Many people think of seaweeds as simply plants that live in the sea. While many types of seaweed do have structures resembling the roots, stems and leaves of plants, and seaweeds do photosynthesize, technically seaweeds fall into an entirely different kingdom: Protista. Often referred to more scientifically as marine algae, seaweeds lack the specialized tissues found in plants responsible for transporting food and water. 

In most field guides, seaweeds are divided into three main groups: green, brown and red. These classifications are based largely on the primary pigments present in the seaweeds’ cells. Brown seaweeds, which fall within the phylum Phaeophyta, actually range in color from light to dark brown to almost black to yellowish or olive when tinged with green. As a group, they tend to be the most conspicuous algae of north Atlantic rocky intertidal shores. One common species in New England is knotted wrack (Ascophyllum nodosum). On a recent visit to the beaches in Marshfield, I found this species drooped in large mats across the large boulders exposed at low tide. 

With long, tough, leathery fronds extending up to 3 feet or more in length, this species can often present a challenge for people traversing the intertidal zone. My Peterson Atlantic Seashore Guide described it well: “Consider that you are in effect walking over a thick bed of wet noodles concealing sharp rocks, deep crevices, and other pitfalls, and use appropriate care when exploring any expanse of rock covered by this slippery weed.” To other rocky intertidal species, such as crabs, snails, and amphipods, these dense mats are extremely beneficial: the moisture trapped under the fronds reduces the risk of desiccation while they wait out the return of the sea, and the thick cover provides protection from aerial and land-based predators (e.g., gulls). 

Along each long strand there are rabbit-eared branchlets, as well as air bladders (i.e., the “knots”). Air bladders help the seaweeds float towards the water’s surface when they are submerged, thereby facilitating photosynthesis. The whole seaweed is securely attached to the rocks by a holdfast, to anchor it in the heavy surf and pounding waves.