Rockweed (Ascophyllum nodosum) is common large brown seaweed, dominant on sheltered rocky shores. The species has long strap like fronds with large egg-shaped air bladders at regular intervals. The fronds of Ascophyllum nodosum are typically between 0.5 and 2m in length.
The inflated bulbs at the end of the blades along with a midrib in the blade are major distinguishing characteristics of rockweed. The bulbs are filled with air and mucilage that allows the blades to float exposing them to sunlight and allowing water to move through the branches.
The bumps on the bulbs are reproductive structures called pits or conceptacles. Special cells within the pits produce eggs and sperm which are released into the water column and fuse producing a baby plant. The new plant then grows directly into an adult. Reproduction occurs through the year. Other seaweeds have a more complicated reproduction cycle where one form produces eggs and sperm while another form produces spores.
The plants drift in large, spherical masses in sheltered waters, grow slowly and can live to be several decades old. Individual fronds can become up to 15 years old before breakage.