plankton -
A generic term for all the organisms that float in the sea. A single organism is known as a plankter.
Wikipedia
Plankton are any drifting organisms that inhabit the pelagic zone of oceans, seas, or bodies of fresh water. Plankton are defined by their ecological niche rather than phylogenetic or taxonomic classification. They provide a crucial source of food to larger, more familiar aquatic organisms such as fish and cetacea.
OmegaWiki Dictionary
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plankton Small organisms (animals, plants, or microbes) passively floating in water.
