- Harbor seals have a well-developed sense of hearing, especially in the water.
- Research shows that under water, harbor seals respond to sounds from 1 to 180 kHz with a peak sensitivity of 32 kHz.
- In the air, hearing ability is greatly reduced; harbor seals respond to sounds from 1 to 22.5 kHz, with a peak sensitivity of 12 kHz. (The average hearing range for humans is 0.02 to 20 kHz.).
- Harbor seals have large eyes. Their vision under water is better than a human's, but inferior on land. Lenses are enlarged and almost round, adapted for focusing on light that is refracted upon entering water. The lenses are not as well-adapted for sight in air.
- Harbor seals' eyes are adapted for sight in dark and murky water.
- Like the eyes of other pinnipeds, harbor seals' eyes contain high numbers of rod cells - photoreceptor cells that are sensitive to low light levels.
- Harbor seals have a well-developed tapetum lucidum, a layer of reflecting plates behind the retina. These plates act as mirrors to reflect light back through the retina a second time, increasing the light-gathering ability of the rod cells. (The tapetum lucidum is the same structure that makes a cat's eyes appear to "glow" when reflecting light at night.)
- Under water, the pupils dilate (expand) into a wide circle to let in as much light as possible. In bright light, the pupils constrict to a slit.
- Mucus continually washes over the eyes to protect them. Unlike most land mammals, pinnipeds lack a duct for draining eye fluids into the nasal passages. When a harbor seal is out of the water, mucus surrounding the eyes gives them a wet, "tear-rimmed" look.
- Good vision does not seem to be essential to harbor seal survival; scientists have found blind but otherwise healthy individuals, including mothers with pups, at sea.
- Harbor seals probably do not have color vision.
- A harbor seal uses its sensitive vibrissae to find food, especially in dark, deep waters, or at night. A substantial nerve system transmits tactile information from the vibrissae to the brain.
- Each vibrissa can move independently. Under water, a harbor seal thrusts its vibrissae to and fro in a sweeping movement by pushing its mobile upper lip in and out.
- Prey moving under water creates vibrations that the seal may detect with its vibrissae. Studies have shown that harbor seals are able to detect and follow the hydrodynamic wake of a miniature submarine by using their sensitive vibrissae to sense water movement. Hydrodynamic trail-following is probably a way for a seal to locate and catch fish in low visibility conditions
Little is known about a harbor seal's sense of taste.
Researchers believe that harbor seals have an acute sense of smell on land. This sense may be important for mothers identifying their pups.