Here in most of the US, days are darker and temperatures are dropping, as humans are wrapping themselves in scarves and hats. While many of us are preparing for skiing and sledding season and are donning our fuzziest sweaters, we may not be aware of what other living creatures are doing to prepare for winter. While we’ve heard about snoozing bears and bird migrations, what does sea life do when the colder weather hits? Where do fish go when it freezes?
Totally Tolerant
Creatures such as some shark species that have been around for millions of years have learned to adapt to different surroundings, and are not bothered by cooling temperatures. Sharks living in icy waters are known as endotherms, with the biological ability to raise their blood temperature to complement their surroundings. Endothermic creatures are usually large, with well-insulated bodies. They store fat immediately beneath their skin, which helps generate warmth. Their diets contain an abundance of high-energy, easy-to-digest foods that constantly keep their body temperatures high.
Unique to endotherms is the ability to shiver when cold. This rapid, rhythmic contraction of skeletal muscles creates its own source of heat by the physics of muscles burning energy.
Dolphins, sea lions and walruses are also endotherms. In fact, pudgy newborn harbor porpoises pack an astounding 43 percent of their total body mass in blubber!
Movin’ On Down the Road
Not every sea creature increases its body weight to protect itself from icy waters. Like many land animals (and heat-loving humans), some fishes and aquatic life migrate. Many animals that frequent coastal waters - from herrings to great white sharks - head south for warmer waters during winter.
Humpback whales, for example, migrate about 3,000 miles on average; one of the longest migratory journeys of any mammal on Earth. Humpbacks migrate to give birth. The water in Antarctica, where humpbacks spend their summer months, is just too cold for newly born baby whale calves to survive. They use their time up north to get nice and fat on their mother’s milk, which allows their little bodies to build up strength and withstand the colder waters on the journey home. Humpback whales return to Antarctica, when water temperatures warm up, for food! Antarctica is teaming with delicious krill, and the whales spend their summer feeding and building up their strength for the next winter migration.
Southern right whales are similar to humpbacks, in that they feed in Antarctica in the summer and then migrate north to Australia to breed and give birth (especially in southern corners of Australia, around the Great Australian Bight).
Atlantic herring migrate in schools to areas where they feed, spawn, and spend the winter. This superfish swims rapidly, allowing it to migrate over great distances in a short period. And, speaking of great distances, Adélie penguins travel about 11,000 miles to migrate to their winter-feeding grounds, the longest migration of any penguin.
Let This Sink In
Some animals have to take special steps to withstand lowering temperatures. In areas with extreme freezing temperatures, it’s often hard for creatures to find warmer waters. These creatures, known as ectotherms, have become accustomed to lowered temperatures and, as such, they have developed ways to stop themselves from freezing.
Ectotherms are cold-blooded animals, which means that the animals’ regulation of body temperature depends on external resources, such as sunlight or warmed rocks. Ectotherms include most fishes, amphibians, reptiles and invertebrates. The body temperatures of aquatic ectotherms are usually very close to their aquatic environments.
For ectothermic creatures to survive the cold winters, they have developed specific strategies to ensure they can keep warm. Using the technique of basking, creatures such as turtles lie in the heat to increase their body temperatures. When their body temperatures become cooler, they move around less and slower, to help conserve heat and energy. In this resting state, fishes' hearts slow down, their need for food and oxygen decreases, and they move very little. Certain species of cod, flatfish and polar fish have a reduced metabolic rate and produce antifreeze molecules called glycoprotein to reduce the freezing point of their body fluids.
Burrowing strategy is also used to protect creatures from cold air and cooling waters. The layer of ice that forms on top of a lake, pond, river, or stream provides some insulation that helps retain water temperature. Because warm water sinks in very cold freshwater, fish in these water bodies often gather in groups near the bottom. Some species, like koi, may burrow into soft sediments and go dormant like frogs and other amphibians, but the majority of fish simply school in the deepest pools and take a "winter nap."
Most turtles burrow into the mud in winter, and become inactive during the coldest months. However, some turtles have an unusual ability to survive very long periods of time without oxygen. These turtles enjoy their normal activities throughout the winter.
So, next time you’re feeling chilled, and you’re ready to crank up the heat, be grateful that you don’t have to pack on extra pounds or find a warm pool of mud to borrow into!
