where 23 species of subterranean snails can be found among other subterranean fauna. It is our duty as individuals and a community to save such natural treasures, not only for the generations to come but because the nature itself.
Kristijan Cindrić – email@example.com
- Habdija, I., Habdija Primc, B., Radinović, I., Špoljar, M., Matoničkin Kepčija, R., Vujčić-Karlo, S., Miliša, M., Ostojić, A. & Sertić Perić, M. (2011): Protista – Protozoa, Metazoa – Invertebrata, Strukture i funkcije. Alfa, Zagreb. pp. 584.
- Ozimec, R., Bedek, J., Gottstein, S., Jalžić, B., Slapnik, R., Štamol, V., Bilandžija, H., Dražina, T., Kletečki, E., Komerički, A., Lukić, M. & Pavlek, M. (2009): Crvena knjiga špiljske faune Hrvatske. Ministarstvo kulture, Državni Zavod za zaštitu prirode, Zagreb. pp. 371.
- Radoman, P. (1983): Hydrobioidea a superfamily of Prosobranchia (Gastropoda), I Systematics. Serbian Academy of Sciences and Arts, Monographs DXLVII. pp. 256.
- Schütt, H. (2000): Die Höhlenmollusken der Omblaquelle. Nat. Croat., Vol. 9, No. 3: 203-215.
- Slapnik, R. & Ozimec, R. (2004): Distribution of the genus Zospeum Bourguignat 1856 (Gastropoda, Pulmonata, Ellobiidae) in Croatia. Nat. Croat., Vol. 13, No. 2: 115–135.
- https://en.wikipedia.org/wiki/Micromollusk, pristupljeno 2.3.2017.
- https://en.wikipedia.org/wiki/Zospeum_tholussum, pristupljeno 3.3.2017.
Genus Congeria inhabits the Dinaric karst in Slovenia, Croatia and Bosnia and Herzegovina. The entire genus was considered extinct until 1934, when their shells were found in theStinjevac spring near Vrgorac in Dalmatia. Based on them, Slovenian malacologist Ljudevit Kušcer assumed that Congeria still lives in the depths of the underground. However, first live specimens were found in the Žira ponor in Popovo polje in Herzegovina by Jože Štirn. It was only in 1962 that the Slovenian malacologist Jože Bole described a new species Congeria kusceri.
Today, there are three species of Dinaric cave clams (Congeria kusceri, Congeria jalzici and Congeria mulaomerovici), and all of them are obligate subterranean species. These three species from the genus Congeria are the only underground bivalve in the world. They are tertiary relicts, remnants of the Tertiary fauna which were able to survive to the present day by occupying underground habitats. They were thus able to survive the changes on the surface which led to the extinction of other species of Congeria. Live specimens of Congeria kusceri were found at eight localities in the Neretva River basin in south Croatia (three localities) and Herzegovina (five localities). Species Congeria mulaomerovici is known from three localities in Sana Rriver watershed in southwestern Bosnia. Species Congeria jalzici was found at three localities of Lika River and its tributaries in Croatia and at one locality of Kupa River watershed in Slovenia.
Congeria kusceri Bole, 1962 and Congeria jalzici Morton & Bilandžija, 2013
Shells of the Dinaric cave clam size up to 20 mm, they are oval and the front end is elongated and tapering to a triangular and curved top. Shells of Congeria jalzici are a bit smaller and their size is around 13 mm. Both shells are very prominent and rounded, and without reefs. Morphology of the shell is extremely variable and it is hard to distinguish between Congeria jalzici and Congeria kusceri. Both the tissue and the shells are depigmented, and statocysts as well as light receptors are reduced. It lives solitary or in groups. It is attached to walls of underground channels by byssal threads. Feeding with fine organic particles occurs by filtering the water. The species is unisexual and fertilization is internal. Females guard the fertilized eggs on the gills until the development of the veliger larvae, which after a short planktonic phase attaches to the hard surface or small bivalves are released directly from the female’s gills. This species spends its entire lifespan in the underground.
In the Red Book of Croatian cave fauna, Congeria kusceri is listed in the IUCN category of CR (critically endangered), which means that the risk of its disappearance in natural habitats is very high. The causes of its endangerment are manifold: pollution of groundwater due to the use of pesticides and fertilizers in agriculture, due to contamination from municipal and industrial wastewater and illegal waste disposal. Changes in groundwater regime occur due to hydrotechnical interventions including meliorations of karst fields, power plant constructions, uncontrolled extraction of groundwater reservoirs, etc. This finally leads to the alterations of direction of groundwater flow and/or groundwater level changes. Backfilling of the caves due to illegal construction and intensive urbanization is another problem for the survival of Congeria kusceri.
According to the Nature Protection Act (Official Gazette, . 80/13) Congeria kusceri and Congeria jalzici are strictly protected taxa. In addition to being protected by the Croatian law, the Dinaric cave clam is listed in the Annexes II and IV of the EU Habitats Directive. According to Directive, there are five Special Areas of Conservation for Congeria kusceri in Croatia: Northern Velebit National Park, Matica-Vrgoračko polje, Ličko polje, Jasena ponor, Neretva delta and Markov ponor.
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