Loch Creran harbours a diverse range of marine habitats and species. The site has been designated for the outstanding biogenic reefs (reefs built by living creatures) of the polychaete worm
Serpulid worms form and live inside white calcareous tubes that intertwine forming fragile aggregations rising out of the muddy seabed that can grow to almost 1 m high and 3 m wide. Within Loch Creran the remarkable reefs grow most abundantly in depths of 6 – 10 m. When the animals are feeding, a feathery double horseshoe of crimson tentacles emerges to transform the reef with a brilliant display of colour. If disturbed, all the tentacles snap back instantly into the tubes which are sealed by a scarlet plug. The intricate serpulid reef habitat provides protected living space for a variety of animals and plants not normally found on the surrounding muddy seabed including red seaweeds, feathery hydroids, sponges, sea squirts and clams. Brittlestars hide within the reef, their fragile arms entwined around the tubes. Small crabs and other worms are also concealed in the spaces. Below, in the shelter provided by the reefs, there are squat lobsters, sunstars, hermit crabs and queen scallops covered with vivid orange and yellow sponges. There is an amazing abundance of life associated with the
Close to the bridge over the Creagan Narrows in the upper basin, reefs of the horse mussel, Modiolus modiolus, cover large areas of seabed at depths of 13 - 25 m. The mussels produce strong, sticky byssal threads, which secure them to stones and shells, anchoring the animals in the strong currents of the channel. The reefs grow as new mussels settle and attach to older ones, trapping stones, shells and sediment in the process. These low-lying beds provide a solid platform for the attachment and shelter of a wide variety of organisms. During the summer, red algae and sea firs grow profusely over the horse mussels. Sponges and sea squirts settle on the hard substratum and sea urchins graze on the attached algae. Predators such as starfish feed on the mussels, whilst whelks scavenge, taking both carrion and living bivalves. Worms find a home in the trapped sediment and the reefs provide a nursery ground for the young of other bivalves such as scallops and clams.