When was the last time that you woke up thirsty in the middle of the night? Maybe you got out of bed and turned on the light. Groggy with eyes blinking from the brightness, you staggered to the kitchen. After drinking a glass of water, you turned off the light, but the hazy afterimages in your vision forced you to rely on your memory and touch to return to your bed. You might consider this scenario a minor inconvenience, but consider if you had to repeat this every three minutes for the next four hours?
Sea otters run through this routine regularly, but instead of going to the kitchen for a glass of water, they dive 30 to 100 feet to find and catch an unsuspecting meal at the seafloor. Light attenuates more quickly under water than in air, so even if the sun shines brightly at the water’s surface, shallow-diving sea otters hunt in semi-darkness. Unlike seals, sea lions, and whales, sea otters do not eat under water, so they bring what they catch to the bright light at the water’s surface to process and consume. After a quick spin to wash off lingering debris from their hard-shelled invertebrate snack, sea otters dive back to the dim seafloor to search for the next bite. While sea otters may get a reprieve from stark light transitions between dawn and dusk, they still successfully forage, groom, and socialize in darkness throughout the night [1-3]. Prolonged seasonal shifts are a fact of life for sea otters in Alaska, who experience nearly 24 hours of daylight in the summer and about 18 hours of night in the winter.
Caption: The tapetum lucidum in a young, female sea otter reflects the camera flash as a green-yellow circle within the pupil at dusk. The tapetum lucidum in the same sea otter viewed with a special camera reflects a gradient of turquoise, green, yellow, orange, and reddish-brown The presence of the tapetum lucidum in the upper portion helps reflect light from the lower visual field (as in human eyes, images are upside down on the retina and the brain flips the image perceived by the eyes). Researchers have been unsure how to interpret observations of wild sea otters in the context of vision for decades. Sea otter prey is often visually camouflaged or buried, and the digging methods that sea otters use to extract them can actually make the surrounding water cloudy [4,5]. However, some sea otters seem to seasonally shift their foraging activity to daylight hours , which suggests that vision improves sea otters’ ability to find and capture prey, thereby conserving energy. As the folks at Sea Otter Savvy can attest, sea otters are highly visually attuned to disturbances and direct their gaze toward the offender when deciding whether to settle back in for a nap or leave the area.
So what is a sea otter to do? Vision takes a lot of energy to maintain, and researchers have consistently found that conserving energy is one of the top priorities that drives sea otter behavior. Have sea otters reduced their reliance on vision or have they developed specialized visual adaptations that help them navigate a highly variable and wide-ranging light environment?
Recent research by my team has helped to fill in longstanding gaps to help us answer these questions . We examined how parts of the sea otter eye—the pupil, retina, and tapetum lucidum—contribute to vision. We found that the retina resembles those of other nocturnal mammals with a dense array of photoreceptors (light-sensitive cells) that is heavily dominated by highly light-sensitive rods. Sea otters have a well-developed tapetum lucidum (Latin for “bright tapestry”) at the back of their eye, which acts like a mirror to reflect light to the retina. Humans and other primates do not have a tapetum lucidum, but it causes the eyeshine you notice in your dog or cat at night. Together, these findings indicate that sea otter eyes are sensitive enough to see reasonably well in bright light and low light, both in air and under water.
Size range of the circular pupil in an adult, male sea otter, photographed across day (left photo) and night (right photo). The scale bar in the upper-right-hand corner represents 1 mm.Sea otter pupils are highly mobile in size, but not as much as seals and sea lions , and the pupil’s constriction response is slightly slower than in other species measured [9-11] . Just like a camera’s aperture, the pupil’s size and shape determines how much light is transmitted to the lens then refracted to the photoreceptors and tapetum lucidum. These observations suggest that although sea otters can see over a wide range of light levels, they may be limited in adjusting to the rapid and extreme change in light conditions when diving during the day.
However, sea otters’ ability to see across a wide range of light levels doesn’t necessarily mean that sea otters see clearly across light levels. When sea otters rest at the water’s surface, their eyes focus light the same way as human eyes: light bends as it travels from the air through the eye due to differences in optical density. When light bends just the right amount, it converges at the photoreceptors on the retina, resulting in a clear image. Even if you have clear vision in air, have you ever tried opening your eyes under water in a swimming pool and noticed that your vision is blurry? Since the optical densities of water and your eye structures do not differ much, light bends less as it passes through the eye. The result? Light converges behind the retina, resulting in a blurry image. Swim goggles remedy the blurry image for humans, since light travels through the air in the goggles before reaching the eyes.
Special staining techniques help us to better visualize the sea otter retina, in which the photoreceptors resemble a galaxy of colored stars! Highly light-sensitive rods (in blue) dominate the retina relative to cones (in green).In ideal conditions in air, sea otters can see about as clearly as seals, sea lions, and walruses [12-22], which is about 7x less clear than humans with 20/20 vision. If you are near-sighted, and your prescription is between -2.00 and -3.00, a sea otter may see about as clearly as you do without your corrective lenses (although keep in mind this is a very rough approximation). We have evidence that sea otters see just as clearly under water as they do in air , so what is the sea otter version of swim goggles? Sea otters can squeeze their lenses through their pupils into a more rounded shape when diving . The more rounded shape bends the light more, which compensates for the reduced bending effected by water, and light properly reaches the retina, enabling a clear image. Although humans can change the shape of our lens when focusing on near and far objects (termed accommodation), our abilities and those of most other vertebrates measured are nowhere near as impressive as sea otters. In fact, sea otters have one of the highest accommodations measured in vertebrates, rivaled only by freshwater otters and diving birds like cormorants [24-27].
Since the pupil plays a key role in reshaping the lens of sea otters when under water, what happens to sea otter vision during low light levels when the pupil dilates to capture more light for the retina? Similar to trying to blow a bubble with an open mouth, if the lens is squeezed through a wider aperture, it will be less round and bend light less, resulting in a blurry image. This means that the sea otter version of swim goggles only works to produce a clear image under a narrow range of light levels.
Darkened daytime conditions like those experienced during California's 2020 wildfires may have triggered sea otter nighttime foraging superpowers. Photo by Sharon HsuSo, how does the combination of pupil, lens, photoreceptors, tapetum lucidum, and accommodation all factor into the visual world of wild sea otters? In air, sea otters likely use vision during the day in social interactions and to detect and avoid dangerous situations. Even though sea otters don’t have highly acute vision to resolve fine detail, they can likely ascertain danger based on contrast due to brightness or color differences (yes, sea otters can see color , similar to a red-green colorblind human!. Current predators of sea otters, including bald eagles, coyotes, and brown bears, tend to hunt diurnally from air or land, as did a former predator of sea otters—humans—prior to the last century [29-31]. When foraging under water, sea otters surely use vision during brighter light to detect non-buried prey and associated environmental features, and they may use vision in dimmer light to detect large environmental features associated with prey. However, sea otters are unlikely to use vision in low light to detect non-buried prey or fine details of the underwater environment.
Like all living organisms, sea otters are not restricted to a single sense for any one behavior. We’ve known anecdotally for decades that sea otters can compensate for vision loss, since they can forage just as successfully during low-light levels and periods of poor water visibility, and when hunting for buried prey. Recent research has confirmed that this ability likely results from highly sensitive whiskers and paws, which can discriminate fine detail as well as humans and other touch specialists, but incredibly faster .
We still have so much to learn about sensory biology and its influence on behavior, not only in charismatic marine mammals like sea otters, but also in species less traditionally considered just because they seem very different than humans. All kinds of life—from insects to plants to bacteria—use some form of light, sound, chemicals, touch, electricity, and/or magnetic fields to survive. Examining sensory abilities across diverse life forms reminds us that what we humans consider as objective aspects of this world, based on how we view, smell, feel, and hear, are in reality the most inherently subjective concepts of all.
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