Sea-urchins like the green sea-urchin and strongylocentrotus droebachiensis, can eat large quantities of bottom-living Irish moss. The plants are usually 2-3 years old when they are eaten or torn away by waves or ice. It is uncommon that they live for more than 4-5 years.
When irish moss recieves little sunlight it is usually dark red in colour, but attains a light yellow or green colour when exposed to stronger sources of light. The white patches at the bottom of the two plants are bryzoas, and the white dots further up are tube building polychaetes.