Yes, jellyfish are generally considered plankton.
What are Plankton?
Plankton are a diverse collection of organisms that live in large bodies of water and are unable to swim against the current. The term "plankton" comes from the Greek word "planktos," which means "wanderer" or "drifter." Plankton can be bacteria, archaea, algae, protozoa, or animals.
Why Jellyfish are Considered Plankton
Jellyfish, in their early life stages, are typically small and drift with the ocean currents. This drifting behavior classifies them as plankton. Even larger jellyfish, while capable of some movement, are still largely at the mercy of ocean currents, reinforcing their classification as plankton. Although some jellyfish species can be quite large (as noted, some can be 130 feet long!), their dependence on currents for transportation means they still fall under the plankton category.
Zooplankton: The Animal Plankton
Jellyfish, being animals, are classified as zooplankton. Other examples of zooplankton include copepods, krill, and larval stages of many marine animals. It's interesting to note that even some animals related to humans (chordates), while in their early larval stages, exist as transparent and soft-bodied zooplankton.
