sea oak

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sea oak The common name for the brown seaweed Halidrys siliquosa (family Cystoseiraceae), a perennial species with regular, alternate branching, and a characteristic zigzag axis. Stalked air bladders resembling seed pods are present; these may be up to 5 cm long and are divided internally by septa into 10 or 12 compartments. They are common and widely distributed at or below low-water mark, or in deep rock pools on sheltered coasts.