Sea Star Structures

In this lesson, students focus on the phenomena of adaptations of specifically one predator in the rocky shore ecosystem: the sea star. They investigate how sea stars have different structures that help them survive in their environment so they can reproduce and pass along their genes to offspring.

• Sea stars are one of the most successful animals on the rocky shore because they have a variety of adaptations that help them survive in their environment.
• Organisms have adapted different structures and behaviors that improve their chances of reproducing and passing on their genes to offspring.

• Why is it beneficial for sea stars to be able to push one of its stomachs, called the cardiac stomach, outside of its own body and into the shell of animals like mussels?
• How do the tube feet of sea stars help them survive?
• How do sea stars use their feet to help them eat?
• How do sea stars protect themselves from predators?
• How does the ability of sea stars to reproduce asexually through fragmentation help sea stars survive?
• What are the benefits of sea stars reproducing sexually compared to asexually?
• Why is genetic diversity important for the survival of a species?

Misconception: Different species “get along” in an ecosystem.  

Fact: Species compete for resources and eat one another for energy and nutrients.

Sea star : a predatory animal with five or more rays (arms) extending from a central disc

Sieve plate : an entrance for water into a sea star’s body

Spines : sharp extensions on the top of a sea star’s body that are used for protection

Tube feet : small tubes on the mouth-side of a sea star that it uses to move and grasp surfaces or prey

Water-vascular system : a series of water-filled canals that function in sea star locomotion and feeding

How Sea Stars Reproduce

Because sea stars cannot move very quickly, they cannot very often escape predators. One adaptation that many kinds of sea stars have is the ability to regenerate their bodies when necessary. They do this through the process of fragmentation.

Fragmentation is a form of asexual reproduction. A new individual develops from a part of a parent that broke off and forms a complete new organism. Sea stars are able to do this because most of their vital organs are found in their arms.

Sexual Reproduction

The ability to regenerate is a useful adaptation in the harsh rocky shore ecosystem. Scientists are still trying to figure out exactly how sea stars are able to re-grow new limbs and sometimes entire bodies.

Sea stars can also reproduce sexually through spawning. Spawning happens when males release sperm and females release eggs into the environment.

To help ensure that the sperm and the eggs will find each other, sea stars gather in groups. Organs in the males’ arms fill with sperm. Organs in the females’ arms fill with eggs. When it’s time to spawn, males and females release large numbers of sperm and eggs into the water. Some female sea stars release millions of eggs into the water. When an egg combines with a sperm, it forms an embryo. The fertilized embryos are free-swimming animals that turn into adult sea stars.

These reproductive behaviors help sea stars pass along their genetic information to future generations. This ensures that their species will continue and not die out.

For the hands-on activity of this lesson, students dissect sea stars to analyze how the internal and external structures of these predatory animals allow them to survive and adapt to the phenomena of rocky shore ecosystems. Students use information from their dissection, lab manuals and class dialogue to figure out how organisms have adaptations that help them survive in this ecosystem by accessing energy, escaping predation, and reproducing.