Inoculative freezing and the problem of winter survival for freshwater macroinvertebrates
Due to the thermal buffering of their environment, aquatic invertebrates are less likely than their terrestrial counterparts to face temperatures substantially below 0°C. Aquatic invertebrates may not be able to avoid internal freezing by supercooling (remaining unfrozen at temperatures below the freezing point of their body fluids), however, because when their body temperatures reach the freezing point of body fluids, these organisms will likely be in contact with external ice, which may induce formation of internal ice (i.e., inoculative freezing). In this study, a variety of winter-collected, aquatic invertebrates (a clam, Sphaerium sp.; an isopod, Lirceus fontinalis; a mayfly, Stenomena femoratum; a belostomatid, Belostoma flumineum; 2 dytiscids, Ilybius oblitus and Agabus disintegratus) and, for comparison, a terrestrial beetle (Hippodamia convergens) were studied with respect to their low temperature tolerance. No species appeared to lower its freezing point appreciably by accumulating colligatively active solutes in body fluids, and all aquatic species supercooled moderately (-5 to -7°C), but significantly less than the terrestrial beetle (-16°C), before freezing when chilled in a dry environment. However, when chilled in contact with external ice, all animals froze at their melting points or just below (as low as ∼-2°C for the beetles), showing that they are susceptible to inoculative freezing. All aquatic species readily survived exposures to sub-zero temperatures when supercooled, but succumbed to the same conditions when inoculated by external ice. Survival of low temperatures by temperate zone aquatic invertebrates appears to depend upon thermal buffering provided by the aquatic environment, slow progress of ice formation in bodies of water at high sub-zero temperatures, avoidance of contact with external ice, and tolerance of high sub-zero temperatures when freezing does not occur in the animals themselves.