Five-ribbed kelp (Costaria costata)
This large kelp, a member of the brown algae group of seaweeds, has been abandoned by the tides. Torn from its secure site in the upper subtidal zone, it has washed ashore and begun to bleach in the hot summer sun. The holdfast that grips rocks or other hard surfaces is the small light brown branching structure at the right hand side of the plant. Connecting it to the blade is the narrow stipe. The blade contains five ribs that run it’s entire length. Between each rib is a series of puckers which remain dark brown while the tissue in between turns green as it dies. The sugar kelp discussed in an earlier column resembles the five-ribbed kelp but lacks the ribs.
This particular specimen is about three feet long and about one foot wide. However, this annual plant can display a wide range of shapes and sizes depending on the time of year and degree of wave exposure. This specimen is from a sheltered area and has grown wide rather than long. If it had remained attached it could have reached up to six feet in length by the middle of the summer. In areas with high water surge from large waves or high currents the plant would have been very narrow having a width some 8-10 times smaller than the length and with more prominent ribs.
This species of kelp ranges from the Aleutian Islands in Alaska to Monterey Bay in central California and is a preferred food item of sea urchins.
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David W. Jamison is a marine biologist and Boston Harbor resident.