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Ascophyllum nodosum


This plant belongs to a group of tough brown seaweeds that dominate rocky seashores in cooler climates. Firmly attached to the rocks by a disk-shaped holdfast, from which arise several narrow fronds that often grow to 3 ft (1 m) in length, and exceptionally to 9 ft (3 m) in very sheltered situations. Single oval bladders grow at intervals down the knotted wrack frond. The fronds produces about one bladder a year, so knotted wrack’s age can be roughly estimated by counting a series of bladders. The bladders hold the fronds up in the water so that they gain maximum light, which is an advantage in the often turbid waters where knotted wrack grows. This also makes it harder for grazing snails to reach the fronds when the tide is in.


Marine, sheltered rocky shores.

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Maine, USA

Spotted on Jul 30, 2012
Submitted on Aug 2, 2012

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