First record of the freshwater sponges (Spongilla) in the inland rivers of Bulgaria

Abstract Records from freshwater sponges on Bulgarian territory are scarce. Only two exist in the scientific literature, one from the Zhrebchevo Dam and one from the Bulgarian part of the Danube River near the border with Serbia. Inland lotic ecosystems such as channels, streams, and rivers, as well as lentic ecosystems, were not studied for this potential biological indicator. Due to the existing knowledge gap, studies of entire rivers and other aquatic ecosystems focused on the monitoring of biological or physicochemical quality elements should be vigilant for Spongillidaе family representatives since even their random discovery and identification could be of great importance. Such records can be considered interesting for gaining knowledge of their distribution, species richness, habitat preference and particular environmental variables dependencies. We investigated macroinvertebrates and macrophytes from 16 sampling sites in a river with a length of about 200 km and a watershed of 1600 km2 to register two Spongilla lacustris individuals at two sites without any significant hydro-morphological pressure from big dams. Our results could serve for monitoring program optimization that aims to investigate rare protected species dependent on freshwater sponges and on seeking more localities with established Spongilla populations. This can be achieved by directing sampling efforts on sampling sites that have similar habitats and have a hydraulic connection with the found specimens.