Plankton

Plankton are mentioned occasionally in the How to Train Your Dragon Book series.

Description
Plankton is a term applied to a diverse and large group of organisms ranging from plants, animals, protista, bacteria, and achaebacteria. The distinction is anything living that is free-floating in marine waters that cannot swim against the tide or flows of water. This includes anything microscopic, tiny barely visible organisms, to larger creatures such as jellyfish.

Function
Plankton are only mentioned in the Book series as a derogatory descriptor.

In reality, though humans do not consume plankton, it occupies the lowest rungs of the food chain and is essential to many other organisms and ecosystems humans rely on.