It sounds like you're already familiar with the terms "benthic", "planktonic", and "nekton" but it never hurts to review. The following definitions came from a glossary of terms for marine biology. Benthic animals live on the bottom, or are attached to a surface. Lots of benthic critters don't exactly live on the bottom, but are more-or-less permanently attached to other living organisms. Animals that are planktonic live up in the water, away from the bottom or other surfaces, and cannot swim against currents. They drift wherever the currents take them. Nektonic animals are usually larger than plankton, and can swim against currents. Most fishes would be considered nekton.

As you probably already know, seahorses are strange-looking fish. You can see photos of seahorses here. While they are very cute, their bodies are not streamlined and they are not strong swimmers. Most fishes swim either by pushing their tails from side to side or using their fins as paddles. Seahorses' bodies aren't very good at either kind of motion. They can swim in calm water, but it is difficult to imagine their being able to swim against currents, so I wouldn't consider them nekton.

Now, would we call them plankton? They probably would be swept away if they got into moving water, except that they spend most of their time with their tails wrapped around a piece of algae or seagrass. This allows them to stay pretty much in one place. Nah, I wouldn't say they're planktonic, either.

That leaves benthic. We've already established that the term "benthic" can describe animals that live on the bottom or a surface. Seahorses like to hang onto seagrasses, and this keeps them from getting swept up into the water column. I think this means we can call them benthic.

I hope this answers your question!

Allison J. Gong
MAD Scientist