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11.3: Enhydra lutris: Sea Otter

Difficulty Level: At Grade Created by: CK-12

Common Name

  • Sea Otter


The sea otter is the smallest marine mammal species and weighs 49- 99 pounds. They are usually 1.2- 1.5 m long and have light and dark brown fur. The sea otter pup usually weighs about 1-3 pounds at birth. The sea otter is related to weasels, badgers, and minks. They are also the heaviest member of the Mustelidae family.

  • Kingdom: Animalia
  • Phylum: Chordata
  • Class: Mammalia
  • Order: Carnivora
  • Family: Mustelidae
  • Genus: Enhydra
  • Species: E. lutris


The sea otter lives in Russia, Alaska, British Columbia, Washington, Oregon, and California. In the ocean, they burrow themselves in kelp forests during the day, but sleep on the waves during the night. They usually live in waters up to 100 ft. in depth, but they can dive down up to 325 ft. Sea otters occasionally travel to land, but are mostly at sea. Able to give birth and raise their young in water, they can also go without fresh water for as long as they live. Sea otters can live in the water without ever coming out of the water. They can come on land if they want to, but they rarely do because of predators.


Cell Biology

Sea otters have eukaryotic cells, which have a nucleus and many organelles. Organelles are like miniature organs in a cell. Organelles include the nucleus, which contains the genetic material, the mitochondria, which provides the energy, vesicles, which transport materials, vacuoles, which are the storage centers, and lysosomes, which have digestive enzymes. Ribosomes are where the proteins are made.

Sea otters have red blood cells and white blood cells. The red blood cell is shaped with a pocket that traps oxygen and brings it to the rest of the body. Interestingly enough, mature red blood cells do not have nuclei. White blood cells are used for fighting off bacteria, disease, and infection. The sea otter also has nerve cells and skin cells. Nerve cells are long and stringy. They create a line of communication between other nerves and can quickly send signals throughout the body. Skins cells are flat and fit tightly together to protect the body.

Sea otter cells divide in two ways, mitosis and meiosis. Mitosis is when one cell replicates its chromosomes, then divides them equally in two replicated cells. Meiosis produces cells necessary for sexual reproduction. Meiosis creates gametes, or sperm and egg cells.


No one knows exactly where sea otters came from, but they believe they arose from a fish-eating, otter-like mammal about 5 to 7 million years ago near the Pliocene and Miocene periods. Their ancestors were originally land mammals that went into water to hide from predators and look for food. Once their ancestors entered the water, they began developing permanent characteristics to adapt to the water like their waterproof coat, flippers, and webbed feet.


The sea otter’s main predators are the great white shark and humans. People still skin them and eat their meat, but it’s not as popular as it used to be. Their prey includes sea urchins, clams, abalone, mollusks, and snails. There is a cat parasite that is currently infecting sea otters. This parasite is a protozoan and is named Toxoplasma gondii.

Amazingly sea otters can help save the world from climate change. While eating lots of sea urchins, the sea otters help the kelp beds and create less carbon dioxide. Sea urchins feed on kelp beds, which are like our trees, keeping the water clean and removing the carbon dioxide. When sea otters eat the urchins, they save the kelp beds and keep them alive.

Birth takes place around May and June in northern regions and January and February in southern regions. They usually give birth once a year, to only one pup. If there are twin pups, one will be abandoned and left to fend for itself, while the other stays with its mother and hopefully lives a full and happy life.


The sea otter is a mammal and is also a vertebrate. Mammals can adapt quicker to an environment than any other class of animals. They are able to keep their body temperatures warmer or cooler than the environment they are currently in. Their fur is thick and keeps them warm in colder temperatures. The sea otter’s lower jaw is connected right to the skull, while other vertebrates have a different bone connecting them. There are three small bones the carry sound waves through the ear. The diaphragm creates a barrier between the heart and the lungs from the abdominal cavity.


The behavior of the sea otter is unknown to most. Although there have been sea otter taggings, sea otters have very confusing behavior, being passive in certain situations and aggressive in others. The sea otter is diurnal, meaning they are active during the day and sleep at night. Sea otters sleep on the water and hold hands when they sleep so that they won’t float away from each other. They are solitary during the day, but in Alaska they can be seen in groups up to 2,000 otters at a time at night. Collecting food takes place under water. When gathering food, they dive down to the bottom of the ocean and collect clams and sea urchins. They then swim back to the surface and lay on their back, using a rock to crush the shell of their food. Instead of fighting their predators, they run and hide in the safety of the kelp forests. Then they climb up to land and keep safe there.

Their territories are divided by sex, and they only come together when mating season comes along. During mating season, the males have multiple partners. Mating takes place in the water. During mating, the male bites the female’s nose, leaving scars, and pushes her head underwater until they are finished. The average sea otter is pregnant for 4-12 months. Usually, her pup weights around 1.4-2.3 kg (3-5 lb). Usually nursing lasts around 6-8 months. Her pup will usually drink from her two lower nipples as they are floating across the water. The pup will be fully weaned at the age of 6-8 months. Sadly, only 25% of the pups last their first year or life. If so, their mothers will carry the pup’s body for months on end after it death.




  • Alexa Lawrence
  • Zoe White

Supervising Faculty

  • Amy Huff Shah


  • Museum School, San Diego, California


  • Published prior to review.

Edit History

  • Created: April 5, 2013
  • Version 1.0 submitted to CK-12: July 10, 2013
  • CK-12 edits: in progress


  • Middle School (grades 6-8)

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Date Created:

Apr 17, 2012

Last Modified:

Dec 05, 2014
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