ong algal encrusted rocks live sponges

There is a broader spectrum of higher animal taxa in the sea than on land, many marine species have yet to be discovered and the number known to science is expanding annually.[54] Some vertebrates such as seabirds, seals and sea turtles return to the land to breed but fish, cetaceans and sea snakes have a completely aquatic lifestyle and many invertebrate phyla are entirely marine. In fact, the oceans teem with life and provide many varying microhabitats.[54] One of these is the surface film which, even though tossed about by the movement of waves, provides a rich environment and is home to bacteria, fungi, microalgae, protozoa, fish eggs and various larvae.[55]

The pelagic zone between the surface of the water and the seabed contains myriad zooplankton drifting with the currents. Most of these tiny organisms are the larvae of fish and marine invertebrates, the result of the liberation of eggs in vast numbers because the chance of any one embryo surviving to maturity is so minute.[56] The zooplankton feed on phytoplankton and on each other and form a basic part of the complex food chain that extends through variously sized fish and other nektonic organisms to large squid, sharks, porpoises, dolphins and whales.[57] Some marine creatures make large migrations, either to other regions of the ocean on a seasonal basis or vertical migrations daily, often ascending to feed at night and descending to safety by day.[58]

The seabed provides a range of habitats on or under the surface of the substrate that have been exploited by creatures adapted to these conditions. The tidal zone with its periodic exposure to the dehydrating air is home to barnacles, molluscs and crustaceans. The neritic zone has many organisms that need light to flourish. Here, among algal encrusted rocks live sponges, echinoderms, polychaete worms, sea anemones and other invertebrates. Corals often contain photosynthetic symbionts and live in shallow waters where light penetrates. The extensive calcareous skeletons they extrude build up into coral reefs which engineer the seabed. These provide a biodiverse habitat for reef dwelling organisms. There is less sea life on the floor of deeper seas but marine life also flourishes around seamounts that rise from the depths, where fish and other animals congregate to spawn and feed. Close to the seabed live demersal fish that feed largely on pelagic organisms or benthic invertebrates.[59] Exploration of the deep sea by submersibles revealed a new world of creatures living on the seabed that scientists had not previously known to exist. Some like the detrivores rely on organic material falling to the ocean floor. Others cluster round deep sea vents where mineral-rich flows of water emerge from the seabed. A dead whale sinking to the seabed provides food for an assembly of organisms relying largely on the actions of sulphur-reducing chemoautotrophic bacteria. These carcases provide conditions very similar to hydrothermal vents. Such places support unique biomes and many new microbes and other lifeforms have been discovered at these locations

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