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
Joe Dovala / Getty Images

Brittle stars (Ophiurida) are echinoderms, the same family that includes sea stars (commonly called starfish), sea urchins, sand dollars, and sea cucumbers. Compared to sea stars, brittle stars' arms and central disk are much more distinctly separated, and their arms allow them to move gracefully and purposefully in a rowing movement. They reside in all of the oceans of the world and are found in all marine environments, from polar to tropical.

Fast Facts: Brittle Stars

  • Scientific Name: Ophiurida
  • Common Name: Brittle stars
  • Basic Animal Group: Invertebrate
  • Size: Discs range from 0.1–3 inches in diameter; arms lengths range between 0.3–7 inches 
  • Weight: 0.01–0.2 ounces
  • Lifespan: 5 years
  • Diet: Carnivore, Omnivore
  • Habitat: All oceans 
  • Population: Unknown
  • Conservation Status: Not Evaluated

Description

A brittle star is made up of an obvious central disk and five or six arms. The central disk is small and clearly offset from its arms, which are long and slender. 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 vascular system that uses water to control locomotion, respiration, and food and waste transportation, and their tube feet are filled with water. A madreporite, a trap door on the brittle star's ventral surface (underside), controls the movement of water in and out of the star's body. Within the central disk lie the brittle star's organs. Although brittle stars don't have brains or eyes, they do have a large stomach, genitals, muscles, and a mouth surrounded by five jaws.

A brittle star's arms are supported by vertebral ossicles, plates made from calcium carbonate. These plates work together like ball and socket joints (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 vascular system. So, unlike a sea star, whose arms are relatively inflexible, the brittle star's arms have a graceful, snakelike quality which allow the creature to move relatively quickly and squeeze into tight spaces, such as within corals.

Brittle stars are measured by the diameter of the central disc, and the length of their arms. Brittle star discs range in size from 0.1 to 3 inches; their arm length is a function of their disc size, typically between two to three times the diameter although some have lengths up to 20 or more times. The largest known brittle star is Ophiopsammus maculata, with a disk measuring 2–3 inches across, and arm length between 6–7 inches. They weigh between 0.01–0.2 ounces and come in a wide variety of colors. Some are even capable of bio-luminescence, generating their own light.

Species

The World Ophiuroidea Database lists over 2,000 species of brittle stars accepted in the Class Ophiuridea, the taxonomic class which contains brittle stars, as well as basket stars and snake stars (Kingdom: Animalia, Phylum: Echinodermata, Class: Ophiuroidea, Order: Ophiurida). Ophiuroidea is the largest class among extant Echinodermata. Traditionally, brittle stars are in a separate order from basket stars, but the division is under scrutiny as DNA results are being reported and that may change.

Habitat and Range

Brittle stars occur in all the world's oceans from the deep sea to intertidal zones, and including salt and brackish polar areas, temperate, and tropical waters. The region with the highest species richness of brittle stars is the Indo-Pacific region with 825 species at all depths. The Arctic has the lowest number of species: 73. 

In some areas, they are found residing in large numbers in deep water areas such as "Brittle Star City" discovered off Antarctica several years ago, where tens of millions of brittle stars were found crammed together. 

Diet

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

Brittle stars may also feed by lifting up their arms to trap tiny particles and algae ("marine snow") using the mucous strands on their tube feet. Then, the tube feet sweep the food to the brittle star's mouth, located on their underside. The mouth has five jaws around it, and crunched up food particles are transported from the mouth to the esophagus and then 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.

Behavior

Brittle stars can drop an arm when being attacked by a predator. This process is known as autotomy or self-amputation, and when the star is threatened, 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 stars don't move using tube feet like sea stars and urchins do, they move by wriggling their arms. Even though their bodies are radially symmetrical, they can move like a bilaterally symmetrical animal (like a human or other mammal). They are the first radially symmetrical animal documented to move this way. 

When brittle stars move, one lead arm points the way forward, and the arms on the left and right of the pointer arm 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, it efficiently just picks a new pointer arm to lead the way.

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 (for example, 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. Brittle stars reach sexual maturity at about 2 years of age and become full grown by 3 or 4 years of age; their lifespans are about 5 years.

Conservation Status

The International Union for Conservation of Nature (IUCN) does not list any brittle star. The WoRMS Catalog of Life includes a total of over 2,000 species but does not identify any endangered species. Perceived threats include pollution and habitat loss.

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