Sea otters are the smallest marine mammal and lack insulating blubber. They rely instead on the densest fur in the animal kingdom to trap air. This insulation requires a metabolic rate two to three times higher than that of a similar-sized terrestrial mammal to maintain body temperature. This high metabolism forces adult sea otters to consume approximately 20 to 25% of their body weight in prey every day.
The Core Menu of Invertebrates
The sea otter’s diet consists almost entirely of marine benthic invertebrates found on the seafloor in nearshore coastal waters. A staple food item across their range is the sea urchin, a prickly herbivore. Their menu also includes a variety of mollusks, such as clams, mussels, and abalone, which are prized for their caloric density.
They also consume crustaceans, including crabs, marine worms, and sea cucumbers. The specific composition of an otter’s diet depends highly on its geographic location and the local availability of prey. In newly colonized areas, otters tend to target larger, higher-value prey, but as the population grows, they broaden their diet to include a greater diversity of smaller and harder-to-process items.
Specialized Foraging Techniques
To access their calorie-rich diet, sea otters employ remarkable foraging behaviors, starting with dives to the seafloor to collect prey. While they typically forage in shallow waters, they can dive to depths of 100 meters or more and hold their breath for several minutes. Once prey is secured, the otter uses loose folds of skin under its forearms, which act as natural pockets, to temporarily store collected food and even a favorite rock.
The most recognized foraging technique involves tool use, making them one of the few mammals known to employ objects to process food. An otter often brings a rock or hard object to the surface and uses it as an anvil or hammer to crack open the shells of clams, mussels, or snails. They perform this shell-cracking while floating on their back, using their chest as a stable feeding platform. This tool use is especially common among females, allowing them to access harder-shelled prey and minimize dental wear.
Diet’s Ecological Impact
The sea otter’s continuous appetite for invertebrates defines them as a keystone species in the marine ecosystem. By controlling populations of grazing herbivores, they indirectly maintain the health and structure of underwater habitats. A primary example of this influence is their predation on sea urchins.
When sea otters are present, they keep sea urchin numbers in check, preventing the urchins from overgrazing kelp forests. Without this predation pressure, urchins multiply rapidly, consuming the base of the kelp and creating barren seafloors known as “urchin barrens.” By protecting the kelp, the sea otter’s diet supports a more biodiverse environment that provides shelter and food for countless other species.