Brittle stars are related to sea stars but have longer, more flexible and more brittle arms. They are radially symmetrical and have five long arms radiating out from a small central disc. Radial symmetry is helpful to bottom-dwelling animals such as brittle stars because it allows them to move and feed in any direction. There are over 2,000 known species of brittle stars, which exhibit a range of feeding behaviors and habitats -- but they share many physiological characteristics in common.
Brittle stars are omnivores; they eat a wide variety of foods found in the water. Most brittle stars feed on detritus and decaying organic matter. They either wander around the sea bed looking for food or filter food particles from the water. Some species of brittle star are predators and eat small invertebrates such as plankton or small clams, oysters or mussels. Large sea stars may even eat small fish or shrimp. In an aquarium, sea stars eat most meaty marine foods, such as squid, fish, or frozen marine fish food, if they are cut finely enough. Some species of brittle star even wrap themselves around black corals or gorgonians and feed on their host's rich mucus. The host animal allows the brittle star to remain, because the brittle star also cleans its host animal as it feeds. Brittle stars are primarily nocturnal and feed mainly at night.
On the bottom of the central disc of a brittle star is its mouth, surrounded by five teeth. Brittle stars use their long arms to grab food and move it to their mouth. Brittle stars do not have a large mouth for chewing or a long esophagus down which food must travel. Instead, ingested food moves directly into the stomach of the animal. Brittle stars are capable of ejecting their stomachs out through their mouths and using the stomach itself to grab food. If the food particle is small enough, the stomach is taken back in through the animal's mouth.
Digestion and Absorption
Glands around the stomach secrete digestive enzymes into the stomach of the brittle star. These enzymes break the animal's food down into useful molecules. These molecules are then absorbed through the walls of the stomach and move into circulation throughout the animal.
Brittle stars have what is known as an incomplete digestive system; their digestive system contains only one opening. When a brittle star has extracted all the nutrition available from its food, it ejects the indigestible particles back out through its mouth.
- "Life: The Science of Biology Fifth Edition"; Purves et. al.; 1997
- About Fish Online: Brittle Stars
- Online Biology Book: Biological Diversity: Animals III
- Starfish: Echinoderms
- Enchanted Learning: Brittle Star
- Photo Credit Jupiterimages/Photos.com/Getty Images