Micronekton

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. Micronekton shows a diverse range of migration patterns including diel vertical migration over several hundreds of metres from below 400 m to the top 200 m of the water column at dusk and inversely at dawn, reverse migration mid-water migration or non-migration. 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 and escapement, 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, 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 and dragonfishes 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. 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 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. 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.

Bioluminescence

is the production and emission of light from a living organism as a result of a natural chemical reaction, typically the molecular decomposition of luciferin substrates by the luciferase enzyme in the presence of oxygen. Bioluminescence in animals is used to communicate, defend against predation, and find or attract prey. It is mainly generated endogenously or through bacterially-mediated symbiosis, within teleosts. It is common in micronekton.
Many mesopelagic species have counter shading ventral bioluminescent photophores which serve to match the intensity of downwelling light so as to hide from predators lurking below. To conceal itself with bioluminescence, the animal must precisely match its luminescence to the intensity, angular distribution and color of the downwelling light.
Stomiiformes have barbels, ventral arrays, and red and blue suborbital photophores. Lanternfishes have also developed lateral photophores on the sides of their bodies and sexually dimorphic luminescent organs on the tail or head. The sexual dimorphism of bioluminescent signalling and sensory systems may help facilitate sexual encounters in the deep ocean. At the onset of sexual maturity, secondary light organs develop in some of the arms of certain female squids e.g. cranchiids for use in sexual recognition. Females of the octopod Japetella develop a ring of bioluminescent tissue around their mouth just prior to mating and this tissue atrophies once the eggs are spent. In the squid Ctenopteryx siculus, males develop a large photophore within the posterior region of their body at sexual maturity. Bioluminescent signaling by micronekton also carries some degree of risk for it may expose the organism to a predator.

Ecology

Foraging patterns

Crustaceans show omnivorous feeding patterns since they prey on zooplankton, such as euphausiids and copepods, and are also known for occasional herbivory. All squids have carnivorous foraging patterns. Most mesopelagic fishes are carnivores. Some mesopelagic fishes, for example Ceratoscopelus warmingii, have some herbivorous feeding strategies, and can thus be classified as omnivores. Mesopelagic fishes mostly feed at night or dusk, with a few species being acyclic.

Role in food webs

Micronekton plays an important role in oceanic food webs by connecting top predators such as tunas and billfishes to lower trophic level zooplankton. Crustaceans, cephalopods and mesopelagic fishes generally have overlapping isotopic niche widths suggesting some degree of similarity in their diet with low level of resource partitioning and a high level of competition among these broad categories. In low productive environments, predators such as swordfish were shown to forage on larger-sized squids since micronekton prey density is reduced and the costs associated with finding prey are higher than the energy intake when consuming smaller-sized micronekton. Crustaceans and mesopelagic fishes generally occupy trophic level 3, smaller-sized squids occupy trophic level 3 to 4 and large nektonic squids such as Ommastrephes bartramii occupy trophic level 5.

Behaviour

Swarming

Crustaceans, such as krill, may form several aggregation types, from high to low densities distributed throughout the water column, that are influenced by current velocities, direction, mean depth, and predator foraging. Cephalopods may form large schools of neritic and oceanic species with millions of individuals, or small schools with a few dozens of individuals or may be found as isolated territorial individuals. Some mesopelagic fishes form schools or are aggregated in scattering layers while others are dispersed

Swimming

Krill individuals of 45.4 mm in length can maintain horizontal sustained swimming speeds of 0.2 cm s⁻¹ and are able to swim into currents for several hours at speeds of 0.17 cm s⁻¹. Krill are able to dart rapidly backwards to escape predators. Cephalopods such as Illex illecebrosus are able to swim continuously. During daytime, mesopelagic fish often hang motionless in the water column with head up or down in a state of torpor. Myctophids have sustained swimming speeds of approximately 75 cm s⁻¹, with larger individuals having higher rates than smaller ones. At night, fishes in the upper layers of the water column are active and swim horizontally, while those which stayed at depth are immobile and vertically oriented. Mesopelagic fishes are capable of rapid evasive movements to escape predators.
However, crustaceans, cephalopods and mesopelagic fishes can adapt their swimming speeds, with the fastest swimming during escape, intermediate during foraging and lowest speed during migration:

Swimming speeds	Crustaceans	Cephalopods	Mesopelagic fishes
During escape	20 to 50	20 to 70	20 to 50
During foraging	2 to 10	30	2.5 to 30
During migration	0.5 to 3	1 to 3	1.5 to 10

Reproduction and growth rate

Sexual differences in gonads of krill first occur in subadults, and secondary sexual characteristics develop progressively in the late sub-adult stage. The reproductive cycle of krill usually spans from December to April.
Cephalopods have a wide range of reproductive strategies and may spawn once or more than once, with the latter including: polycyclic spawning, with eggs laid in separate batches during the spawning season and growth between the production of egg batches, multiple spawning, with group-synchronous ovulation, monocyclic spawning and growth between egg batches, intermittent terminal spawning, with group-synchronous ovulation, monocyclic spawning and no growth between egg batches, continuous spawning, with asynchronous ovulation, monocyclic spawning and growth between egg batches. Cephalopods typically grow fast and mature rapidly, with their life cycle generally terminating with reproduction.
The age of mesopelagic fishes can be determined from their otoliths and their growth rate can be calculated from the von Bertalanffy growth equation. Most mesopelagic fishes become sexually mature one year after hatching in highly productive areas, and more than two years in low productive areas. Most tropical myctophids and smaller gonostomatids are believed to have a one-year life cycle compared to mesopelagic fishes from colder waters which have a longer life cycle. In temperate and subtropical regions, myctophids spawn mainly from late winter to summer. The spawning season for Gonostomatids differ among species, with Sigmops elongatus spawning in spring and summer, Gonostoma ebelingi in early fall, Gonostoma atlanticum during all seasons in the subtropical central Pacific, and Gonostoma gracile in fall and winter in the western Pacific. Other mesopelagic fishes such Maurolicus muelleri, Vinciguerria nimbaria and Vinciguerria poweriae spawn mainly in spring and summer.