I've posted this photo before, and talked about the encrusting organisms that live on it. Today I want to focus on the substratum itself, the green algae Ulva. Belonging to the phylum Chlorophyta, or the green algae, this group is considered the basal group for land plants. They share the same chlorophyll pigments, store starch, and posses cellulose cell walls.
Ulva is an interesting algae, it grows in large sheet-like patches only two cells thick. The sheets are often ruffled and fluted making its common name, sea lettuce, an apt descriptor. It attaches to rocks, docks, and other structures by a very small disk.
It also has a very interesting life cycle termed alternation of generations. The diploid generation makes spores which grow into a haploid adult without fertilization. The halpoid adult (n) makes gametes which are fertilized and grow into a diploid adult (2n). Repeat the cycle. There are lots of algae that have this type of life cycle, but Ulva is one of the only ones where you cannot tell if the adult is haploid or diploid by looking at it.