The green algae are also dominant in the mid-level tide pools, with the brown algae growing around the perimeter. In the image above Ulva lactuca (sea lettuce) has colonized this pool, but quite often you'll also find Enteromorpha. Blue mussels will carpet the bottom and crevices of mid-level tide pools. If you find a tide pool that has no green algae, but lots of Irish moss (Chondrus crispus) you will most likely observe that there are many large periwinkles in the area that have eaten all of the green algae. If a pool has lots of green algae be sure to look for juvenile crabs hiding beneath the canopy of green algae. They often keep the periwinkles away by preying on them.