Sponges (Porifera) are animals that live in aquatic habitats. Numerous marine sponges are known from caves filled with sea water. However, the only specialized representative in freshwater subterranean habitats is species from the Spongillidae family: Ogulin cave sponge (Eunapius subterraneus). The species is endemic to Croatia and it is described from the surroundings of the city of Ogulin.
The Ogulin cave sponge grows up to 1 cm in diameter and, by grouping with more individuals, makes egg-shape or plate-shape habitus from 1 to 8 cm in size. It is not yet clear which factor is decisive for the development of a particular habitus form. The habitus is likely influenced by the flow rate of water in submerged cave channels or lakes. The surface of their body is very smooth and soft, with large osculum and atrium, and white in colour. Skeleton is made from a net of mostly single spicules connected with little spongin.
Like all other sponges, they live a sessile life, meaning that they are attached to cave walls or under big stones where they live their entire life. The Ogulin cave sponge lives in a slower water flow with a temperature range between 7 and 11 °C. All populations are found only in groundwater where they inhabit total darkness, except in one locality where on one occasion only, specimens were found in the twilight zone. The species has so far been recorded at 21 localities, including the Cave system Lukina jama – Trojama, the deepest cave in Croatia and the only locality in the Adriatic basin.
Groundwater in the karst areas is heavily influenced by external factors, including pollution and devastation. Cave animals can only survive within a narrow range of environmental conditions. This makes them very sensitive to any changes, but also excellent indicators of condition changes in cave habitats. The most common indicator showing the degree of habitat vulnerability is the decrease or complete disappearance of a cave population. Examples of this are the Đula – Medvedica cave system next to Ogulin and the Špilja u Kamenolomu Tounj cave, the localities in which once-stable cave sponge populations could not be found during recent research. In both cases, there is a major negative human impact on the environment and the caves, which resulted in both caves being placed on the list of 10 most endangered karst phenomena in the world.
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