The open sea is a pelagic ecosystem (Pelagic is a term that refers to a part of the sea or ocean, that is not near the shore nor near the bottom), in wich the living components are plankton an nekton.
Plankton (singular plankter) are a diverse group organisms that includes microalgaes (phytoplankton) -both first gif- and animals (zooplankton) that float along at the mercy of the sea’s tides and currents.
Several structural features and behaviors have evolved to keep afloat organism that are not strong swimmers. some plankton species display a number of interesting adaptation that help them avoid predation. Many planton and nekton swim by cilia, flagella, appendages and undulatory tails
Some are babies (correct term is larva) that will grow into strong-swimming, non-planktonic adults. Others will remain plankton for their entire lives, also jellyfish. Most of the plankton in the ocean are micro-algaes, It is estimated that 80% of the oxygen on earth is produced by phytoplankton.
Marine organisms produce over half of the oxygen that land animals need to breathe.
Humans and almost all other animals depend on oxygen in the atmosphere or water to respire—that is, to produce energy at the cellular level necessary for survival. Most sea animals extract oxygen directly from ocean water, while land animals breathe air from Earth’s atmosphere, which consists of about 21 percent pure oxygen.
Oxygen has not always been a given element in the air; in fact, its presence is a relatively recent development in Earth’s history. Until around 600 million years ago, our atmosphere was composed of less than five percent oxygen, instead being mainly a nitrogen and carbon dioxide mixture dating back to Earth’s formative volcanic activity over four billion years ago.
Fortunately for us, organisms evolved that could use carbon dioxide, along with solar radiation, to produce metabolic energy and oxygen—a process called photosynthesis. Whlie we may think of photosynthesis as the life process of land plants, algae and a variety of other microscopic organisms called phytoplankton had been using photosynthesis long before terrestrial plants appeared…
Ctenophores (Greek for “comb-bearers”) have eight “comb rows” of fused cilia arranged along the sides of the animal. These cilia beat synchronously and propel ctenophores through the water. Many ctenophores, like various other planktonic organisms, are bioluminescent, able to give off light.