Several species of red algae cover the subtidal area in a dense turf. It is often so thick that they prevent the brown seaweeds and attached animal species like barnacles and mussels from settling. The two primary species of red algae are Chondrus crispus and Mastocarpus stellatus. Both are low growing seaweeds that attach to the substrate with a holdfast. Because they grow in such dense mats, water is retained when the tide goes out providing a moist habitat suitable for invertebrates like amphipods to survive the brief period before the tide returns.