Sea Lettuce - Ulva lactuca

Sea lettuce is a thin, flat, green algae that grows from a discoid holdfast on rocks or on other algae. The margin is ruffled and often torn. It can reach 18 cm (7 in) long and up to 30 cm (12 in) across. The membrane is only two cells thick, is soft and translucent.

Ulva lactuca is edible, but has a strong flavor that can be lessened by soaking in water.

I found this sea lettuce floating free in the low intertidal zone.