Harbor Seal: Phoca vitulina

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Physical Characteristics and Appearance
Anatomical Structures
Adaptations for An Aquatic Environment


Physical Characteristics and Appearance

In order to identify adaptations of the Phoca vitulina, we must first understand physical structure and appearance. Harbor seals have thick, short, coarse hair that sheds every year after breeding season. This process of “molting” lasts about 1-2 months. The first molt occurs close to birth, and then again after the pup is one year old (SeaWorld, 2005). Harbor seals lose hair as they age, as with humans and land mammals alike. Hair is covered by an oil secreted in the glands of skin and acts as a waterproofing mechanism rather than insulation. A layer of thick blubber covering the entire body of a harbor seal keeps it warm in even the coldest regions of the world (The Brown Reference Group PLC, 2010). Insulation is especially important to seals because of the high heat capacity of water. A high heat capacity means that it takes more heat energy away from the seal, dictating the need for insulation. General coloration of harbor seals varies from grey, silver, black, or brown and is often dependent upon environment. Rings and/or spots have been observed on the back with fewer on the underside. harbor seals can be characterized into two groups via coloration, the “light phase” or “dark phase.” Members of the light phase are white, yellowish, or grey with dark spots. Dark phase seals are brown or black with dark spots and rings (Cale, 2012). In some areas, such as San Francisco Bay, the iron and selenium deposits are thought to cause follicle change in the harbor seals’ fur, making the fur a reddish/rust color (SeaWorld, 2005). For more information on the differing habitats of harbor seals, visit our Habitat page.

White, grey, and black harbor seal coloration. Photo Credit: Marine Mammal Institute, OSU        Coloration is affected greatly by habitat. Photo Credit: © The Marine Mammal Center
White, grey, and black harbor seal coloration. Photo Credit: Marine         Coloration is affected greatly by habitat. Photo Credit:
Mammal Institute, OSU.                                                                       © The Marine Mammal Center 

Male harbor seals generally reach adult size at about 4.6-6.6 feet and weigh between 154-375 pounds. Female harbor seals, as with many species, are smaller in size, but only by a minor size and weight difference. They grow to be about 3.9-5.6 feet in length and weigh between 110-331 pounds (SeaWorld, 2005). The male and female sizes and weights do overlap, so it is essential to understand that although this pattern is consistent, it is not constant. Rather, it is used as a template to identify males versus females. Also, these numbers can be generalized to a number of habitats, with minor differences accounted for. Body shape is quite similar across several species of seals. The Phoca vitulina is rounded and “fusiform,” meaning that it tapers off at both ends to form the head on one end and the hind flippers at the other (SeaWorld, 2005). Since body shape generally follows the same pattern for the majority of seals, we rely on differing anatomical structures to distinguish among species. 

Anatomical Structures

Harbor seals share several similar features in body structure as with other land mammals. The first similarity is the general structure of the skeleton. Phoca vitulina possess an endoskeleton, complete with osseous bone and cartilage. Their lumbar vertebrae are stronger and thicker than any other bones which creates stability for the limb bones. These limb bones are shorter than in land mammals, yet very powerful (The Brown Reference Group PLC, 2010). The first set of limb bones are the foreflippers. Foreflippers share the same general skeletal features as a land mammal, with some modifications. Foreflippers have five digits, equal in length. Claws average 1-2 inches in length and are adapted for grooming and defense. Functions of the foreflippers include steering in water, holding prey, and supporting body weight on land (SeaWorld, 2005).

The harbor seal also has a second a set of limb bones in the hind flippers. Hind flilppers have five bony digits as in the foreflippers. The only difference is that each digit varies in size, as in a human hand. These digits are webbed, covered with hair, and have claws. Hind flippers are used to propel the seal in water because of their higher resistance to water and act similar to a rudder on a boat. Movement on land does not utilize hind flippers. Instead, it involves flopping on their bellies and has often been compared to that of a caterpillar. A short, flat tail is tucked between the two hind flippers (SeaWorld, 2005).

Moving toward the other end of a harbor seal, we find the head. Harbor seals have a rounded head with forward facing eyes. The advantage of forward facing eyes is binocular vision. With binocular vision, harbor seals can perceive their environment more accurately (SeaWorld, 2005). Large mouths allow seals to encompass prey, while the 34-36 teeth are adapted to either tear or crush food depending on the prey (SeaWorld, 2005). Front teeth are used for gripping and separating food, such as fat and muscle, while back teeth are used for crushing shells and bones. Moving outside of the mouth, the upper lip and cheek region are covered with vibrissae, or whiskers. Attachment to muscles creates a constant blood and nerve supply, allowing the whiskers to continually grow. As members of the “true family” of seals harbor seals lack ear flaps. Instead, they have ear openings that close when diving (SeaWorld, 2005). Other adaptations for an aquatic environment, including diving, are discussed in the following section.

Binocular vision is an advantage to harbor seals. © Ingrid Overgard - The Marine Mammal Center                    Jaw expansion by young pup. Photo Credit: © Adam Ratner - The Marine Mammal Center
Binocular vision is an advantage to harbor seals.                                                      Jaw expansion by young pup.
© Ingrid Overgard - The Marine Mammal Center                                                        Photo Credit: © Adam Ratner -
                                                                                                                              The Marine Mammal Center

                                                     Ear openings on side of harbor seal head. Photo Credit: Robin Riggs, Aquarium of the Pacific                   
                                                     Ear openings on side of harbor seal head.
                                                     Photo Credit: Robin Riggs, © Aquarium of the Pacific.

Adaptations for An Aquatic Environment

Studying the anatomical structures involved in swimming and diving of the Phoca vitulina help us to better understand how they have adapted throughout time. Harbor seals swim by using all four flippers. As mentioned earlier, the hind flippers function as a propeller, while the foreflippers are used to steer. With these adaptations, seals can reach speeds up to 12 miles per hour, but typically swim much slower than this. They are able to swim forward and upside-down but not backward (The Brown Reference Group PLC, 2010). When diving, harbor seals utilize these flippers to efficiently chase prey, escape predators, and play. Harbor seals can dive deeper than 650 feet. Although seals generally only dive for a few minutes and remain in shallower waters they are able to dive for longer periods when necessary. As the harbor seal dives deeper, seals experience diving asphyxia due to the low concentration of oxygen. This asphyxia is a combination of hypoxia (lowered oxygen levels), hypercapnia (increasing carbon dioxide), and acidosis (an aggregation of hydrogen) (Elsner et al. 1995). A number of adaptations allow for harbor seals to continually engage in this diving behavior without serious harm or death.

Prior to diving, we see the importance of the first adaptation, the myoglobin of muscle fibers. Myoglobin binds oxygen tightly and allows muscles to absorb oxygen from the blood and store it. This storage of oxygen can be accessed while in low oxygen areas during long dives (Mirceta et al. 2013).
As a harbor seal begins his/her dive, the next few adaptations become important. Blood is quickly redirected to the areas that require it most (to the heart, lungs, and brain) (SeaWorld, 2005). Cardiovascular responses such as bradycardia (slowed heart action) and reduced cardiac output occur. When a harbor seal begins his/her dive, heart rate lowers immediately. Vasoconstriction (the constriction of blood vessels) increases blood pressure and contributes to the theme of oxygen conservation. Other organ circulation, including the kidneys and visceral circulation, is either reduced or ceased (Elsner et al. 1995). Overall, adaptations for efficient use and conservation of oxygen creates the perfect environment for a harbor seal dive.

                                                        Harbor seal pup returning from a dive.
                                                        Harbor seal pup returning from a dive.
                                                        Photo Credit: Hugh Ryono, © Aquarium of the Pacific.



The most important sense of a harbor seal is touch via whiskers. As mentioned earlier, harbor seal whiskers are used to “feel” their environment (Denhardt et al. 1998). Each whisker is part of a hair follicle with anywhere between 1,000 and 1,600 nerves (The Brown Reference Group PLC). As the whisker encounters an object or is moved by water, tiny sensory organs in the follicle recognize pressure change. These organs send impulses to the brain, which can tell the seal a variety of information. This information is especially important when hunting in the dark. Fish leave a “hydrodynamic trail” when swimming (also known as a wake) which lasts for several minutes (Denhardt et al. 2001 ). Seals are able to use their sensitive whiskers to tell the size and velocity of prey and track the fish, but only over short distances (Lasley, 2011 and Davies, 2011). “They can measure the height of objects, they can discriminate different shapes, and they can very accurately determine an object's surface” (Zimmer, 2001). This adaptation can also be life-saving when avoiding a predator or finding their way home.

                                                    Vibrissae (whiskers) on a harbor seal.
                                                    Vibrissae (whiskers) on a harbor seal.
                                                    Photo Credit: © Aquarium of the Pacific.

Taste can also be life-saving to a harbor seal. Harbor seals are able to taste the differences in levels of salt in seawater. Once a harbor seal is familiar with an area, they begin to form associations with salt content and types of fish available. When hunting or migrating, harbor seals can use their sense of taste to tell location and abundance/type of fish (The Brown Reference Group PLC, 2010).

Although harbor seals mainly rely on their other senses, they do have visual capabilites as well. The visual structures of a harbor seal involved in sight actually change when moving from air to water and vice versa. Throughout history, harbor seals have evolved sharp eyesight to see through the transparent air (Zimmer, 2001). In air, harbor seals utilizes two lenses. The cornea protects the lens from light damage by reflecting some of it. In water, this adaptation of sharp eyesight does not benefit the seal. The cornea has a close reflective capacity to water and becomes unnecessary, so it disappears (The Brown Reference Group PLC, 2010).

Underwater Hearing
Hearing is not as essential in a harbor seal as touch, taste, and sight are, but it can still help them percieve environmental stimuli. Due to the fact that a seal’s body tissue and the surrounding water are about the same density, sound waves pass through body tissues easily. First, they pass through the head and through the less dense bones of the skull. From there, the waves are conducted to the middle ear and reaches the inner ears at about the same time (The Brown Reference Group PLC, 2010).


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