Marine Vertebrates
Marine vertebrates, classified (following WoRMS) under the Kingdom Animalia, Phylum Chordata and Subphylum Vertebrata, are among the most structurally complex organisms. The four main marine superclasses and classes in Vertebrata are:
Agnatha (jawless fishes)
Gnathostomata
Class Cephalaspidomorphi (lampreys)
Class Myxini (hagfishes)
Pisces (formerly Osteichthyes (now a synonym)) (bony fishes)
Class Actinopterygii (ray-finned fishes)
Class Elasmobranchii (formerly Chondrichthyes) (cartilaginous fishes: sharks, rays and skates)
Class Holocephali (cartilaginous fishes: chimaeras, ratfishes)
Class Sarcopterygii (lobe-finned fishes: lungfishes, coelacanths)
Tetrapoda
Class Aves (birds)
Class Mammalia (mammals)
Class Reptilia (reptiles)
Superclasses Agnatha and Pisces are all a form of fish. Superclass Agnatha contains the 105 species of jawless fish such as lampreys and hagfish.
Superclass Pisces contains at least 27,712 species of bony fishes including Class Actinopterygii which contains well over 20,000 species of ray-finned fishes, Class Elasmobranchii with at least 928 species of sharks, rays and skates, Class Holocephali with at least 928 species of chimaeras, and Class Sarcopterygii which contains the lobe-finned fishes such as lungfishes and coelacanths.
Under Superclass Tetrapoda, important marine classes include Class Aves (the birds) containing at least 9,842 species, Class Mammalia (all mammals) with at least 4,835 species, and Class Reptilia (the reptiles) which contains at least 3,082 species, which are all have species common in both terrestrial and marine environments.
Superclass Agnatha
This superclass includes the "primitive" jawless fish such as lampreys and hagfishes in the Classes Cephalaspidomorphi (lampreys), Myxini (hagfishes), and Pter
Class Actinopterygii
There are tens of thousands of species of bony fishes found in both marine and freshwater environments.Note that the plural form of fish (“fishes”) is used when referring to more than one species of fish. Bony fishes come in all shapes and sizes and live in all marine zones. They range in size from the bluefin tuna that measure up to 3 m to the stout infant fish, which measures about 7 mm. Most fish are slightly endothermic, meaning that they are able to regulate the temperature of their bodies, but not to the extent seen with mammals.
Endothermic bony fishes belong to the Suborder Scombroideiwhich includes about 122 species such as albacores, bonitos,cutlassfishes, frostfishes, hairtails, kingfishes, scabbardfishes, seerfishes, tuna, and wahoo. Endothermy uses a lot of energy but results in greater muscle control, better nerve signals, and improved digestion.
Class Elasmobranchii
This class consists of cartilaginous fish such as sharks, rays, and skates, whose skeletal structures are made up primarily of cartilage. This class contains some of the first marine species to develop paired fins. They all have 5-7 gill slits and lack swim bladders. Their large livers hold a large volume of oil that aids in buoyancy.
Because they lack a swim bladder, most sharks must swim constantly to avoid sinking. Constant swimming also aids in maintaining the flow of water and oxygen over the gills through their mouths. Their skin is covered in denticles rather than scales giving it a rough sandpaper-like feel. They reproduce through internal fertilization.
Class Holocephali
This class contains cartilaginous fishes such as chimaeras and ratfishes.
Class Sarcopterygii
This class contains the lobe-finned fishes such as the lungfishes and the recently discovered coelacanths.
Marine Birds (Class Aves)
Marine birds are characterized by a variety of adaptations to marine conditions. These adaptations include the ability of feathers to resist water, the presence of salt glands, curved bills, and webbed feet. Many marine birds are able to dive into the water to capture prey and some birds, like penguins, are able to swim into deeper water.
The preening gland releases waxes and water repellant fats that create a protective shield on the bird to prevent water from saturating the feathers. This protective shield also keeps the bird insulated and, when combined with feathers made out of keratin, the birds are essentially waterproof. Salt glands allow marine birds to drink salt water and expel the excess salt from their bodies. Salt glands work by condensing salt from the blood into the sinuses allowing the bird to sneeze out the excess. Some marine birds push out salt directly from salt glands.