The Capitella capitata is a polychaete worm that grows up to 10 cm in length. It is blood-red in colour and has the most earthworm-like appearance of all polychaetes. The species is sedentary and fragile, with a flexible body.
Capitella capitata occurs on muddy sand, gritty sand, fine sand or rich mud on the lower shore to sub-littoral. It may be found under pebbles or small stones, with the burrows at or near the surface of the sediment.
It is an opportunistic specie tolerant of stressful conditions, and often found in polluted waters (sewer discharges, hydrocarbons, metals ...) where it out-competes less tolerant species. A large abundance of C. Capita can be seen as an indication of polluted waters .