Starfish: Types, Characteristics & Anatomy

Lesson Transcript
Instructor: Wendy McDougal

Wendy has taught high school Biology and has a master's degree in education.

Starfish are colorful echinoderms with five arms found in marine habitats, but they are not actually fish. Explore the types, characteristics, and anatomy of common starfish. Updated: 11/27/2021

The Stars of the Ocean

If you've ever had the chance to peer into a tide pool, you've most likely caught a glimpse of a spectacular array of marine life. Tide pools are like nature's fish bowls, often full of crusty barnacles, billowy sea anemones, and a five-armed favorite: the starfish. Known as a sea star in the scientific world, the starfish is not really a fish at all. Closely related to the sea urchin and sand dollar, a starfish belongs to the phylum Echinoderm and the class Asteroidea. In this lesson, learn more about the unique starfish and gain an understanding of its life in the ocean.

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  • 0:00 The Stars of the Ocean
  • 0:45 Types of Starfish
  • 1:51 Starfish Anatomy
  • 3:48 Characteristics
  • 5:18 Lesson Summary
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Types of Starfish

There are around 2,000 different species of starfish found in the oceans, and we will take a look at a few examples. Although most species have five arms, some may have ten, twenty, or even up to forty arms. The common starfish represents the typical species, with five arms and often found in an orange color. Common sunstars resemble more a sun than a star, with a large central disc and many arms radiating outward.

The sand star bears resemblance to its cousin, the sand dollar. This starfish lacks suction cups on its tube feet, and spends its time buried under the sand. The largest species is the sunflower sea star, which has 16-24 arms and can have an arm span of up to three feet. One of the most unique is the crown-of-thorns sea star. This is a venomous species that can give quite a sting. It feeds on coral reefs and when too prolific, can devastate these marine environments.

Starfish Anatomy

Starfish exhibit radial symmetry, meaning that their arms are arranged evenly in a circle around a central disc - like spokes on a bike wheel. Although starfish don't have eyes or even a brain for that matter, they do have one sensory tentacle at the end of each arm. This tentacle is sensitive to chemical and physical changes, and has a photosensitive spot that helps them detect movement.

If you've ever had the chance to touch a starfish, you know that it has a tough, spiny outer covering. This layer contains calcium carbonate, which gives them a leathery feel. Sometimes this layer is also covered with spikes, giving starfish extra protection from predators that may not want to chew their way through that unpalatable exterior. Another very important feature of this layer is the presence of specialized gills, which make gas exchange possible for the starfish.

On the underside of a starfish we find hundreds of tiny suction cup-like structures called tube feet. These tiny tube feet allow starfish to cling to rocks as well as move along the ocean floor, hunting and scavenging.

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