Speaking of Nature: Seaweed isn't a plant
Bill Danielson photo
This photo shows a species of seaweed called “Spiralled Wrack” (Fucus spiralis). Can you figure out how many snails are hiding in this seaweed?
Some of my favorite summertime memories from my childhood revolve around visits to the ocean. I can remember wonderful trips to Cape Cod and the coast of Maine where it seemed that every sight, sound and smell was something new. You could smell the ocean before you got within sight of it and once you hit the beach, it was just fun, fun, fun.
Sandy beaches were good for sand castles and burying people, but I think my favorite type of shoreline was the gently sloping rocky shore that had tide pools. When the tide was high, you couldn’t see much of these areas, but when the tide went out, they held hidden wonders; creatures that were temporarily trapped and easy to watch.
Tiny little fish darted from side to side, snails glided over the rocks, polychaete (pronounced poly-keet) worms slithered to-and-fro and small crabs skulked in the shadows just daring you to try to pick them up. And always, in every tidal pool, there seemed to be at least one piece of seaweed. Sometimes there was enough to completely fill the pool, but usually there was just enough to give the fish and the crabs somewhere to hide.
When I was young, I was convinced that seaweed must be slimy and disgusting, but if you actually dig down and stoke up your courage enough to touch a piece, you’ll realize that it’s not slimy at all. In fact, it feels a little more like rubber than anything else, which is sort of strange for a plant, don’t you think? Well I have a secret to share with you. Seaweed isn’t actually a plant at all!
This is where we delve into the world of science; a world that embraces technicalities. To be classified as a plant, an organism must be multicellular and it must have cell walls with cellulose. It turns out that seaweed has neither of these characteristics. Instead, seaweed falls into a group of organisms called protists, but you would probably be more familiar with the name algae.
A large seaweed “plant” isn’t a single organism, but rather a huge colony of single-celled organisms that are living and cooperating together. They even show the beginnings of some simple specialization, with certain members of the group forming a holdfast (resembling roots) and others resembling leaves, but each cell in the collective is an organism unto itself.
That being said, there are still many different types of seaweed. Some are large, some are small. Some are green, while others are a mustard-yellow, or a wine-red. Some have thin, flimsy “leaves,” while others have thicker pads that contain chambers filled with air. These allow the seaweed to float into an upward pose while underwater, which gets it as close to the sun as it can manage.
This is important for although seaweeds are not plants, they do make their own food with the help of the energy in sunlight. As a result, it is quite important for them to be in clear, shallow water where the rays of the sun can penetrate. The shallowest water is the best for this approach to living, but in the ocean there is that little complication called the tides that needs to be dealt with.
Seaweed “plants” have to be able to withstand life outside the water for extended periods of time. They have to be able to endure the baking sun in the winter and exposure to sleet, snow and freezing temperatures in the winter. Don’t forget that there are two low tides every day of the year.
So, if you find yourself near the ocean this summer and especially if you find yourself near a rocky area with some tide pools, make sure that you do a little exploring. Look for fish and snails and crabs, but don’t forget to take a close look at any seaweed you might find. It might turn out that seaweed is just a bit more interesting than you ever gave it credit for.
Bill Danielson has worked as a naturalist for 16 years. In that time, he has been a national park ranger, a wildlife biologist and a field researcher. He currently works as a high school chemistry and biology teacher. To contact Bill, or to learn more about his writing, visit www.speakingofnature.com