ORIGINAL PAPER Clearance rates in the Arctic bivalves Hiatella arctica and Mya sp. Received: 4 October 2002 / Accepted: 18 January 2003 / Published online: 27 February 2003 Ó Springer-Verlag 2003 Abstract Filtration was studied in two Arctic clams, Hiatella arctica and Mya sp., collected in Young Sound, Northeast Greenland. Clearance rates were determined as a function of ambient temperature and algal cell concentration, using the clearance method and feeding with a unicellular flagellate. For both species, clearance rates increased with increasing temperature from <)1 up to 4–8°C. At higher temperatures, filtration ceased and the clams closed their valves. Clearance rates were also determined in temperate specimens of H. arctica collected on the west coast of Sweden. For these speci- mens, clearance rates increased with increasing temper- ature from 0 to 18–20°C. When weight-specific clearance rates were compared between the two populations and between species, there were no differences at 1°C. Clearance rates in Arctic H. arctica were maximal at algal cell concentrations corresponding to 2.5–8 lg chlorophyll a l -1 . Temperature compensation in Arctic bivalves is discussed and it is concluded that adaptations to constant low temperatures consist of a lower mini- mum temperature, for active filtration. Low clearance rates due to low temperatures did not seem to limit growth, under the prevailing conditions in Young Sound. Introduction Sessile suspension-feeders like bivalves, ascidians and sponges, constitute an important component of macro- zoobenthic communities in Polar regions (Arntz et al. 1994; Sejr et al. 2000). Despite the extreme physical conditions, abundant and diverse assemblages have been established but with lower production to biomass ratios compared to the temperate regions (Brey and Clarke 1993). In contrast to the Antarctic, Arctic macro-zoo- benthos include a high proportion of species with a distribution that ranges over both Arctic, sub-Arctic and temperate regions (Dunton 1992). Such species are use- ful for comparative studies. The low growth rates typically found in Arctic macro-benthos (e.g. Bluhm et al. 1998; Sejr et al. 2000) may be caused by two factors: either the constant low temperatures around or below 0°C, or resource limita- tion caused by the low annual primary production. In field studies, it can be difficult to distinguish between the effects of either limiting factor, as they are both present. Originally, it was believed that animals living at very low temperatures should show a relatively high level of standard metabolism, thereby leaving less capacity for growth. However, many studies have demonstrated that Polar marine ectotherms have low standard metabolism (Peck 1998, 2002). All processes involving a change in free energy are affected by temperature. Unless an evolutionary compensation (e.g. through speciation) has altered primary physiological and biochemical process- es, a marked rate decrease would be expected in ectotherms living at low temperatures (Clarke 1998). The rich and diverse benthic fauna of Polar regions suggests an adaptation to low temperatures, although studies of isolated enzymes have shown typical thermal dependency (for review see Clarke 1998). On the whole-body level, the acquisition of food to fuel metabolic processes is considered a key process in relation to growth. Compensation for rate reduction in suspension-feeders caused by low temperatures can be Polar Biol (2003) 26: 334–341 DOI 10.1007/s00300-003-0483-2 Jens Kjerulf Petersen Æ Mikael K. Sejr Jens E.N. Larsen J.K. Petersen (&) Æ J.E.N. Larsen Department of Marine Ecology, National Environmental Research Institute, Frederiksborgvej 399, P.O. Box 358, 4000 Roskilde, Denmark E-mail: jkp@dmu.dk M.K. Sejr Department of Marine Ecology, University of Aarhus, Finlandsgade 14, 8200 Aarhus N, Denmark