How Not to Freeze to Death — an Invertebrate Story of Survival
As the seasons change from fall into winter, we all feel that bitter cold deep in our bones. Thankfully we have warm blood, clothing, and shelter from wet, cold winter conditions. But what about sea life? While many of us may know that big mammals, like whales and seals, have thick blubber to keep them warm through colder temperatures, how do fish and invertebrates continue to thrive as temperatures in the oceans drop?
Fish and invertebrates (think sea stars, snails, and anemones) have unique adaptations to keep them alive and well through the winter months. Many invertebrates that live in temperate regions, like the BC coast, or polar regions have evolved in colder water over thousands of years, and best thrive in cool temperatures to begin with. In fact, many animals that live in cold water face challenges from an increase in temperature. Cold water animals are already well adapted to live in near freezing oceans.
Did you know that seawater can drop below 0 °C without freezing into ice? Many areas of the ocean drop below 0 °C, especially in polar, ice-covered areas where the water just below the ice can sit around -1.8 °C. Saltwater has a lower freezing point than freshwater. The sodium chloride (salt) in the water disrupts the process of ice crystals forming at a molecular level, so the water must be even colder for ice to form. Luckily for many marine animals, deeper ocean water is very stable at ~ 2 °C and does not dip below 0 °C. Animals that spend most of their time fully submerged in the deep don’t have to worry about their bodies coming close to freezing. It is shallow water or partial exposure to freezing air that can be bad news.
Why is freezing so bad for marine life? Cold temperatures can cause issues in many biological processes and can cause intense damage to cells. Most tissues and cells of living organisms have water inside of them, and as we know, when water gets very cold it turns to ice. Ice crystals are small and sharp, so having them inside of a cell can puncture the cell wall, causing damage and likely death to the cell. No amount of thawing can reverse this type of damage. In most animals, when enough cell damage occurs, ice crystal formation is deadly.
So how do fish and invertebrates survive frigid temperatures? (And do they have any tips for us?) There are two main ways to survive the cold: being resistant to freezing, and dealing with freezing. In cold areas, fish and invertebrate animals tend to focus on preventing ice crystal growth, in REALLY cold areas (frozen shorelines and the Arctic region) most animals have evolved to deal with the inevitable growth of ice in their bodies. Animals that are resistant to freezing typically experience cold temperatures irregularly or for short amounts of time. Animals that are adapted to deal with freezing experience very cold, sub-zero temperatures often and for extended periods of time.
Freezing resistant animals use a few different tactics to avoid any ice crystals from forming in their cells. Their goal is to withstand the cold enough to avoid the creation of ice. Many invertebrates and fish have a high level of salt in their bodies from living in the ocean. As mentioned above, salt lowers the freezing point of water, so having seawater in their body or a high level of salt in their blood is a great place to start when trying not to freeze. Ice forms best around a starting point, like a particle or protein, often called an “ice-nucleating agent”. Many animals rid their bodies of these agents by not consuming them or intentionally excreting them to make it much more difficult for ice to form.
The animals out there living in seawater below 0 °C have some very cool tricks to survive having sharp ice crystals frequently forming in their cells. One of the coolest tricks is the use of cryoprotectants (essentially antifreeze components) which are present in the blood and are the primary way polar fish and invertebrates deal with subzero temperatures.
Some fish, including the Notothenioidei family common in Antarctica, have developed special antifreeze proteins in their blood. Antifreeze proteins prevent multiple ice crystals from forming in tissues and lowers the freezing point in their bodies! It is really interesting that ice crystals still form in their bodies, but the antifreeze proteins prevent more from growing and act almost like a cushion around the ice crystals. Their tissues have adapted to dealing with the ice crystals so the ice doesn’t cause them harm as it would in other animals.
Similar adaptations are found in invertebrates that live in the intertidal zone where shallow water and exposure to cold air at low tide can cause intense temperature fluctuation. Mussels, barnacles, and snails are the main survivors here. They are not only capable of withstanding the temperature changes that come with being both in the water and out of water over the course of a day, they also survive seasonal temperature variations of the water and air over the course of a year. This is especially cool in Arctic regions where temperatures span a range of about 60 °C throughout a year. These invertebrates release chemical compounds in their tissues along with antifreeze proteins to prevent or withstand minor ice crystal formation. Some species of oyster and mussel have even been found to alter their fat tissues in response to cold, changing the structure so they are less likely to freeze.
While the cold, coastal winter is upon us, feel comforted in your warm sweaters and socks knowing you are not a salty barnacle in the intertidal zone or a Notothenioidei living just below the ice in Antarctica, with antifreeze pulsing through your veins.
By Jordan Hawkswell