Asterias rubens ("northern sea star") can be purple, reddish brown, orange, or yellow. It commonly feeds on blue mussels, restricting their lower range on the shore.
Field Guide to Marine Invertebrates - Echinoderms
Strongylocentrotus droebachiensis ("green sea urchin") is found in the lower intertidal zone in tide pools and is abundant in the subtidal zone. In a natural cycle, large groups of urchins eat their way through kelp beds to create “urchin barrens” where seaweeds are scarce; then, the urchin population decreases because of lack of food, allowing the kelp beds to recover. Recent overfishing of urchins on the Maine coast caused notable increases in kelp populations near the shore.
Photos by Sarah Hall.