Rockweed

They are called rockweed because they anchor themselves to rocks and other submarine structures, forming thick mats.

Fucoid seaweeds are vital to the ecology of intertidal zones. They are the principal food source for numerous molluscs and also eaten by some fish species. They also provide a range of organisms with a refuge from predators, and protect them from heat and ultra-violet light when tides are low.

Eighteenth century mariners took care to note the presence of drifting clumps of rock weed because it could signify they were entering seas affected by storms or strong currents, or that they were possibly in the vicinity of an island or submerged rocks.