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       Temperature, salinity, and the availability of oxygen, light, and nutrients shape the marine ecosystems. Thousands of different invertebrates make their home in the ocean, often providing food for larger species. For instance, there are about 5,000 species of sponges, of a wide variety of colors and shapes. These animals have the least complex body structure of all multi-celled creatures, consisting of an outer layer of tissue and an innerlayer of either silica (SiO2) or calcium  carbonate (CaCO3). The echinoderms, which include starfish, sea urchins, and sea cucumbers, are a group of exclusively marine bottom-dwelling invertebrates, characterized by hard, spiny skin. Around 6,000 species dwell in the world’s salt waters, often in rocky pools around beaches.

       The cnidarians are another important marine invertebrate group and include the jellyfish, corals, and sea anemones. These organisms are characterized by their soft, watery bodies. The molluscs, which are found on land and sea, include the gastropods (limpets, slugs, and snails), cephalopods (octopus, squid, and cuttlefish), and bivalves (clams, mussels, cockles, oysters, and scallops). Most crustaceans, of which there are about 39,000 species, are marine and include crabs, lobsters, shrimps, barnacles, and woodlice. The copepods, which are tiny crustaceans ranging from 0.05 to 1.8 cm in length are particularly abundant. There could be hundreds of thousands of copepods in a cubic meter of surface water and they are an important source of food for predators like marine worms and the smaller jellyfish. Marine worms are another diverse group, many of which are completely different from the well-known earthworm. For instance, the arrow worm is a major predator of the copepods. It is also known as the chaetognath worm, which means “bristle jaw.” The worm darts and grabs its prey in its jaws. The nematodes, or thread worms, are among the tiniest of creatures in the marine environment, being just a fraction of an inch long. They live in sediments and feed off bacteria. As a group, the nematode worms have not been studied much, and researchers believe there may be thousands of species remaining to be discovered. At the other extreme are the tube worms found on the ocean floor, which may be up to 1.8 m in length. These creatures are so unusual that they have been placed in a phylum of their own. They have no mouths, but their bodies are filled with chemosynthetic bacteria, which extract energy from minerals rather than sunlight, and these provide much of their food supply.

       Birds, mammals, and fish all live in or around the marine environment, but fish are the only vertebrates that are purely aquatic and are found in both freshwater and saltwater around the world. Around half of the 25,000 known species of fish live in the marine environment, mainly in shallower, warmer waters. Around 1,000 fish species occupy the open ocean. Fish that live in the deep ocean are generally black, brown, or gray, without the silvery camouflage that characterizes those living nearer the surface. Some are buoyant and swim up and down the depths of the water, searching for food, while others, including sharks and rays, are heavier and sink if they cease swimming. They tend to stay in place, catching food as it passes, or making just short hunting excursions.

       Whales are marine mammals, breathing with lungs, and the largest animals in the oceans. There are two suborders: the baleen whales, which do not have teeth and filter feed on massive amounts of plankton; and the toothed whales, which feed on fish and squid. Toothed whales have the remarkable ability to navigate through the ocean by echolocation. The whales, sharks, and giant squid represent the top end of the marine ecosystem.

In the open ocean, the ecosystems are vertically stratified. That is, different plants and animals are found at different depths. Sunlight penetrates to a depth of only 3 ft (0.9 m) or so in the cloudy waters of an estuary, compared to up to 300 ft (90 m) in the clearer waters of the open ocean. In this relatively light region, known as the, there is a net primary production of food by photosynthesis carried out by 4,000 or so species of phytoplankton.

       The column of water extending down from the surface to a depth of about 4 km is called the pelagic zone and is composed of the epipelagic (top), mesopelagic (middle or twilight), and bathypelagic (bottom) zones. Below this, extending to about 6.4 km, are the abyssal and hadal zones. Hadal comes from the Greek word for unseen, abyssal from the Greek word for bottomless. There is no light in these zones close to the ocean floor other than what is emitted from the organisms themselves. Thousands of species, including bacteria, squid, and fish, emit flashes of light by a process known as bioluminesence to distract predators and locate prey.

       The oceans and seas contain some specific ecological niches. For instance parts of the ocean floor, or benthos, contain hydrothermal vents, where jets of water containing sulfur (S) compounds gush up, heated by the magma beneath the ocean floor. The ecological communities of microbes, worms, and mussels living around hydrothermal vents were known only since 1977. They can withstand temperatures of up to 660°F (350°C), and the microbes are capable of extracting biochemical energy from the sulfur compounds.

       Shallow areas of the seas and oceans have rich ecosystems too. The sea floor slopes gradually from the shore out to the deeper ocean. The area near the shore is called the littoral zone, and many fish and shellfish are found here. Filter feeders such as mussels and barnacles live in rocky-bottomed shores, while soft-bottomed beaches are home to scavengers like shrimps and polychaete worms. The number of species increases nearer to water.