A well-defined marine order of the class Crustacea. These planktonic malacostracans are closely allied with, and sometimes included in, the Decapoda. The order contains two families, one with a single species, the other with about 84 species divided among 10 genera. Euphausiids are predominantly pelagic organisms of the open ocean, but a few species are neritic. All are shrimplike in appearance (see illustration).
Euphausiids form a significant proportion of the total planktonic biomass. This is especially true in higher latitudes and most pronounced in the Antarctic Ocean, where Euphausia superba, which attains a body length of some 2.4 in. (60 mm), occurs in vast numbers.
Distribution and ecology
All species are strictly marine and do not occur in freshwater or brackish environments. Species are found in all oceans and seas, except the brackish Baltic and Black seas.
Known as krill to whalers, they constitute the diet of many whales, particularly the baleen whales. The main whale feeding grounds coincide with the areas of greatest concentration of euphausiids: areas of convergence, backwaters, vortices of mixed layers, centers of gyres, and fronts. They contribute to the diet of many other animals such as seals, herring, sardines, many birds, and even humans. Their concentration in areas of high productivity is correlated with their own diet, which consists mainly of phytoplankton, small crustaceans, and detrital matter. Their feeding appendages, which consist of not only the mouthparts but also the anterior pairs of thoracic legs, are elaborate and can be used in several ways; this enables the animal to filter food particles from the water, collect organic material from the surface of the sediment, or catch live prey.
Most euphausiids live at considerable depth during daylight hours, and many undertake extensive diurnal vertical migrations to the surface layers at night. Their vertical position in the water column is thought to be photoregulated; during the day most species live at depth under conditions of blue-green light of low intensity. A few species live in the bathypelagic environment, having been captured at depths greater than 6000 ft (2000 m); these species have small eyes and attain considerable size, the maximum recorded body length of one species being 6 in. (150 mm).
Euphausiids, with the exception of Bentheuphausia amblyops, possess photophores that emit a brilliant blue-green light.
The eyes are compound, bilobed in some genera, and contain three pigments: a carotenoid (astaxanthin), a melanoid, and a photolabile substance that is probably a visual pigment. The spectral sensitivity of the eye is greatest to blue-green light. See also: Photoreception
Respiration is by means of foliose, digitiform gills located at the bases of the second to eighth thoracic appendages.
The blood is a pale, leukocyte-bearing fluid with hemocyanin as the respiratory pigment. The heart is compact and has two pairs of ostia. See also: Respiratory pigments (invertebrate)
The male copulatory organs are extremely complex and form the main criterion for specific identification. The phenomenon of swarming is common in many genera and is usually associated with reproduction. The male transfers a spermatophore to the spermatheca of the female at copulation. The early embryos usually live freely in the sea, but those of some 25 species are carried attached to the ventral regions of the thorax.
The larval stages are numerous. A nauplius hatches from the egg membranes and develops to a metanauplius. Three calyptopis stages follow during which the abdomen and stalked eyes develop. All species pass through these stages. The following developmental sequences, some 6–12 molts as furciliae, vary between species and within species in different localities and at different times. Sexual maturity is normally achieved at an age of 1 year, although some species such as E. superba and those living in the meso- and bathypelagic environments require 2 or more years. One species, Thysanoessa longicaudata, produces two generations each year in warmer parts of the North Atlantic.
The possibilities of exploiting the Antarctic E. superba and other species in other regions have been investigated in some detail. Estimates of the possible yield of a commercial fishery for E. superba range 27–180 × 106 tons (30–200 × 106 metric tons) per year. This species forms large surface and subsurface aggregations that can be located visually or by echosounder. Ring and purse nets operated from small catchers or stern trawls operated from larger ships can be used to capture the swarms.
The euphausiids can be processed to yield large quantities of protein. Krill pastes and meal can be manufactured to feed domestic animals or used in therapeutic diets. Krill sausages, krill-stuffed eggs, and shrimp butter are already marketable products in Japan, Russia, and Germany.
In addition to Euphausia superba, some 30 species of euphausiids form surface or subsurface swarms or aggregations and are also therefore potentially exploitable resources. Euphausia pacifica is marketed for human consumption and also used, as is Meganyctiphanes norvegica in the North Atlantic, as food for fish in mariculture systems. See also: Crustacea; Decapoda