 New students, in this topic we shall discuss the ectotherms living in cold and freezing environments. Living in cold environment has certain challenges. For example, the problems in maintaining metabolism and freezing threat. These challenges are because ectotherms cannot maintain their body temperatures and their body temperatures vary with the ambient temperature. So if there is a cold temperature or freezing temperature, their bodies should adopt that temperature, that is freezing temperature, that is not so living in such an environment is a challenge. New students, survival requires maintaining adequate metabolic rate. However, enzymatic activity becomes very slow in cold environments. Many animals living in cold environments have evolved specialized enzymes which can function optimally at lower temperatures. These enzymes, in comparison to their homologous enzymes, perform similar functions but can perform those functions at lower or colder temperatures. New students, in order to control enzymatic function, avoiding freezing threat is comparatively a challenging task. The function of ice crystals, which is called nucleation, is almost lethal if it is in a cell. Crystals rupture cells and tissues, and cells are destroyed. No animal can survive the complete freezing of its tissue water. Now we shall discuss few strategies of animals to avoid freezing threat. We just mentioned that ice crystal formation within these cells is damaging. So if ice crystals are formed in the body of an animal but not in the cells, so that is not lethal, that is less damaging. So the adaptiveness of animals is that if ice crystals are formed in freezing environment, then they become extra servular fluid and the ice crystals or nucleation process is stopped in the cells. This means that animals have a lot of adaptions and these adaptions use three major methods that animals use. First, to keep in their bodies ice nucleating agents i.e. INS. Second, to keep in their bodies antifreeze substances i.e. cryo-protectants. And third phenomenon is supercooling. We shall discuss all these three methods in detail. Ice nucleating agents are such agents which accelerate crystal formation i.e. nucleation. There are many such animals which keep ice nucleating agents in extra servular fluid. This type of INS helps to freeze extra servular fluid as quickly as compared to the intracellular fluid. Such substances include many inorganic substances, organic residues, different proteins and some microorganisms which are kept in extra servular fluid and work as ice nucleating agents. Dear students, when ice crystals are formed in extra servular fluid, then the entire extra servular fluid does not freeze. In fact, ice crystals are formed in it. Some parts of the extra servular fluid stay in the fluid form. That becomes more concentrated with solutes. When there are more solutes, then the water in the cell comes out of the extra servular space. When this happens, the intracellular fluid in the cell lowers its freezing temperature. It will not freeze on a normal freezing temperature, but it will require a lot of cooling. The result is that the cells are protected from making ice crystals. So, they can withstand freezing temperatures. While using the ice nucleation mechanism, the animals that survive, for example, we have a mage, our fly, which is a chironomus. It is found in the freshwater larvae which survives at freezing temperatures up to minus 32 degrees centigrade without freezing. Dear students, the second mechanism is to keep anti-free substance. Cryoprotectants are substances that are present in body fluids of some cold climate ectotherms. In these substances, in these cryoprotectants, there are different kinds of compounds such as glasrol, sorbitol and some anti-freeze proteins. These substances lower the freezing point of the fluids. For example, if the glasrol is present in the solution, the freezing point of the solution goes up to minus 17 degrees centigrade. If the glasrol is kept in this way, it means that the body will not freeze on a zero degree centigrade. Rather, it will have to go up to minus 17 degrees centigrade. This way, it is saved from freezing. Dear students, another or third most common method employed to avoid freezing is the supercooling. Supercooling is possible in some animals. This is a freeze avoidance strategy in which the body is completely avoided by an eating agent. No ice nucleating agent body is present. This is the pattern or mechanism used. Because ice nucleation, i.e. ice crystal formation, that requires some ice crystals as seeds or as nuclei. If any ice crystal is not present, then freezing does not start. When there is no ice nucleating agent in the body, then freezing does not start. Body fluids will be cooled below the freezing point, but they will not freeze. Some Arctic bottom-dwelling fishes are like this, which are supercooled without freezing. This is because they live in deeper waters where there is no ice. Ice is always on the surface. The water below is cold, but it does not freeze. If a fish comes to the surface and touches an ice surface, then immediately the nucleation process starts and it freezes and dies. Similarly, larvae of parasitic wasps, which are breccan sapphies, can be supercooled to minus 47 degrees centigrade without ice crystal formation.