Variable sizes, shapes and colours. Many are transparent but can be red, black or silver. The defining characteristic is that they cannot swim against a current. The word plankton is from the greek and means wanderer or drifter. Much plankton is microscopic but they can also reach immense sizes, up to 2m.

Free floating in the ocean.

Varied, some will feed on "plant" plankton, others on zooplankton and still others do not feed as plankton, only when they reach adulthood.

Zooplankton are divided into species that spend all their lives as plankton, the holoplankton and those who only spend part of their lives as plankton the meroplankton.