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The underground world of karst: Dinaric cave tubeworm


The Dinaric cave tubeworm inhabits the submerged speleothems in the Pit opposite Torak in Krka NP

The Dinaric cave worm, Marifugia cavatica Absolon & Hrabe, 1930, is a paleoendemic species of the Dinarides. This stygobiont species inhabits the underground freshwaters of the Dinaric karst from Italy to Albania, and is the only known representatives of the polychaetes, tubeworm family Serpulidae, to live a subterranean life. It is believed that during the Pliocene, in inhabited surface lakes.

This is a sessile organism, living attached to the rock (or other hard substrate). It builds a limestone tube around its body, about 1 mm in diameter, and up to 6 cm long. Along the tube is an irregularly toothed and interrupted ridge, with irregularly spaced rings. It extends its feathery branches outside the tube to catch and filter organic matter from the water. In unfavourable conditions, it retracts the entire body back into the tube, and closes the operculum. This adaptation has enabled it to survive low water levels underground, and even longer periods without water. It typically inhabits waters in the temperature range of 4 to 19°C. Depending on the speed of the current in the ground waters, it can live individually or in colonies. Dense colonies are built in calmer flow area, with numerous other aquatic cave organisms living between the limestone tubes of this tubeworm. There are some 70 localities of this species known in Croatia, and with the exception of the K2 cavern on the island of Brač, all are on the mainland.

It is interesting that this species inhabits all localities where other subterranean sessile species are found, such as the Dinaric cave clam Congeria kusceri Bole, 1962 and the Ogulin cave sponge Eunapius subterraneus Sket and Velikonja, 1984.

In Croatia, M. cavatica is a strictly protected species. Its colonies are a unique habitat in the National Habitats Classification, H Marifugia deposits.

As a sessile species, it is directly threatened by changes to the water regime following different hydrotechnical works, and different types of pollution that can enter into the ground water.

In speleodiving research of the structure Pit opposite Torak in 1998, the first locality of this stygobiont polychaete in Krka National Park was found. In this cave, the Dinaric cave tubeworm colonizes submerged speleothems. The Pit opposite Torak was submerged with the rising water levels in Visovac Lake and the growth of the travertine barrier at Skradinski buk. The dive survey explored the structure to a depth of 17 metres, and confirmed a connection between all three lakes in the pit.

Photo: Vedran Jalžić