Tolerance to freezing and supercooling of interstitial turbellaria and polychaeta from a sandy tidal beach of the island of Sylt (North Sea)

Abstract

Tolerance experiments on freezing and supercooling (without ice formation) were designed to determine correspondence between tolerance to low temperatures and spatio-temporal distribution of one turbellarian and five polychaete species from sandy beaches of the North Sea island of Sylt. Freezing tolerances are always less than supercooling tolerances. Dinophilus gyrociliatus is significantly more sensitive to freezing (LD50 value after 30 min of freezing:-3°C) than the others, whereas Stygocapitella subterranea is significantly less sensitive (50% mortality at-15.7°C after 30 min). The supercooling tolerances differ considerably among the species. The sequence of tolerances (LD50 values) is as follows: Microphthalmus sczelkowii (-2.9°C after 4 h); D. gyrociliatus (<1 h at-8°C); M. listensis (5.6 h at-8°C); Protodriloides symbioticus (8.2 h at-8°C); Notocaryoplanella glandulosa (66 h at-8°C); S. subterranea (72 h at-8°C). Species of sand flats (d. gyrociliatus, M. listensis, P. symbioticus) have lower tolerances than those of the beach slope (N. glandulosa, S. subterranea). Among the latter, tolerances increase with distance of the distributional area from low tide level. S. subterranea, a species occurring at the uppermost position in the intertidal, proves to be best adapted to both freezing and supercooling. Species preferring deeper regions of the beach (M. sczelkowii) show lower supercooling tolerances than surface dwelling forms. Northern species usually have higher tolerances to cold than southern ones, reaching their distribution limits near the island of Sylt.