Animals in the Sea
There are 11 phyla of animals on land and 28 phyla of animals in the sea. But it was not always that way in the ocean. As many as five mass extinctions, probably caused by asteroids and/or comets that struck the earth, transformed the ocean ecology in the half-billion years since the emergence of multicelled life. Life in the ocean changed from SESSILE (non-moving organisms) to MOTILE (organisms capable of movement on their own). Whole branches from the evolutionary tree were lopped off while others flourished. The greatest transformation occurred 251 million years ago causing the extinction 95 percent of species in the oceans (and most vertebrates on land). Research now suggests that it was followed by an explosion of complexity in marine life, one that has persisted ever since. More motile organisms arose which resulted in more interactions and, ultimately, more complexity.
The Protozoa are single-celled organisms and are not technically classified as animals. Examples of commonly recognized protozoans are the amoeba, paramicium and the euglena, organisms frequently studied in high school science classes. Some protozoans are parasitic but the large majority in the ocean are free-living organisms. They move by using flagella, cilia or by flexing their bodies.
Common marine protozoans are the foraminifera. The Globigerina is a foraminifera that contributes a large amount of calcium carbonate from their shell to the calcareous oozes on the ocean floor.
Radiolarians are also protozoans with a silica shell which contributes to siliceous oozes.
Some zooplankton spend their entire life cycle as zooplankters. These are called holoplankton. Other zooplankters are the larval form in an organism's life cycle. These are called meroplankton.
Zooplankton, known as krill, are the food that baleen whales strain from the sea. Krill are shrimp-like crusteans. Krill comprise the largest biomass of any species in the ocean and exceed the biomass of all humanity by 5 times. Krill are the primary consumers of phytoplankton. They capture about 10% of the biomass of the producers they consume.
About 70% of all zooplankters are tiny shrimp-like organisms called copepods.
See the charts below.
Its name is derived from the many openings on its surface - "bearer of windows".