Body Structure
Brittle Starfish are echinoderms of the class Opiuroidea. They have a central disk,which is made up of several fine scales, and their skin is hard and spiny. Connected to the central disk are five arms, each composed of top-side plates that allow the starfish to have excellent flexibility and quickness. The mouth is on the underside of the body, or central disk, and houses five movable jaw segments, making the mouth opening look like a star. Having no anus, all food matter that is not digested is excreted out of the mouth. The starfish have thousands of tube feet on each arm. These feet don’t have suckers, but instead push material aside as a way of moving. Brittle starfish do not have a brain, only a simple ring of nerve cells that move information around the body. The tube feet around the arms sense light and smells, and in aquariums they can sense fish food as soon as it hits the water. The starfish’s arms, tube feet, and organs can all be regenerated, and this is needed when the frail arms are easily broken or ripped off by attacking predators, usually being fish. The starfish can even eject its legs at will, to either get away from predators or when close to death; Brittle stars sometimes eject their legs to save the central disk.
Habits
They are nocturnal, coming out at night to get food. As a way of eating, Brittle starfish push their stomach out through their mouth to eat. They are scavengers, eating detritus, algae, and plankton. Some larger species will feed on clams, oysters, mussels, small fish, or shrimp.Some echinoderms, such as sea cucumbers and Brittle stars are thought to be considered passive suspension feeding organisms. However, they are known to wave their arms and create mucus nests between arms to capture phytoplankton or other suspended particles, and also wave its arms out in the open, capturing plant particles from the sediment surface. Brittle starfish have a peaceful temperament, leaving corals, most fish, and other reef invertebrates alone. However, they have been known to hide on rock ledges and drop down on fish, capturing and eating them.
Habitats
Brittle starfish like to burrow; they have been discovered burrowed into sand and mud. They've also have been found on sponges and corals, in kelp forests, and under and on rocks and living rocks. In addition, they prefer structures to hide under. Due to these things, they have been thought to prefer rocky crevasses and sandy regions. In aquariums they help keep sand clean and stirred up, providing oxygen to beneficial bacteria living in the sand. In various locations around the world they have been found in dense groups, sometimes an inch thick of Brittle stars piled on top of each other; however, single Brittle starfish have also been found just by themselves. Certain studies show that Brittle stars live closely together because it’s convenient and beneficial, and not because they came to a suitable feeding area by chance. Low predation is thought to allow these dense beds of Brittle stars.
