Seaweed, algae found around coasts from the shore to fairly deep water. Most common are the brown algae, or wracks. Some, such as bladderwrack, clothe the rocks between tides; others live up to 40 ft (12m) deep. The large brown algae (kelps) sometimes form thick beds of long, tangled fronds, with tough, well-anchored stems. Gulfweed is another widespread species. Delicate green and red seaweeds live mainly in rock pools. Seaweeds provide oxygen through photosynthesis. Some seaweeds also provide food and shelter for sea animals, and many are used for food, fertilizer, iodine, and gelatin.