Brittle Stars of the Sea

Brittle stars are an echinoderm with whip-like arms

Blue Deep Water Brittle Star, Ophiothrix spiculata, Anacapa Island, Channel Islands, Pacific, California, USA
A brilliant blue brittle star. Joe Dovala / Getty Images

Brittle stars are echinoderms - so, they are related to sea stars (commonly called starfish) although their arms and central disk are much more distinct than those of sea stars. Since brittle stars are in the Class Ophiuroidea, they are sometimes referred to as ophiuroids.

The World Ophiuroidea Database lists over 1,800 species of brittle stars accepted in the Order Ophiurida, the taxonomic order which contains brittle stars.

Description and Anatomy

Brittle stars range in size from a few millimeters to several inches.  They can be a range of colors, and some are even capable of phosphorescence

Brittle stars have a relatively small central disk, with long, slender arms. They have tube feet on their underside, like sea stars, but the feet do not have suction cups at the end and are not used for locomotion - they are used for feeding and to help the brittle star sense its environment.  Like sea stars, brittle stars have a water vascular system, and their tube feet are filled with water. The water is brought into the body using the madreporite, which is on the brittle star's ventral surface (underside). 

Within the central disk lie the brittle star's organs - it doesn't have a brain, but it has a large stomach, genitals, muscles, and a mouth surrounded by 5 jaws.

A brittle star's arms are supported by vertebral ossicles, which are plates made from calcium carbonate.  These plates work together like ball and socket joints (e.g., like our shoulders) to give the brittle star's arms flexibility.  The plates are moved by a type of connective tissue called mutable collagenous tissue (MCT), which is controlled by the nervous system.  So, unlike a sea star, whose arms are relatively inflexible, the brittle star's arms can have a graceful, snakelike quality, which allows them to move relatively quickly and squeeze into tight spaces (e.g., within corals).

Brittle stars can drop an arm when being attacked by a predator. When this happens it is called autotomy, or self-amputation, and the nerve system tells the mutable collagenous tissue near the base of the arm to disintegrate.  The wound heals, and then the arm regrows, a process which can take weeks to months, depending upon the species.

Brittle Star Locomotion

Brittle stars don't move using tube feet like sea stars and urchins do - they move by wriggling their arms. Brittle stars are a radially symmetrical animal, but they can move like a bilaterally symmetrical animal (e.g., like a human or mammal). This is remarkable because they are the first radially symmetrically animal documented to move this way. 

When brittle stars move, one lead arm points the way forward, while they arms on the left and right coordinate the rest of the brittle star's movements in a "rowing" motion so that the star moves forward. This rowing motion looks similar to the way a sea turtle moves its flippers. When the brittle star turns, instead of turning its whole body like we have to, it efficiently just picks a new lead arm, which leads the way.

Classification

Feeding

Brittle stars feed on on detritus and small oceanic organisms such as plankton, small mollusks, and even fish - some brittle stars will even raise themselves on their arms, and when fish get closer, they wrap them in a spiral and eat them.

The mouths of brittle stars are located on their underside. Brittle stars may also feed by filter feeding - lifting up their arms to trap small particles and algae with mucous strands on their tube feet. Then, tube feet sweep the food to the brittle star's mouth.   The mouth has 5 jaws around it. Food goes from the mouth to the esophagus, to the stomach, which takes up much of the brittle star's central disk.  There are 10 pouches in the stomach where the prey is digested.  Brittle stars don't have an anus - so any wastes must come out through the mouth. 

Reproduction

There are male and female brittle stars, although it is not obvious which sex a brittle star is without looking at its genitals, which are located inside its central disk. Some brittle stars reproduce sexually, by releasing eggs and sperm into the water. This results in a free-swimming larva called an ophiopluteus, which eventually settles to the bottom and forms a brittle star shape.

Some species (e.g., the small brittle star, Amphipholis squamata) brood their young. In this case, eggs are held near the base of each arm in sacs called bursae, and then fertilized by sperm that has been released into the water.  The embryos develop inside these pockets and eventually crawl out.

Some brittle star species may also reproduce asexually through a process called fission. Fission occurs when the star splits its central disk in half, which then grows into two brittle stars. 

Habitat and Distribution

Brittle stars may be found in both shallow and deep water around the world, including polar areas, temperate waters, and tropical waters. They may even be found in brackish waters. They may be found in large numbers in some areas, including deep water areas - such as "Brittle Star City" discovered off Antarctica several years ago. 

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