Marine Fish

Apart from some 50 or so species of lampreys and hagfishes, fish species are divided into two unequal-sized groups, the cartilaginous fishes including chimaeras, and sharks and rays (class Elasmobranchii),and the bony fishes (Osteichthyes). Some 60% of all known living fish occur in marine habitats. They range in size from an 8mm long goby Trimmatom nanusin the Indian Ocean to the 15m whale shark Rhincodon typus and occur in virtually all habitats, from shallow inshore waters to the abyssal depths.

There are around 850 species of elasmobranchs including many of the largest fish species and top predators in many marine ecosystems. There tend to be more shark species at lower latitudes, but at family level richness tends to be higher on the edge of the tropics. The bony fishes are a remarkably diverse group, with an enormous range of morphological, physiological and behavioural adaptations. By far the largest and most diversified Order is the Perciformes. This is the largest of all vertebrate orders and dominates vertebrate life in the ocean, as well as being the dominant fish group in many tropical and subtropical freshwaters.

Most fish are strictly marine but some, such as many salmonids and sturgeons, ascend rivers to breed. Those that breed at sea but spend their lives otherwise in freshwater, such as most eels in the family Anguillidae (order Anguiliformes), are referred to as catad-romous. Species with a wide salinity tolerance are referred to as euryhaline while those with narrow tolerances, are referred to as stenohaline.