Marine Biology

Oxbow theory

TYPES OF NEKTON

Types of nekton There are three types of nekton viz. chordates, molluscs and arthropods. The majority of the nekton species consists of carnivorous organisms which find their prey over a wide area.

1.      Chordates

Nekton belonging to the chordates form the largest group, having  bones or cartilage. This group includes sharks, bony fish,  whales, porpoises, dolphins, seals, turtles, snakes, and sea birds. There are  more  than  25,000 species of fishes, higher than any other group of vertebrates.     

i.           Fishes

Fishes represent the major constituent of the nektononic vertebrates . Based on the habitat they inhabit, these are grouped into holoepipelagic and meroepipelagic. Holoepipelagic fishes are those that spend their entire lives in the epipelagic region. These fishes include most of the tropical and subtropical fishes which often lay floating eggs and have epipelagic larval life. The meroepipelagic fishes spend only part of their life in the epipelagic region and visit this region to find their prey. They may spawn in inshore or fresh water regions. Herbivorous species like the anchovy (Engraulis encrasicolus), however, play a major role in upwelling areas of the sea where enormous quantities of fish can be harvested.      

ii.            Reptiles

The nektonic marine reptiles include sea snakes and turtles. These are cold blooded animals. Though the turtles are  true pelagic animals they often visit the shores for breeding .Their limbs are modified to form paddles, which enable them to swim fast, and they can also drift motionless. The leathery turtle (Dermochelys) and the hawk- billed turtles (Chelonia imbricata) are carnivores, and known to feed on medusae, crustaceans, cephalopods and fishes, while the green turtle (Chelonia mydas) feeds largely on seaweeds and eel-grass.

The sea-snakes are represented by more than sixty species  distributed abundantly in the Indo-Pacific region. Important genera include Enhydrina, Pelamis, Microcephalophis and Laticauda. They feed mainly on the fish. They are viviparous and mostly venomous.

iii.          Seabirds (Aves)

Seabirds (Aves): Seabirds, which are warm blooded nektonic animals, play a significant role in the food chain dynamics of the marine ecosystem. Especially their interaction with marine fishes and fishing is regarded as very  important."Seabird" is  rather a loose term traditionally used to cover those birds which obtain at least part of their food from the sea by travelling some distance over or under its surface. They typically breed on offshore or coastal areas like cliffs, dunes, skerries or remote islands. There are 274 species  of seabirds, comprising mainly penguins, albatrosses, fulmars, petrels, shearwaters, pelicans, cormorants, skuas, ducks, terns and auks. Seabirds are characterized by long life (high adult survival rate), higher age at first maturity and breeding, slow reproductive rate and intense care for the offspring.

iv.          Mammals

Mammals: The mammals include the seals, dugongs, manatees, dolphins, porpoises and whales, of which the whales are the most important. These are the largest members of the nekton. The blue sulphur-bottom whale, a plankton feeder, attains a length of 25 metres and a weight of 60-80 tons. The toothed whales (sperm whale, Physeter catodon, and killer whale, Orcinus orca) are predators feeding on squids and large fish. The whalebone whales (baleen whales) on the other hand feed on plankton, mainly sieving euphausids and others from surface waters.