The vast, white ice surface of Antarctica stretches for over 3 million square kilometers: empty, desolate, and almost completely silent. But appearances can be deceiving. On the coast of this expanse, just a few meters beneath the ice lies a multicolored constellation of life. This remarkably diverse realm is home to over 8,000 species of sea denizens who rely on an arsenal of otherworldly traits to survive. So how do these species not only live, but thrive, in conditions most animals would be unable to bear?
Antarctic waters are some of the most consistently frigid in the world, hovering below 0 degrees Celsius for a large portion of the year. This means burning energy too quickly can be deadly, so survival in this ecosystem looks slow and steady. Freezing temperatures persist even in the summer, but this season brings a rare gift: sunlight. For a brief period, it’s abundant, shining through the ice for 24 hours a day. This infusion of energy drives intense coastal phytoplankton blooms, transforming the waters into a thick green soup.
Marine life both large and small takes advantage of this bounty, including the giant Antarctic isopod. Cousins of the humble pill bug, these crustaceans can reach up to 11 centimeters long, and they never miss the opportunity for a meal. The scavengers eat a wide variety of prey, including other giant isopods. And by slowing their metabolisms, they can make this food last, with one study showing specimens surviving for 50 days without eating. Since this adaptation involves careful conservation of energy, giant isopods spend most of their time stationary or inching across the seafloor. Meanwhile, their tiny amphipod relatives celebrate the summer by releasing offspring alongside the algal explosion, ensuring their young have an abundance of food. And sea cucumbers carpeting the seafloor hoover up the dense plankton with outstretched tentacles, producing nutrient-rich feces that nourish nearby life.
But this big summer blowout doesn’t last long. The first signs of autumn arrive in fine needles of frazil ice. These slowly coagulating crystals form a skin across the surface, then mix with falling snow before freezing into a thin crust of ice. The waters get darker and colder. And in this swiftly dimming world, a set of long, spiky limbs sidle into view. With up to six pairs of legs, these giant sea spiders aren’t arachnids, but rather a related class of marine arthropod unique to the seafloor. In addition to housing some of the animal’s organs, its legs are covered in tiny holes, which grow more numerous as the spider ages. These holes will likely help absorb the dissolved oxygen that saturates these freezing southern waters. Since oxygen fuels growth, many local species have evolved to take advantage of this abundance, and it may be one of the reasons that gigantism is so common in this region. Individual sea spiders, for instance, can grow to the size of dinner plates. But soon, these underwater giants will be moving slower than ever.
As winter settles in, the sea floor becomes even colder. Waters fall to negative 1.8 degrees Celsius. On the surface, the thin icy crust thickens into a layer called nilas, and young sea ice starts forming ridges that block out the sun. The ocean begins to mirror the still landscape above it. Antarctic sea cucumbers and urchins go into dormancy for months, and their metabolic rates fall to the slowest on Earth. Antarctic limpets continue feeding, but at such a gradual pace that they're largely still surviving on energy reserves from the summer. Tiny crustaceans survive off the traces of algae growing on their home’s icy ceiling, raising their young in these expansive winter nurseries.
But this seemingly endless winter won't last forever. As spring comes, light slowly begins to trickle back down through the ice. And week by week, bit by bit, this underwater world will begin waking up to begin its delicate, slow-motion dance once again.
