Luminous shrimp
| Luminous shrimp | ||||||||||||
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Meganyctiphanes norvegica |
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| Systematics | ||||||||||||
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| Scientific name | ||||||||||||
| Euphausiacea | ||||||||||||
| Dana , 1852 |
The luminous shrimp or krill , scientific name Euphausiacea , are an order of crabs with worldwide distribution. All species are marine (they live in the sea) and freely swimming ( pelagic ), less often above the bottom in water close to the ground (hyperbenthic). With 86 species, luminous shrimp are relatively poor in species, but some of them occur in enormous numbers of individuals. The largest species is Thysanopoda spinicaudata with a body length of up to 150 millimeters.
features
Luminous shrimp are crabs with a shrimp-shaped habit, the species are relatively similar and difficult to distinguish from one another. The body is usually transparent with translucent internal organs, and vivid red pigments of varying sizes and intensity are usually also present. Outwardly it is divided into a cephalothorax , which is covered on the dorsal side (on the back) by a continuous carapace and a rear body section, the pleon , which consists of six free segments. In front of the cephalothorax, two stalked complex eyes are visible, which can be either round or bilobed.
Both the thoracopods on the cephalothorax and the pleopods on the pleon are typical, two-branched split legs . The eight pairs of thoracopods have tufted gills on the outside of the first limb, the coxae; these are exposed and not covered by the carapace. On the second limb, the base, sit the two-limbed exopodites and the five-limbed endopodites. The outer limb of the exopodite carries a fan made of floating bristles and is used for locomotion. The walking leg-like appearance Endopodite be beaten mostly under the hull section, seated on them rows of Fiederborsten form a filter basket, with the euphausiid their food from the water filter ; the water flow required for this is generated by the secondary flagella (scaphocerite) of the second antenna pair. In some species the endopodites of the second and third thoracopods are elongated and have long thorns, so they do not take part in the filtering process. In the largest family, Euphausiidae, the endopodite of the eighth pair of legs has regressed, and in many species the seventh partially or completely. The first two of the five pairs of pleopods are transformed into mating organs in the males of the Euphausiidae. The sixth pair, the split in two branches (the Rami) uropods are slightly leaf-shaped, and approximately as long as the taper Telson which carries laterally two extensions; together they form a tail fan.
Luminosity
The luminosity that gives the shrimp its name goes back to special luminous organs , the photophores. Photophores sit on the eye stalks, the coxae of the second to seventh thoracopods and on the abdominal plates ( sternites ) of the first four pleon segments, between the pleopods. A luciferin - luciferase system is used to generate light . The structure of luciferin corresponds to that of the dinoflagellates and is presumably ingested with them through food. The animals emit light pulses that begin intensely and slowly go out over the course of a few minutes. The generation of light is under neuronal and hormonal control; it is triggered, for example, by light pulses from other animals. The glow is mainly used for intra-species communication, for the recognition and orientation of mating partners and for keeping the swarms together.
Development and life cycle
Luminescent shrimp are sexually separated. The male fertilizes the female directly by transferring a spermatophore . Appendices of the first pleopods, known as petasmata, grip like a key in a lock in a pocket-shaped structure; Called thelycum, on the underside of the thorax; the shape of the Petasmata is an important feature of species identification. Depending on the species, the eggs are either released individually into the open water or, in some species, collected in ice bags that the female carries around with her attached to thoracopods. The number of eggs depends on their size and the size of the female and ranges up to around 6,000 eggs. Deep-sea dwellers usually only shed very few, but unusually large, eggs, in the case of the species Thysanopoda minyops, for example, never more than 60.
The larval development of the light shrimp is very complex and the most complicated in the higher crustaceans . The first larval stage, a nauplius , which is only developed in species without ice sacs, is followed by a metanauplius. The following three moults are called the Calytopis stage, in this stage the pleon is elongated, the pleopods appear, and in this stage food intake begins. The following larval stages, called furcilia, are characterized by the separation of the eyes from the carapace. This is followed by one to three juvenile post-larval stages with a morphology similar to that of the adults, until finally sexual maturity is achieved. The duration of the larval period depends on the temperature; it lasts the longest in the Eiskrill Euphausia crystallorphias with 235 days.
The generation time of many species, especially the tropical deep-water inhabitants, is unknown. Species in the temperate latitudes usually develop for two to three months and probably three generations a year, for example Meganyctiphanes norvegica in the Mediterranean. The Arctic and Antarctic species such as Antarctic krill take two years to complete a generation. Usually the animals die immediately after reproduction and are then no more than 6 to 10 months old, but sometimes they can hibernate and live two or rarely even three years. In the case of the slowly growing Antarctic Euphausia superba , even 4 years have been proven.
Light shrimp species usually group together to form large schools. Organized swarms, in which individuals coordinate their orientation and speed, are also known as "schools".
nutrition
Luminous shrimp are filter feeders, the species in cold water predominantly phytoplankton , but they also ingest dead particulate organic matter ( detritus ). Many tropical species also filter zooplankton , so they are zoophagous. The most important prey organisms are copepods . Some deep-sea species are predators, and some filter feeders, especially species with differently shaped, thorn-bearing thoracopods, specialize in certain prey organisms. For the Arctic and Antarctic species, especially their larval stages, the phytoplankton on the porous ice underside are an important part of the food base.
Vertical spread, migrations
Most species of the Euphausiacea live in near-surface water layers. They usually perform vertical hikes every day, often of several hundred meters, occurring in deeper water during the day and in shallow water layers at night. The biological meaning of migrations is either avoidance of energy losses in colder deep water or avoidance of predators. About 15 percent of the species live permanently in mean sea depths of up to 2,000, but never more than 100 meters. Few species are deep-sea specialists and are never found at depths above 1,000 meters, they reach at least 5,000 meters. Deep sea species do not perform vertical migrations.
Ecological importance
In the cold open water of the Arctic seas and the large upwelling areas , four to five species of luminous shrimp dominate, especially here called krill, and are the dominant species of these communities in terms of biomass and energy consumption . Through the vertical migration, dung balls and moulting residues, they transport large amounts of the algal biomass into deeper water layers and thus play a role in the global carbon cycle. Due to their high biomass, they are an essential part of marine food chains . They are the main food of numerous fish species such as Atlantic herring , capelin and blue whiting . Even predatory fish are highly dependent on them; in intestinal analyzes, albacore tuna in the Bay of Biscay contained 10 to 40 percent Euphausiacea as stomach content. Baleen whales like the blue whale are completely dependent on them for their nutrition. In the Antarctic Ocean in particular, Euphausia superba, with an average annual biomass of 379 million tons, forms either directly or indirectly the basis of the entire food web and is essential for the survival of almost all vertebrates living here. Fish, whales, seals, birds and octopus together consume an estimated 250 million tons of krill per year in the Antarctic Sea.
Essentially, only Euphausia superba has economic importance for direct fishing , compare under Antarctic krill .
Phylogeny and Taxonomy
According to morphological arguments, the most likely sister group of the luminous shrimp are the decapods (Decapoda). After comparing the gene sequence of the 28S rDNA , however, the hover shrimp (Mysida) could also be sister taxons.
Two families are distinguished within the order:
- Bentheuphausiidae Colosi, 1917. monotypic, only species Bentheuphausia amblyops . Eyes reduced, without photophores.
- Euphausiidae Dana, 1852 with the following genera:
swell
- V. Spridonov, B. Casanova: Order Euphausiacea. In: Frederick Schram, Carel von Vaupel Klein, M. Charmantier-Daures, J. Forest (Eds.): Treatise on Zoology - Anatomy, Taxonomy, Biology. The Crustacea. Volume 9 Part A: Eucarida: Euphausiacea, Amphionidacea, and Decapoda (partim). Part 1, Brill Scientific Publishers 2010, ISBN 978-90-04-16441-3 .
Individual evidence
- ^ LF Greer, AA Szalay: Imaging of light emission from the expression of luciferases in living cells and organisms: a review. In: Luminescence. 17 (1), 2002, pp. 43-74. doi: 10.1002 / bio.676
- ↑ LB Quetin, RM Ross: Feeding by Antarctic krill, Euphausia superba: Does size matter? In: WR Siegfried, PR Condy, RM Laws (Ed.): Antarctic Nutrient Cycles and Food Webs. Springer-Verlag, Berlin 1985, pp. 372-377.
- ↑ Geraint A. Tarling, Magnus L. Johnson: Satiation gives krill that sinking feeling. In: Current Biology. Volume 16, No. 3, 2006, pp. R83-R84. doi: 10.1016 / j.cub.2006.01.044
- ^ A. Atkinson, V. Siegel, EA Pakhomov, MJ Jessopp, V. Loeb: A re-appraisal of the total biomass and annual production of Antarctic krill. In: Deep Sea Research Part I: Oceanographic Research Papers. Volume 56, No. 5, 2009, pp. 727-740. doi: 10.1016 / j.dsr.2008.12.007
- ^ DGM Miller, I. Hampton: Biology and ecology of the Antarctic krill. In: Biomass Scientific Series. No. 9, 1989, pp. 1-166.
- ↑ Simon N. Jarman, Stephen Nicol, Nicholas G. Elliott, Andrew McMinn: 28S rDNA Evolution in the Eumalacostraca and the Phylogenetic Position of Krill. In: Molecular Phylogenetics and Evolution. Volume 17, No. 1, 2000, pp. 26-36. doi: 10.1006 / mpev.2000.0823
