Sea Star Structures

Science background gives teachers more detailed information on the phenomena students explore in this unit. Below is an excerpt from the science background section on sea star structures.

The Eating Habits of Sea Stars

As the process of cellular respiration shows, finding food is an essential part of the survival of all organisms. This is why many adaptations help an organism find and get food. For example, sea stars are predatory animals with five or more rays (arms) extending from a central disc. They have unusual adaptations that make them extremely effective predators on rocky shores.

When a sea star gets ready to eat a shelled animal like a mussel, it does something unlike almost any other animal on Earth. First, it produces a glue so it can attach its feet to the outside of the shell and forcibly pull the shell apart.

Then comes the really unusual part. It pushes one of its stomachs, called the cardiac stomach, outside of its own body and into the shell. The cardiac stomach then actually digests the animal inside the shell, turning it into a soupy mixture. The stomach then brings the food back into the sea star, where it is further digested by the sea star’s second stomach, called a pyloric stomach. This ability to eat shelled prey such as mussels is an adaptation that allows sea stars to eat prey much larger than the sea star’s mouth

Finding food isn’t the only way that sea stars have adapted to life on the rocky shore. They are one of the most successful animals there because they have also adapted to withstand the rushing waves with hundreds of tiny, suction-cupped tube feet that cling to surfaces. Tube feet are small tubes on the mouth-side of sea stars. They also have a tough outer covering that keeps them from drying out.

Other Sea Star Adaptations

Sea stars also use their feet and a water vascular system to move. The water vascular system is a series of water-filled canals: a stone canal, a ring canal, and a radial canal (central disc). The sea star’s tube feet can also fill with water. By moving water from the vascular system to the feet, the sea star can make a foot move. This process allows the otherwise stiff and lifeless sea stars to move toward their prey at speeds of up to 1.5 meters per minute.

Water enters the vascular system through a sieve plate, which is an entrance for water into a sea star’s body.

Sea stars also have adaptations that help them survive predation. Just as they are predators of many organisms in the rocky shore, they are also prey for various organisms, including many kinds of birds and fish. To help defend themselves, sea stars have spines, which are sharp extensions made of calcium that coat the top of a sea star’s rays (arms).