What Grows As You Watch?

Kelp in the Underwater DomeBull kelp, Nereocystis luetkeana, can grow a foot a day. Each year we can look into the Underwater Dome (above) or in the waters off Pier 60 (below) to see this annual algae growing up toward sunlight. It has no roots, true stems, or flowers, but does photosynthesize like a vascular plant.

Kelp off of Pier 60 at the Seattle Aquarium Since kelp has no roots or other storage organ, each year’s growth begins from spores. These spores occur in patches (see red arrow in picture above) that are lighter in color than the surrounding blades. These spore patches, or sori, orwill drop off the blades and fall to the substrate at the bottom of the bay where they will release their spores. The spores will then transform into female and male cells, which will unite to produce next year’s new crop of kelp.

Sea otters are considered a keystone species whose absence affects the entire habitat. In the ocean proper, they feed on urchins and other organisms. When sea otter populations were driven to near extinction by the fur trade in the 1700s and 1800s, bull kelp began disappearing as well. That’s because the absence of sea otters allowed sea urchins to reproduce unfettered; abundant populations of urchins decimated kelp beds before they were able to mature and produce sori. No sori meant  no spores; no spores meant no young kelp. Small fish, crustaceans and other organisms dependent on this kelp were also affected. When sea otters were reintroduced in the 70s off the coast of Washington, kelp beds again began to flourish.

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