Intracellular freezing and survival in the freeze tolerant alpine cockroach Celatoblatta quinquemaculata
The alpine cockroach Celatoblatta quinquemaculata is common at altitudes of around 1500 m on the Rock and Pillar range of Central Otago, New Zealand where it experiences freezing conditions in the winter. The cockroach is freeze tolerant, but only,to C. -9degreesC. The cause of death at temperatures below this is unknown but likely to be due to osmotic damage to cells (shrinkage). This study compared the effect of different ice nucleation temperatures (-2 and 4 degreesC) on the viability of three types of cockroach tissue (midgut, Malpighian tubules and fat body cells) and cooling to three different temperatures (-5, -8, -12 degreesC). Two types of observations were made (i) cryomicroscope observations of ice formation and cell shrinkage (ii) cell integrity (viability) using vital stains.
Cell viability decreased with lower treatment temperatures but ice nucleation temperature had no significant effect. Cryomicroscope observations showed that ice spread through tissue faster at -4 than -2 degreesC and that intracellular freezing only occurred when nucleated at -4 degreesC. From temperature records during cooling, it was observed that when freezing occurred, latent bleat immediately increased the insect's body temperature close to its melting point (c. -0.3 degreesC). This "rebound" temperature was independent of nucleation temperature.
Some tissues were more vulnerable to damage than others. As the gut is thought to be the site of freezing, it is significant that this tissue was the most robust. The ecological importance of the effect of nucleation temperature on survival of whole animals under field conditions is discussed.