Marine Biology 94, 403-407 (1987) Marine © Springer-Verlag 1987 Niche overlap in mud snails (Hydrobiidae): freezing tolerance J. Hylleberg 1 and H. R. Siegismund 2 1 Department of Ecology and Genetics, University of Aarhus; Ny Munkegade, DK-8000 Aarhus C, Denmark 2 Zoological Institute, University of Munich; Seidlstra6e 25, D-8000 Miinchen 2. Federal Repuhlic of Germany Abstract Tolerance to freezing was measured in four species of hy- drobiid snails exposed to temperatures from 0° to -8°C and salinities from 0 to 30%0 S for up to 7 d. Experiments showed the following increasing order of tolerance to freez- ing: Potamopyrgus jenkinsi (Smith), Hydrobia neglecta Muus, H. ulvae (Pennant) and H. ventrosa (Montagu). Sur- vival decreased with increasing salinity in P. jenkinsi, while the opposite was found in Hydrobia species. H. ulvae sur- vived only slightly better than H. neglecta, while H. ven- trosa was very tolerant to freezing even at low salinities in contrast to the other species of Hydrobia. H. neglecta suf- fered higher winter mortality than H. ventrosa during the severe winter of 1985 in an estuary. Introduction Hydrobiid mud snails are very common in shallow marine and brackish environments. In Denmark it is hardly pos- sible to find a single square meter of sea bed where they are missing, except on black, stinking mud and exposed sandy beaches (Muus, 1967; Hylleberg, 1986). The popula- tion density of these small snails (2 to 5 mm) is often in the range of 20000 to 40000 m- 2 • The snails are deposit feeders and able to digest a variety of the microorganisms associated with sediment particles (Kofoed, 1975; Lopez and Levinton, 1978). Diatoms are the most important food source (Muus, 1967; Fenchel and Kofoed, 1976; Jensen and Siegismund, 1980). Three species from the genus Hy- drobia are found: H. ulvae, H. neglecta, and H. ventrosa; in addition, Potamopyrgusjenkinsi can be common. The species may coexist in the same patch of sea bed, but rarely in equal abundance (Muus, 1967; Fenchel, 1975; Hylleberg, 1986). Hydrobiids display exploitative interspe- cific competition in laboratory experiments (Fenchel and Kofoed, 1976). Since the snails obviously compete for im- portant food items, it is of interest to estimate as many pa- rameters of the fundamental niches as possible in order to judge the integrated responses to environmental variations. Some parameters of the fundamental niches of hydrobiids have previously been reported in Hydrobia species (Hyl- leberg, 1975, 1976; Lassen and Kristensen, 1978). In na- ture, hydrobiids are subjected to a variety of en- vironmental changes which determine the response of the snails. Both salinity and temperature change seasonally. Therefore, the distribution group of interacting species may be restricted severely during a certain season. This may al- ter the importance of biological interactions. The purpose of the present study is to show tolerance to freezing in combination with salinity. Such data were necessary for the interpretation of the distribution of the four species of mud snails in an estuary (Siegismund and Hylleberg, 1987). Materials and methods Experimental determination of freezing tolerances Three experiments were carried out: In the first, Hydrobia neglecta Muus and H. ventrosa (Montagu) were collected from Kysing Fjord (Siegismund and Hylleberg, 1987), H. ulvae (Pennant) from the Wadden Sea (Site 8 in Hylleberg, 1986) and Potamopyrgus jenkinsi (Smith) from the river Mattrup A. The snails were collected in April 1979 after a severe winter, stored at 3°C at 20%0 S (Hydrobia) or fresh water (P. jenkinsi), and provided with surface sediment as food source until used in experiments one month later. Batches of ten individuals were transferred directly from storage tanks to 80 test tubes with 20%0 S sea water (Hy- drobia spp.) and fresh water (P. jenkinsi). The tubes were placed in a _7°C water bath containing anti-freeze fluid, a crystal of ice was added, and the tubes were sealed with parafilm. The lO-ml test tube water immediately froze solid. At 12- to 24-h intervals the tubes were transferred to 3°C for I d; surviving snails were then counted at room