Adaptations of the Deep: Seal Whiskers and Eyes

In the ocean, sunlight decreases rapidly the deeper you are. Only a small amount of light can reach below about 650 feet in the deep-sea area known as the mesopelagic, or twilight zone. And beyond about 3,300 feet, the bathypelagic, or midnight zone, is bathed in complete darkness.

Many seal and sea lion species can dive to the twilight zone, and some can even descend into the midnight zone. Northern elephant seals, for example, are among the deepest diving mammals, capable of swimming to depths of more than 5,000 feet as they migrate, avoid predators and forage for food. But in the dark waters of the deep ocean, how do marine mammals even find their prey?

Seals and sea lions are highly adapted to life underwater as well as on land. With big eyes that include rounded lenses and large pupils that fully open underwater, they adapt very quickly to decreasing light levels as they dive deeper. A special layer in the back of their eyes, called the tapetum lucidum, also helps absorb more light to see better in the dark, murky ocean and find prey.

Additionally, their extremely sensitive vibrissae, or whiskers, can detect vibrations in the water to sense their prey. Each seal whisker has about 1,500 nerve endings gathering and relaying vital sensory information about the animal’s environment. With so many nerve endings, seal whiskers are the most sensitive in the animal kingdom (for comparison, cat whiskers only have about 200 nerve endings each).

This adaptation helps seals and sea lions hunt because their prey create a wake, or movement, in the water as they swim. Sensitive whiskers can pick up on this movement, helping the marine mammal to focus on and follow the fish, squid or other prey. These unique animals can also control their whiskers, protracting and retracting them in a rhythmic motion, to help them search for food.

As elephant seals swim in the pitch-black waters of the midnight zone, their whiskers allow them to track and catch prey even when they cannot see. In fact, largely thanks to their incredible whiskers, some seals with impaired vision or blindness have also been known to thrive in the wild.

As we diagnose, care for and track our patients, we’re not just learning about the specific diseases and injuries that affect marine mammals, we’re also learning about the health of the ocean as a whole. Marine mammals serve as sentinels of the sea, alerting us to the dangers they face that can also affect us.

As the world’s largest marine mammal hospital, the Center is able to collect and process thousands of samples from our patients as part of our ongoing disease investigation research. Results from these samples support our scientists in identifying diseases and pathogens in our patients, investigating the reasons why marine mammals strand, and determining how these factors are connected to ecosystem and human health.

In addition to being a critical adaptation that helps marine mammals to forage, the chemical composition of sea lion and seal whiskers can also reveal a lot to scientists about the animals’ diets in the wild. The Center’s scientists collect a whisker sample from each of our Guadalupe fur seal patients prior to their release, and provide these unique research samples to our partners at NOAA for nutritional analysis for critical monitoring of this threatened species following a recent Unusual Mortality Event (UME).

What we learn from our patients can help reveal alarming ocean health threats, and this knowledge helps us decide what actions to take to better protect these animals as well as future generations. Did you know it’s only because of people like you that this life-saving care and research is possible? Your support helps return marine mammals like Coyote and Sentinel to a healthier ocean home, where they can interact with other wildlife, embark on incredible migrations, and continue diving to the darkest depths foraging for food.