These flies are black in appearance and show large variation in size, with males tending to be larger and more robust and 'hairy' than females.

[1][8] A study of coastal Diptera species at 109 locations distributed around the New Zealand coastline found C. littoralis at 41% of the sites.

[9] Like other species in the family Coelopidae, C. littoralis inhabit beds of decaying seaweed and kelp (wrack) washed up on the coast, and can form large aggregations.

C. littoralis is found in beds of decaying seaweed and kelp (wrack) washed up on the coast, and can also be observed resting on surfaces including cliff faces and driftwood in large aggregations.

Their larvae feed on the decaying algae and its associated microorganisms,[11] and go through three developmental stages (or instar) before pupating on sand further up the beach.