Introduction

I was eating seaweed salad and this question occured to me.

What is “seaweed”?

Seaweed refers to (many species of) multicellular marine algae, where according to me (not quite an expert), algae means “not necessarily plant."1

For simplicity, let's focus on edible seaweed, which falls into 3 common groups:2

  1. green algae, such as green caviar (a.k.a. sea grapes)
  2. red algae, such as nori (of sushi fame)
  3. brown algae, such as kombu (of miso soup fame)

What is “plant”?

You might've heard of the kingdom Plantae. Like I, you might've thought Plantae (“ooh, Latin, look at me”) had a precise, generally accepted definition. Well, you can take that thought, that temptation of a straightforward answer, and throw it in the trash.

From strictest to loosest, the 3 common modern plant definitions include:

  1. Embryophyta, which does not include green, red, nor brown algae
  2. Viridiplantae, which includes green algae and does not include red nor brown algae
  3. Archaeplastida, which includes green and red algae and does not include brown algae

So?

Seaweed sometimes are plants. Green and red algae seaweed sometimes are plants, and brown algae seaweed never are plants.3

Conclusion

Let me ask you a question, dear reader: do you actually care about the minutiae of taxa? I don't; you probably don't. Scientists design scientific words for precision and objectivity, which can come at odds with casual language. For example, the botanical definition of berry excludes strawberries, raspberries, and blackberries, yet includes cucumbers.

Even when we have strong intuition, scientific words may contradict that intuition. Kombu, a type of brown algae seaweed, grows in “forests” (kelp forests) with “roots” (holdfasts), “stalks” (stipes), and “leaves” (fronds). If you looked at kombu growing in the ocean, you would almost certainly intuit it a plant. Yet science defines otherwise.

Neither the scientific meaning nor the casual meaning of words will always work better. Use the casual meaning at a scientists’ conference; you'll get eye rolls. Clarify the scientific meaning at a party with friends; you'll get eye rolls.

Actually, try that last one. See how far you can get in discussing seaweed the next time a friend brings it up. Test the waters, if you will.

  1. Let me save you from this deep, dark hole; no generally accepted definition of algae exists. ↩︎

  2. Note not all green, red, nor brown algae fall under the term “seaweed.” ↩︎

  3. We consider brown algae “heterokonts,” a sub-group of protists. “Protist” captures any eukaryotic organism not an animal, plant, nor fungus. ↩︎