Knobby Star

Knobby Star (Pisaster giganteus)

Contributed by TPERP Victor Knarreborg

Knobby Star

Where to find them: They are found on the Pacific coast of North America, from Alaska to Mexico.

Found among rocks in low intertidal and sub intertidal zones. Usually found around the protected costal lines with low tide. They can often be found attached to rocks, pier supports or in the sand. They are usually found up to 88 meters down in the water.

What do they eat: They are predators and eat many mussels and barnacles. The star’s tube feet use strong suction to pull the mussel’s shell apart. They can pull nonstop for hours until their prey gets tired. Once they open their prey’s shell, even a bit, they extend their stomach out into the prey and digest it. It takes more than six hours to eat one bivalve like a mussel.

They prey on several kinds of sea organisms including barnacles, gastropods, bivalves and limpets.

Who eats them: Benthic fish, large crabs. The knobby star has only a few predators. Sea otters and sea birds feed on giant sea stars. Also, their larvae are subject to being made a meal by certain types of sea snails.

 Size and Color: They can grow as large as 24 inches in diameter. Their color varies from brown to red or purple.

They can be yellow, orange, brown, red or purple. They are a heavy sea star with a large middle disk and five big arms radiating out. They have short white spines on top and tube feet on the bottom. Their mouth is located underneath.

Knobby stars have a dense body with wide arms. Pisaster Giganteus have a surface that is either tan or brown, but you will find a giant sea star with the occasional yellowish or grayish surface. They contain thick, blunt white spines which are surrounded by a blue ring that is very distinctive in this species. It is the spines that the name giganteus refers to. The tips of the spines are swollen and surrounded by brown fuzz and pedicellariae that have a plier-like shape. These pedicellariae are used as a protective mechanism against predators. They have no distinct pattern. They range from 36-48 centimeters from the tip of one arm to the other.

Reproduction: Males and females release their sperm and eggs into the water where fertilization occurs.

Knobby stars have small eggs, and their sperm contain spherical heads. Once their larvae are born, they are bilaterally symmetrical. By the time they mature and reach adulthood they are centered around a set point with radial symmetry to their bodies. The gonads of the giant sea star grow in a winters time just in time for spawning season between the months of March and April.

Interesting facts: If they lose an arm to a predator, they can grow it back. Sea stars eat by inserting their inside-out stomach into a shell and digesting their prey. When the meal is done they retract their stomach back into their bodies.

Phylum: Echinodermata
Class:   Asteroidea
Order:    Forcipulatida
Family: Asteriidae
Scientific Name:   Pisaster giganteus3
Common Name(s): Knobby Star sometimes called the “jeweled star” because of the blue and white knobs all over its body.


  1. Amsel, Sheri. “Asteroidea.” Sea Star (Knobby). Exploring Nature Educational Resource. © 2005 – 2013. June 11, 2013.
  1. Oracle Education Foundation Copyright Agent

3. Ocean Institute Field Guide. Copyright © 2002 Ocean Institute.

  2. “Classification of Southern California Sea Stars”. Retrieved 12 April 2012.
  3. McDonald, Gary. “Pisaster Giganteus”. Retrieved 12 April 2012.
  4. “Pisaster Giganteus”. SiMOn. Retrieved 12 April 2012.

Last revised 03-Sep-14