Hard bottom polychaete assemblages in the Northern Adriatic Sea: diversity and patterns of spatial distribution.

Most studies of the polychaete fauna in the Northern Adriatic Sea deal with soft bottoms, while hard bottom studies are very scarce and the last extensive one was conducted by Amoureux in 1975. The Northern Adriatic Sea is a peculiar and sensitive part of the Mediterranean region, currently undergoing through severe changes caused by the combination of natural and anthropogenic factors. In order to get an updated measure of the hard bottom polychaete diversity in the area and assess their spatial distributional patterns, new studies are necessary. In summer 2007 polychaetes were sampled from hard substrate (by scraping off the hard bottom benthos within a 10x10 cm metal frame) on three stations along the Sveti Ivan Island in the vicinity of Rovinj, at 1.5 m, 5 m and 25 m depths. Altogether 103 taxa were found; four species are new for the Adriatic Sea (namely Lepidonotus tenuisetosus, Nereis perivisceralis, N. splendida, N. usticensis), while 19 are recorded for the first time in its northern part. Some newly recorded species are dominant and typical of warmer areas. Multivariate analyses showed that the polychaete assemblages significantly differed both among depths and sites, the two shallower depths being more similar among each other. The depth influenced polychaetes horizontal distribution patterns, since significant differences of both abundance and faunal composition characterised the shallowest assemblages (1.5 m), while at 25 m spatial variability was only due to the abundance. Uncommonly, the lowest horizontal variation of polychaetes distribution was observed at 5 m, although it was expected at 25 m depth due to a higher environmental stability. The high number of newly reported species confirms the importance of regular taxonomic updating of the polychaete fauna in the Northern Adriatic.