This has given us a good fossil record. The chalk beds of Georgia and Mississippi as well as the white cliffs of Dover, England, attest to the vast quantities of these tiny animals. Radiolarians are similar to their relatives, the forminiferans , both in skeletal shapes and protoplasmic food trapping ability. But their skeletons are made of a more resistant material called silica, a valuable filter and abrasive agent used in such products as toothpaste and for filtering certain liquids. Their bodies litter the ocean floor covering approximately 3 million square miles of tropical oceans.
Copepods are so numerous that it is estimated that they compose seven out of every ten zooplankters.
Their huge numbers have led some biologists to suggest that there are more of these animals in the world than all the multi-cellular animals combined. All of their food consists of phytoplankton. They are related to crustaceans, among whose members are crabs, lobsters, shrimp, and barnacles. For all their smallness, some large inhabitants of the oceans are dependent on them for food. The 45 foot long Basking Shark and the 60 foot long Whale Shark head the list of those in the oceanic food chain that feed on these copepods.