J. Mar. Biol. Ass. U.K. (2007), 87, 1089–1094 Printed in the United Kingdom Journal of the Marine Biological Association of the United Kingdom (2007) doi: 10.1017/S0025315407055142 Benthic hydroids (Cnidaria: Hydrozoa) from Svalbard waters—biodiversity and distribution The aim of the present study was to update the existing knowledge of benthic hydroids from Svalbard waters. The material for the study was collected at several locations in the Svalbard region. The list of hydropolyps from the investigated area includes about 100 species. A total of 42 taxa was noted in the present study, 18 of which were reported for the first time in this area. One species is likely new to science. The analyses address the zoogeographical status of the species and the diversity of microhabitats. Arctic-boreal species were the main component of the hydroid assemblage, and eight types of substrata were colonized by the hydroids. Their highest biodiversity occurred on algae and bryozoans. Biological studies and faunal records used to monitor the Arctic region are of increasing importance in light of climate change and its inf luence on benthic community structure. INTRODUCTION Hydrozoa often have complex life cycles in which benthic asexual stages of hydroids alternate with free-swimming sexually reproducing medusae. As opposed to medusae, hydroids are mostly colonial and often epiphytic or epizootic. They are often the first colonizers of a vacant substratum (Hughes et al., 1991), and once established they may prevent the settlement of other colonizing animals by successfully defending the space (Young & Chia, 1987) and achieving a high biomass. They play an important role in the passage of energy from pelagic to benthic systems (Bouillon et al., 2004) by catching small plankton organisms and particulate organic matter and transferring them to the benthic domain. Studies of benthic hydroids are difficult for many reasons. The identification of hydroids poses serious challenges due to these organisms’ small size, morphological similarity, and wide phenotypic variation within species. They are often mistaken for algae or are even overlooked in benthic assemblages (Gili & Hughes, 1995). Knowledge of faunal distribution in the Arctic region is of increasing importance while that pertaining to the Svalbard region is key as this archipelago is located on the frontline of climate change. Climate warming in the polar regions is responsible for decreasing macrofaunal diversity which is linked directly to receding glaciers and the resulting high input of inorganic sedimentation (Wlodarska-Kowalczuk & Weslawski, 2001). It is necessary to collect as much biological information as possible in order to document and understand the changes occurring in ecosystems under dynamic Arctic conditions. Many studies have focused on Arctic Hydrozoa collected during polar expeditions (Broch, 1909, 1916, 1917; Kramp, 1911, 1932, 1943; Stepanjants, 1989; Schuchert, 2001a); nevertheless, knowledge of Svalbard hydroids is very limited. The first records of hydroids from this area date back to the 19th Century (Boeck, 1860; D’Urban, 1880; Marktanner-Turneretscher, 1895; Bonnevie, 1899). Some general information concerning the occurrence of hydroids in Svalbard waters was reported in several studies of benthic communities (Römer & Schaudinn, 1900; Odhner, 1915; Jäderholm, 1916; Rylov, 1923; Christiansen, 1965; Różycki & Gruszczyński, 1986), and these studies provided the foundation for compiling a list of 99 Svalbard hydroid species (Palerud et al., 2004). A critical re-evaluation of this list is necessary, however, in light of modern taxonomy and new records. Marta Ronowicz Institute of Oceanology, Polish Academy of Sciences ul. Powstancow Warszawy 55, Sopot 81-712, Poland. E-mail: martigor1@o2.pl Figure 1. Sampling sites in the Svalbard Archipelago. Black circles indicate the number of samples taken in each locality.