Holoplankton

Holoplankton are organisms that are planktic (they live in the water column and cannot swim against a current) for their entire life cycle.

Examples of holoplankton include some diatoms, radiolarians, some dinoflagellates, foraminifera, amphipods, krill, copepods, and salps, as well as some gastropod mollusk species.

Adaptations include flat bodies, lateral spines, oil droplets, floats filled with gases, sheaths made of gel-like substances, and ion replacement.

When predators release a chemical in the water to signal zooplankton; cyclomorphosis allows holoplankton to increase their spines and protective shields.

Pelagic cnidarians (jellyfish and related species) have nematocysts on their tentacles that eject a coiled microscopic thread very rapidly.