Daphnia magna
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''Daphnia magna'' is a small
plankton Plankton are the diverse collection of organisms found in Hydrosphere, water (or atmosphere, air) that are unable to propel themselves against a Ocean current, current (or wind). The individual organisms constituting plankton are called plankt ...
ic
crustacean Crustaceans (Crustacea, ) form a large, diverse arthropod taxon which includes such animals as decapods, seed shrimp, branchiopods, fish lice, krill, remipedes, isopods, barnacles, copepods, amphipods and mantis shrimp. The crustacean group ...
(adult length 1.5–5.0 mm) that belongs to the subclass
Phyllopoda Branchiopoda is a class (biology), class of crustaceans. It comprises Anostraca, fairy shrimp, clam shrimp, Diplostraca (or Cladocera), Notostraca and the Devonian ''Lepidocaris''. They are mostly small, freshwater animals that feed on plankton ...
.


Description

''D. magna'' is a typical water flea of the genus ''
Daphnia ''Daphnia'' is a genus of small planktonic crustaceans, in length. ''Daphnia'' are members of the order Anomopoda, and are one of the several small aquatic crustaceans commonly called water fleas because their saltatory swimming style resembl ...
''. The females reach up to 5 mm in size, the males about 2 mm, thus they are among the largest species in the genus. The body is protected by a translucent
carapace A carapace is a Dorsum (biology), dorsal (upper) section of the exoskeleton or shell in a number of animal groups, including arthropods, such as crustaceans and arachnids, as well as vertebrates, such as turtles and tortoises. In turtles and tor ...
made of
chitin Chitin ( C8 H13 O5 N)n ( ) is a long-chain polymer of ''N''-acetylglucosamine, an amide derivative of glucose. Chitin is probably the second most abundant polysaccharide in nature (behind only cellulose); an estimated 1 billion tons of chit ...
, a transparent polysaccharide. It has a ventral opening and five pairs of thoracic limbs, used to help the filtering process. Spike rows run along the back of the carapace. The intestine is hook-shaped and has two digestive ceca. The head has two antennae and a large compound eye. Adult females can be distinguished from those of otherwise similar species such as '' D. pulex'' by the absence of a comb on the abdominal claw and the presence of two distinct combs on the
abdomen The abdomen (colloquially called the belly, tummy, midriff, tucky or stomach) is the part of the body between the thorax (chest) and pelvis, in humans and in other vertebrates. The abdomen is the front part of the abdominal segment of the torso. ...
. The males are smaller than the females and have larger first antennas, a diagnostic feature that distinguishes them from small females.


Ecology

''D. magna'' is a key species in many lentic habitats. It can be found in lakes and shallow ponds rich in organic matter sediment. Numerous natural predators are known and can lead to plastic phenotypic responses. In the presence of
kairomone A kairomone (a coinage using the Greek καιρός ''opportune moment'', paralleling pheromone"kairomone, n.". OED Online. September 2012. Oxford University Press. http://www.oed.com/view/Entry/241005?redirectedFrom=kairomone (accessed 3 Octobe ...
s, ''Daphnia'' spp. develop conspicuous protective structures as an elongated spine and a large body size. In response to temperature differences, ''D. magna'' has shown both phenotypic plasticity and the ability to genetically evolve to deal with the heat stress of warmer, urban pond waters. Populations of Daphnia have also developed increased resistance to local pesticides. Another factor that affects both phenotypic and behavioural plasticity of ''D. magna'' is the salinity of its surroundings. ''D. magna'' is an important primary consumer and prey of many planktivorous fishes. Other invertebrate predators are the larvae of the phantom midge ''Chaoborus'' and hemipterans (''
Notonecta ''Notonecta'', or the common backswimmer, is a genus of backswimmer insects in the family Notonectidae. Species in this genus include: *'' Notonecta borealis'' *'' Notonecta glauca'' *'' Notonecta hoffmanni'' *'' Notonecta indica'' *'' Notonec ...
'') and ''
Triops ''Triops'' is a genus of small crustaceans in the order Notostraca (tadpole shrimp). The long-lasting resting eggs of several species of ''Triops'' are commonly sold in kits as a pet. The animals hatch upon contact with fresh water. Most adult-st ...
.'' The large size of the adults protects them from
predation Predation is a biological interaction where one organism, the predator, kills and eats another organism, its prey. It is one of a family of common feeding behaviours that includes parasitism and micropredation (which usually do not kill the ...
from some planktivorous
invertebrate Invertebrates are a paraphyletic group of animals that neither possess nor develop a vertebral column (commonly known as a ''backbone'' or ''spine''), derived from the notochord. This is a grouping including all animals apart from the chordate ...
s.


Distribution and habitat

''D. magna'' is widespread in the Northern Hemisphere and in particular in the
holarctic The Holarctic realm is a biogeographic realm that comprises the majority of habitats found throughout the continents in the Northern Hemisphere. It corresponds to the floristic Boreal Kingdom. It includes both the Nearctic zoogeographical reg ...
. It can be found in
fresh Fresh or FRESH may refer to: People *DJ Fresh (born 1977), UK-based drum and bass artist *DJ Fresh (producer), US-based R&B producer born Marqus Brown Arts, entertainment, and media Films * ''Fresh'' (1994 film), a crime film * ''Fresh'' (200 ...
and
brackish Brackish water, sometimes termed brack water, is water occurring in a natural environment that has more salinity than freshwater, but not as much as seawater. It may result from mixing seawater (salt water) and fresh water together, as in estu ...
water bodies of different sizes, from lakes to ponds and
ephemeral Ephemerality (from the Greek word , meaning 'lasting only one day') is the concept of things being transitory, existing only briefly. Academically, the term ephemeral constitutionally describes a diverse assortment of things and experiences, fr ...
rock pool A tide pool or rock pool is a shallow pool of seawater that forms on the rocky intertidal shore. Many of these pools exist as separate bodies of water only at low tide. Many tide pool habitats are home to especially adaptable animals that ...
s near the sea. ''D. magna'' tolerates higher levels of salinity (up to one-fifth the salinity of sea water) than most other species of the genus. ''D. magna'' is mainly found in the pelagic zone of water bodies, as it feeds primarily on suspended particles in the
water column A water column is a conceptual column of water from the surface of a sea, river or lake to the bottom sediment.Munson, B.H., Axler, R., Hagley C., Host G., Merrick G., Richards C. (2004).Glossary. ''Water on the Web''. University of Minnesota-D ...
(mainly algae, but also bacteria and
detritus In biology, detritus () is dead particulate organic material, as distinguished from dissolved organic material. Detritus typically includes the bodies or fragments of bodies of dead organisms, and fecal material. Detritus typically hosts commun ...
). Nevertheless, compared to other species of ''Daphnia'', it is more often found in association with the substrate where it is able to exploit benthic food sources as
periphyton Periphyton is a complex mixture of algae, cyanobacteria, heterotrophic microbes, and detritus that is attached to submerged surfaces in most aquatic ecosystems. The related term Aufwuchs (German "surface growth" or "overgrowth") refers to the col ...
and
sediment Sediment is a naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of wind, water, or ice or by the force of gravity acting on the particles. For example, sand an ...
.


Nutrition

The main feeding strategy of ''D. magna'' is the filtering of suspended particles. A specialized filtering apparatus, formed by the thoracic appendices, generates a water current within the thoracic opening of the carapace, which permits the collection and the ingestion of unicellular algae, bacteria, and
detritus In biology, detritus () is dead particulate organic material, as distinguished from dissolved organic material. Detritus typically includes the bodies or fragments of bodies of dead organisms, and fecal material. Detritus typically hosts commun ...
. ''D. magna'' can also feed on periphyton and detritus, an ability that can offer a competitive advantage to this species over strictly pelagic filter feeders in some environments where suspended food sources might be temporally limited.


Reproduction

As most of the other species of the genus ''Daphnia'', ''D. magna'' reproduces by cyclical parthenogenesis. This form of reproduction is characterised by the alternating production of asexual offspring (clonal reproduction) and at certain time sexual reproduction through haploid eggs that need to be fertilised. The asexual eggs (up to few dozens per clutch) are diploid and usually develop into females, or in response to adverse environmental stimuli, into males. Asexual eggs hatch in the female brood pouch 1 day after being laid and are released after 3 days. Juveniles go through four to about six moults before becoming mature over a period of 5–10 days. An adult female produces one clutch with up to 100 eggs every 3–4 days until her death. It can live over 3 months in the laboratory at 20 °C. In response to unfavourable environmental conditions (which could lead to the freezing or the drying up of the pond), the same female can produce haploid resting eggs (usually two at a time), which when fertilised by males, are wrapped within a protective shell called an ephippium. These resting eggs enter a phase of diapause and are able to resist long periods of adverse environmental conditions over a long period of time. Hatching is triggered in response to specific stimuli such as increasing photoperiod and temperatures. The hatchlings from resting eggs develop exclusively into females. Some clones of ''D. magna'' that do not produce males reproduce by automictic parthenogenesis, in which two haploid cells produced by meiosis fuse to produce a female zygote without fertilisation. This tends to make the resulting daughters homozygous, which may be deleterious.


Behavior

The name "water fleas" might come from the typical swimming behavior of ''Daphnia'' species which is reminiscent of a series of jumps. The movement of the big second antennae generates an upward movement of the whole animal followed by its sinking (hop and sink). Although less common than for other lake dwelling species, vertical and horizontal migration patterns of this species have been observed. Diel vertical migration (DVM) consists in the daily movement of animals from the upper water layers, where they spend the night, to the deep and dark layers, where they spend the day. This behavior reduces exposure of diurnal visual predators (such as many fish) by finding refuge in the dark near the bottom and then feeding undisturbed during the night in the food-rich upper water layers. The basis of this behaviour is phototactic behavior (movements of entire organisms to, or away from, a light source). In ''D. magna''
phototactic Phototaxis is a kind of taxis, or locomotory movement, that occurs when a whole organism moves towards or away from a stimulus of light. This is advantageous for phototrophic organisms as they can orient themselves most efficiently to receive l ...
behavior has an innate component (genetic) and an inducible component (for example in the presence of fish kairomones). In Diel horizontal migration, ''D. magna'' finds refuge within submerged plant-beds near the shore during daytime and migrates to open waters during the night. Cases of reversed migration patterns are documented when the risk of visual predation during the day is higher at the bottom or in the littoral zones (for example in the presence of fish that hunt within submerged plant‑beds). ''D. magna'', as the smaller ''D. pulex'', is able to switch to a feeding behavior, termed browsing behavior, when suspended food is scarce. This feeding strategy consists in the stirring up of sediment particles from the bottom with the use of the second antennae and by the subsequent filtration of the suspended particles.


Parasitism

''D. magna'' has become a model system to study the evolution and ecology of host-parasite interaction. Animals collected from natural habitats are frequently infected. Many parasites that infect ''D. magna'' have been identified and studied (Table 1), ''D. magna'' shows parasite-induced behavioural characteristics that can differ among genotypes.


Microbiota

''D. magna'' can be looked at as a complex
ecosystem An ecosystem (or ecological system) consists of all the organisms and the physical environment with which they interact. These biotic and abiotic components are linked together through nutrient cycles and energy flows. Energy enters the syste ...
, colonized by a community of
commensal Commensalism is a long-term biological interaction (symbiosis) in which members of one species gain benefits while those of the other species neither benefit nor are harmed. This is in contrast with mutualism, in which both organisms benefit fro ...
,
symbiotic Symbiosis (from Greek , , "living together", from , , "together", and , bíōsis, "living") is any type of a close and long-term biological interaction between two different biological organisms, be it mutualistic, commensalistic, or parasit ...
and
pathogen In biology, a pathogen ( el, πάθος, "suffering", "passion" and , "producer of") in the oldest and broadest sense, is any organism or agent that can produce disease. A pathogen may also be referred to as an infectious agent, or simply a germ ...
ic
microorganism A microorganism, or microbe,, ''mikros'', "small") and ''organism'' from the el, ὀργανισμός, ''organismós'', "organism"). It is usually written as a single word but is sometimes hyphenated (''micro-organism''), especially in olde ...
s called microbiota. The close proximity of the microbiota to its
host A host is a person responsible for guests at an event or for providing hospitality during it. Host may also refer to: Places * Host, Pennsylvania, a village in Berks County People *Jim Host (born 1937), American businessman * Michel Host ...
allows for a tight interaction, capable of influencing
development Development or developing may refer to: Arts *Development hell, when a project is stuck in development *Filmmaking, development phase, including finance and budgeting *Development (music), the process thematic material is reshaped * Photograph ...
, disease resistance and nutrition. The absence of microbiota in ''D. magna'' has been shown to cause a slower growth, a decrease in fecundity and a higher mortality compared to ''D. magna'' with microbiota. The gut microbiota changes upon death and its complexity is reduced and stabilized in case of starvation.


Model organism

''D. magna'' presents numerous advantages when used as experimental organism. Its transparency allows for the observations of its inner anatomical structures at the
microscope A microscope () is a laboratory instrument used to examine objects that are too small to be seen by the naked eye. Microscopy is the science of investigating small objects and structures using a microscope. Microscopic means being invisibl ...
, while its reproduction through cyclical
parthenogenesis Parthenogenesis (; from the Greek grc, παρθένος, translit=parthénos, lit=virgin, label=none + grc, γένεσις, translit=génesis, lit=creation, label=none) is a natural form of asexual reproduction in which growth and development ...
allows to generate clones (asexual reproduction) or to perform crossing between strains (sexual reproduction). In the research field, ''D. magna'' is considered easy to keep in the laboratory. Some of its advantages for experiments are a fast
generation time In population biology and demography, generation time is the average time between two consecutive generations in the lineages of a population. In human populations, generation time typically ranges from 22 to 33 years. Historians sometimes use this ...
, limited storage usage, easy and cheap feeding and simple maintenance. ''D. magna'' is used in different field of research, such as
ecotoxicology Ecotoxicology is the study of the effects of toxic chemicals on biological organisms, especially at the population, community, ecosystem, and biosphere levels. Ecotoxicology is a multidisciplinary field, which integrates toxicology and ecology ...
,
population genetics Population genetics is a subfield of genetics that deals with genetic differences within and between populations, and is a part of evolutionary biology. Studies in this branch of biology examine such phenomena as adaptation, speciation, and pop ...
, the evolution of sex,
phenotypic plasticity Phenotypic plasticity refers to some of the changes in an organism's behavior, morphology and physiology in response to a unique environment. Fundamental to the way in which organisms cope with environmental variation, phenotypic plasticity encompa ...
,
ecophysiology Ecophysiology (from Greek , ''oikos'', "house(hold)"; , ''physis'', "nature, origin"; and , '' -logia''), environmental physiology or physiological ecology is a biological discipline that studies the response of an organism's physiology to enviro ...
(including global change biology) and host-parasite interactions. Historically, ''D. magna'' allowed researchers to test some interesting theories and conduct pioneering studies: *
Macrophage Macrophages (abbreviated as M φ, MΦ or MP) ( el, large eaters, from Greek ''μακρός'' (') = large, ''φαγεῖν'' (') = to eat) are a type of white blood cell of the immune system that engulfs and digests pathogens, such as cancer cel ...
s attack invading parasites as a part of cellular immunity ( Metchnikoff
Nobel prize The Nobel Prizes ( ; sv, Nobelpriset ; no, Nobelprisen ) are five separate prizes that, according to Alfred Nobel's will of 1895, are awarded to "those who, during the preceding year, have conferred the greatest benefit to humankind." Alfr ...
in 1908) * Ecotoxicological studies (
Einar Naumann Einar Christian Leonard Naumann (13 August 1891 – 22 September 1934) was a Swedish botanist and limnologist who was professor of limnology at the University of Lund. Naumann worked during the summers at the Fishery Station in Aneboda (Smålan ...
) Other recent experiments used the resting eggs of ''Daphnia'' present in a pond
sediment Sediment is a naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of wind, water, or ice or by the force of gravity acting on the particles. For example, sand an ...
to reconstruct the evolutionary history of that population in relation to one of its
parasites Parasitism is a Symbiosis, close relationship between species, where one organism, the parasite, lives on or inside another organism, the Host (biology), host, causing it some harm, and is Adaptation, adapted structurally to this way of lif ...
( ''P. ramosa''), a nice example of resurrection biology. In ecotoxicology ''D. magna'' is specified to be used in the
OECD Guidelines for the Testing of Chemicals OECD Guidelines for the Testing of Chemicals (OECD TG) are a set of internationally accepted specifications for the testing of chemicals decided on by the Organisation for Economic Co-operation and Development (OECD). They were first published in ...
, Tests No. 202 "''Daphnia'' sp., Acute Immobilisation Test", and Test No. 211 "''Daphnia magna'' Reproduction Test". Test No. 202 is a 48-hour
acute toxicity Acute toxicity describes the adverse effects of a substance that result either from a single exposure or from multiple exposures in a short period of time (usually less than 24 hours). To be described as ''acute'' toxicity, the adverse effect ...
study, where young ''Daphnia'' are exposed to varying concentrations of the substance under test and the EC50 determined. Other ''Daphnia'' species than ''D. magna'' may occasionally be used, but labs mostly use ''D. magna'' as
standard Standard may refer to: Symbols * Colours, standards and guidons, kinds of military signs * Standard (emblem), a type of a large symbol or emblem used for identification Norms, conventions or requirements * Standard (metrology), an object th ...
. Test No. 211 is a 21-day
chronic toxicity Chronic toxicity, the development of adverse effects as a result of long term exposure to a contaminant or other stressor, is an important aspect of aquatic toxicology. Adverse effects associated with chronic toxicity can be directly lethal but are ...
test, at the end of which, the total number of living offspring produced per parent animal alive at the end of the test is assessed, to determine the lowest observed effect concentration of the test substance.


References


External links

* * {{Taxonbar, from=Q672531 Cladocera Crustaceans described in 1820 Freshwater crustaceans of Europe Animal models