Fish that spend their lives in the epipelagic are termed holoepipelagic. These include white tip sharks, spiny marine bream, yellow fin tuna, striped marlin, oarfish, and ocean sunfish.
The second group of oceanic fish is called meroepipelagic. These fish spend only part of their life in the epipelagic zone. This is a more diverse group and includes fish that spend their adult lives in the epipelagic but spawn in inshore waters (ribbon halfbeak, herring, whale shark, dolphin, ) or in fresh water (salmon).
Another component of the oceanic nekton is the marine mammals. These include whales (order Cetacea), and the seals and sea lions (order Carnivora). Other marine mammals include manatees and dugongs (Order Sirenia), and sea otters (order Carnivora) but these occupy inshore waters.
Nektonic reptiles of the oceanic are almost exclusively turtles and snakes. There are also marine iguanas in the Galapagos Islands and marine crocodiles around some Australian and Indonesian Islands, but these are littoral and inshore animals.
Gas filled cavities (lungs) help float all air-breathing nektonic animals. Other means used by marine mammals to increase buoyance are bone reduction and the presence of a layer of lipids (fats or oils). Large amounts of lipids are also present in nektonic fish that do not have swim bladders (sharks, mackerels, bonito). In addition to these static means of producing buoyancy, some nektonic animals have hydrodynamic mechanisms for producing buoyancy during movement. (Formation of lifting surfaces in the anterior region.) Examples are pectoral fins and flippers and the presence of a heterocercal tail.
Oceanic nekton also have adaptations for locomotion, surface resistance, defense (cryptic coloration), sensory systems (ecolocation). Some have osmotic adaptations to allow for absorption of fresh water.
Special adaptations of Marine Mammals
1. Large size - less loss of body heat (reduces surface to volume ratio)
2. Insulating layers of blubber or fat
3. Adaptations of the circulatory system for reducing heat loss
Respiratory modifications for diving
1. Have the ability to hold their breath for extended periods of time making deep dives possible.
2. They are able to dive with their lungs empty avoiding problems of buoyancy and bends (nitrogen bubbles in the blood)
3. Their blood is rich in hemoglobin and other respiratory pigments, and the muscles are rich in myoglobin (another respiratory pigment in the muscles).
4. During deep dives, sphincters in arteries shut off blood to parts of the body so it only goes where it is needed.
5. The heart slows, and the muscles can tolerate a greater oxygen debt than terrestrial animals.
6. When the animal surfaces and breathes, a very rapid O2-CO2 exchange occurs.
1. The kidneys reabsorb water and they excrete a very concentrated urine
2. Fatty insulating layers may also play a role in water storage.
3. Most water intake comes from the fish they eat.