engleski

First record of freshwater jellyfish Craspedacusta sowerbii in a Mediterranean karstic river Krka (Croatia) and a promising method for polyp detection

Craspedacusta sowerbii is a freshwater hydrozoan species considered to be invasive in the EU. The species originates from Eastern Asia and has colonised all continents with the exception of Antarctica. Craspedacusta sowerbii is mainly detected in a stage of hydromedusae, albeit there are evidences that species has much wider distribution, persisting in many water bodies in a stage of hydropolyp. Whereas hydromedusae of this species can influence several trophic levels in plankton, little is known on the hydropolyp influence. The data on C. sowerbii distribution in Croatia are very incomplete, with only few scientific studies. Current research presents first record of C. sowerbii in a karstic Mediterranean river Krka, within National Park Krka, Croatia. The hydrozoan was initially detected during a broader study of tufa deposition and periphyton development within NP Krka. Later on, the study was extended to studying C. sowerbii distribution. Aiming to cover a broad range of the hydrozoan traits, artificial substrates (glass slides) were submerged in lotic and lentic habitats at three differing sites (i.e., at tufa barrier Skradinski buk, and within two upstream barrage lakes – natural Visovac Lake, and man-made Brljan Lake). The slides were retrieved every 1-2 months during 2014 and 2015. Natural substrate (moss covered tufa and cobbles) within the sites was also sampled and checked for the presence of polyps. Observations were done on live material. Craspedacusta sowerbii was detected only in tufa barrier lotic environment, contrary to some prior studies’ findings on the species preference for stagnant water habitats. We detected polyps, both single and in colonies, and frustules. The latter are produced through budding of polyps, evidencing asexual reproduction. Artificial substrates had around 15× higher densities of polyps compared to natural substrates (average 170 ind./dm2 vs. 11 ind./dm2, respectively). Accordingly, C. sowerbii might prefer substrates with less developed periphyton. It is, however, possible that the artificial glass-substrate provided better possibility for polyp detection. Polyps are small and difficult to detect under stereomicroscop, while glass slides allowed microscopic observations. Method used in this investigation might be a novel inexpensive tool to monitor C. sowerbii in water bodies, allowing detection of polyp stage. Hydropolyps were observed to mostly feed on aquatic insect larvae. They seemed uninterested on protozoans as pray as some ciliates (e.g., Vorticella spp. and Stentor coeruleus) were observed attached and intact on body stalks of polyps. These findings give new information on the ecology of C. sowerbii in benthic form, and its potential ecological niche. Further studies are needed to test its invasive potential and impact, both in benthos and in plankton.

hydropolyp; artificial substrate; invasive species

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