Complex sentence: These beds may extend over several hundred square metres of mixed sediment with sand and pebbles and contain more than a million brittle stars, either "O. nigra" or "O. fragilis", or a mixed community of the two.
Split into simpler sentences: These beds may extend over several hundred square metres of mixed sediment with sand and pebbles and contain more than a million brittle stars. These can be either "O. nigra" or "O. fragilis", or a mixed community of the two.