Brittle stars are highly successful marine invertebrates belonging to the class Ophiuroidea. Their name comes from the Greek words for “serpent” and “tail,” aptly describing their whip-like arms. Often overshadowed by sea stars, brittle stars are among the most abundant creatures in the deep sea and inhabit nearly all marine environments across the globe.
Defining Characteristics and Classification
Brittle stars belong to the Phylum Echinodermata, along with sea urchins, sea cucumbers, and sea stars. This phylum is characterized by five-part radial symmetry and an internal skeleton made of calcium carbonate plates, called ossicles. The Class Ophiuroidea contains over 2,000 species and is distinguished by its unique body plan.
The defining feature of a brittle star is the clear boundary between its central body disk and its five, slender arms. All internal organs, including the stomach and reproductive organs, are contained solely within this central disk, never extending into the arms. The arms are long and flexible, often reaching many times the disk’s diameter, which dictates the brittle star’s movement and lifestyle.
Anatomy and Specialized Movement
The architecture of a brittle star’s arm is supported by a column of large, interlocking ossicles, known as vertebral ossicles, which resemble vertebrae. These skeletal elements articulate using muscle-controlled ball-and-socket joints, granting the arms immense flexibility in all lateral directions.
This sophisticated structure allows the brittle star to move rapidly across the seafloor by wriggling its arms in a serpentine or rowing motion. Movement uses a functionally bilateral pattern: one arm points the direction of travel while the other four arms push and pull the body forward. While the water vascular system is present, the tube feet are reduced and lack the suckers found in sea stars. These tube feet are not used for locomotion; instead, they serve sensory functions, detecting light, odors, and chemical cues.
Ecology, Feeding, and Regeneration
Brittle stars are cosmopolitan, found in every marine habitat from the intertidal zone to the abyssal depths. Over half of the known species live at depths greater than 200 meters. They play a significant role as “ecosystem engineers” in benthic environments, often dominating the seafloor in sheer numbers. Their diet is highly varied, reflecting their diverse habitats.
Feeding Strategies
Brittle stars employ several feeding strategies. Many species function as detritivores or scavengers, using their tube feet to move small organic particles (detritus) into their mouths. Others are suspension feeders, such as basket stars, which extend their highly branched arms into the water column to trap plankton and bacteria using a mucus coating. Some brittle stars are active predators, consuming small crustaceans and worms, sometimes everting their stomach to digest prey outside the body.
Regeneration and Autotomy
A remarkable survival trait of Ophiuroidea is their ability to regenerate lost body parts with exceptional efficiency. The common name “brittle star” comes from their defense mechanism called autotomy, where they voluntarily shed an arm when attacked or disturbed. This self-amputation is facilitated by a unique substance called mutable collagenous tissue, which can rapidly change its stiffness. The lost arm is quickly regrown; in some wild populations, up to 99% of arms show signs of active regeneration.
This regeneration process is highly effective, allowing the brittle star to escape a predator by sacrificing only an arm, not its life. While arm regrowth can be slow in cold environments, the animal typically resumes normal feeding and locomotion quickly. This ability to rapidly recover from injuries is a major factor in the success of brittle stars.
Distinguishing Brittle Stars from Sea Stars
While both brittle stars (Ophiuroidea) and sea stars (Asteroidea) share pentaradial symmetry, their differences are substantial. The most immediate visual distinction is arm attachment: brittle star arms are slender and whip-like, joining a distinct central disk, while sea star arms are thicker and merge gradually into the central body area.
Movement also differs: brittle stars use muscular arm flexure to crawl quickly, whereas sea stars rely on thousands of suction-cupped tube feet to glide slowly. Brittle stars also possess an incomplete digestive system, featuring a mouth that serves for both feeding and waste expulsion, lacking the anus present in most sea stars.