Chondrus crispus, commonly known as Irish moss is a dark, reddish brown seaweed. It will often iridesce blue and green in direct sunlight when submerged. While Irish moss is found in great abundance in the subtidal zone, it will also grow in mid and lower tide pools, carpeting the bottom. Irish moss is harvested commercially for a compound called carrageenan (kara-ghee-nan). Carrageenan is extracted from the seaweed after a drying and boiling process. It can be found in common household products and foods such as toothpaste, ice cream, milk shakes, and sauces as a gel or thickening agent. Try this lab exercise.

Mastocarpus stellatus, pictured above is very similar in appearance to Chondrus crispus, hence its common name "false Irish moss". The difference is in the morphology of the blades. Mastocarpus has slightly curled blades with bumps or nodules at the tips of the blades. These bumps contain reproductive cells. Mastocarpus is also more tolerant of freezing temperatures and is often found slightly higher in the subtidal zone than Chondrus. Mastocarpus spp. prefers growing on vertical, fast-draining surfaces.