A micronekton is a group of organisms of 2 to 20 cm in size which are able to swim independently of ocean currents. The word 'nekton' is derived from the Greek νήκτον, translit. nekton, meaning "to swim", and was coined by Ernst Haeckel in 1890.
Overview Micronekton organisms are ubiquitous in the world's oceans and they can be divided into broad taxonomic groups. The distinction between micronekton and micro-, meso- and macro- zooplankton is based on size. Micronekton typically ranges in size from 2 to 20 cm, macro-zooplankton from 2 mm to 2 cm, meso-zooplankton from 0.2 to 2 mm and micro-zooplankton from 20 μm to 0.2 mm. Micronekton represents 3.8-11.8 billion tons of mesopelagic fishes worldwide, approximately 380 million tons of Antarctic krill in the Southern Ocean and a global estimated biomass of at least 55 million tons of a single group of Ommastrephid squid. This diverse group assemblage is distributed between the sea surface and approximately 1000 m deep (in the mesopelagic zone). Micronekton shows a diverse range of migration patterns including diel vertical migration over several hundreds of metres from below 400 m (deeper layers) to the top 200 m (shallower layers) of the water column at dusk and inversely at dawn, reverse migration (organisms stay in the shallow layer during the day) mid-water migration (organisms stay in the intermediate layer, i.e. between 200 and 400 m) or non-migration (organisms stay in the deep layer at night and shallow layer during the day). Micronekton plays a key role in the oceanic biological pump by transporting organic carbon from the euphotic zone to deeper parts of the oceans It is also preyed upon by various predators such as tunas, billfishes, sharks, marine birds and marine mammals.
Taxonomic groups
Generally, the taxonomy of global existing micronekton is not yet complete due to the paucity of faunal surveys, net avoidance (organisms sensing the approach of the net and swimming out of its path) and escapement (animals escape through the meshes after entering the net), and gear in-adaptability. New species are continually being discovered and described in new regions of the world's oceans. Crustaceans are highly diverse, with a single group, the decapods, consisting of 15,000 species in around 2,700 genera. Euphausiids consist of 10 genera with a total of 85 species. Hyperiids are also widely distributed in the world's oceans with approximately 233 species across 72 genera. Cephalopods comprise less than 1000 species distributed across 43 families. They occur in all marine habitats such as benthic, burrowing on coral reefs, grass flats, sand, mud, rocks; are epibenthic, pelagic and epipelagic in bays, seas and the open ocean. Bristlemouths (Gonostomatidae), largely Cyclothone, account for more than 50% of the total vertebrate abundance between 100 and 1000 m. Twenty-one species of bristlemouths have been described globally. Lanternfishes are the secondmost abundant marine vertebrates, having diversified into 252 species. Hatchetfishes (Sternoptychidae) and dragonfishes (Stomiidae) are other common mesopelagic taxa in the deep-sea environment.
Anatomy and physiology
Crustaceans The crustacean body is divided into three sections: head, thorax and tail. They typically have 2 antennae and a varying number of pairs of thoracic legs called pereiopods (or thoracopods). Crustacean species such as Systelaspis debilis and Oplophorus spinosus have specific visual pigments thought to facilitate congener recognition. The oplophorid genera Systellaspis, Acanthephyra and Oplophorus secrete luminous fluids as part of their distress response.
Cephalopods Cephalopods are soft-bodied animals with a cranium and, in most forms, a mantle/fin (cuttlebone or gladius) as primary skeletal features. They have highly developed central nervous systems with well-organized eyes. Cephalopods can be divided into four main groups: squids, cuttlefishes, octopuses and chambered nautiluses, which have distinguishable morphological features. Squids can have chromatic vision through the presence of various visual pigments.
Mesopelagic fishes Few anatomical and physiological studies of mesopelagic fishes have been conducted, except for research of the swimbladder of these organisms. The deepest-living mesopelagic fishes have no swimbladder. Most species inhabiting the upper mesopelagic zone have gas-filled swimbladders (which aid in buoyancy). Other species have a gas-filled swimbladder when young which becomes filled with fat with age. Polyunsaturated wax esters are common in muscle or adipose tissue of lanternfishes, posing an obstacle to human consumption. Lanternfishes possess retina with a single pigment capable of absorbing bioluminescent light ranging from 480 to 492 nm at a distance of up to 30 m in the deep ocean.
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