Echinoderms
Common brittle star
Ophiothrix fragilis
Epipelagic zone
Forms living carpets of thousands of fragile, spine-covered brittle stars that filter plankton from the current after nightfall.
🗺️ Geographic range
Northeastern Atlantic, from the Lofoten Islands and Iceland in the north to the Mediterranean and Azores in the south, and along the West African coast to the Cape of Good Hope; very common around the British Isles and the North Sea.
📊 Population
Locally extremely abundant, forming 'beds' or carpets of up to 2,000 individuals per square meter on current-swept soft seabeds; its abundance fluctuates with temperature and food availability, sometimes spiking after mild winters.
Habitat
- Atlantic Ocean · High seas / open water · Epipelagic zone · Permanent
🥚 Reproduction
Separate sexes, annual mass spawning, external fertilization, planktonic larvae drifting for two to four weeks before settling.
Behaviour
- Diet
- Omnivorous: phytoplankton, diatoms, detritus, and occasional scavenging of dead organic matter.
- Hunting
- Passive suspension feeder: raises its arms into the current to trap suspended particles on its tube feet and spines, then carries them to its mouth.
- Communication
- None known.
- Social life
- Gregarious, forms very dense aggregations; nocturnal, hides in crevices by day and emerges to feed at night.
Physiology
- Anatomy
- Small central disc covered in spines, five long, thin, extremely fragile arms lined with rows of glassy, toothed spines; highly variable coloration (violet, red, yellow, grey).
- Respiration
- Gas exchange via bursal sacs located at the base of the disc.
- Vision
- No eyes; has photoreceptive cells and tube feet sensitive to light and touch.
- Adaptations
- Autotomy: it can voluntarily shed an arm gripped by a predator and fully regenerate it; its calcified, glassy spines make it unpalatable.
⚡ Its extraordinary abilities
- Can voluntarily sever one of its own arms to escape a predator, then fully regrow it
- Aggregates into ultra-dense living carpets, up to 2,000 individuals per square meter, forming a synchronized filter-feeding 'reef'
🏆 Records
Among the highest population densities ever documented in echinoderms
Forms some of the highest population densities ever documented among brittle stars, with beds reaching up to 2,000 individuals per m² around the British Isles and Ireland.
If you cross paths with this animal…
Risk: None, completely harmless, no venom or bite.
Behaviour: Shy: retreats into crevices as soon as it's disturbed by light or touch.
Advice: Observe and photograph without handling it, as grasping it will almost always cause it to self-amputate an arm.
💡 Did you know?
- Its Latin name means 'fragile': a single touch can be enough to make it shed one of its spiny arms through autotomy.
- Some seabeds host populations of this species at densities of up to 2,000 individuals per square meter, one of the densest known aggregations among echinoderms.
