Daphnia

Daphnia

Overview
Daphnia are small, plankton
Plankton
Plankton are any drifting organisms that inhabit the pelagic zone of oceans, seas, or bodies of fresh water. That is, plankton are defined by their ecological niche rather than phylogenetic or taxonomic classification...

ic crustacean
Crustacean
Crustaceans form a very large group of arthropods, usually treated as a subphylum, which includes such familiar animals as crabs, lobsters, crayfish, shrimp, krill and barnacles. The 50,000 described species range in size from Stygotantulus stocki at , to the Japanese spider crab with a leg span...

s, between 0.2 and 5 mm in length. Daphnia are members of the order
Order (biology)
In scientific classification used in biology, the order is# a taxonomic rank used in the classification of organisms. Other well-known ranks are life, domain, kingdom, phylum, class, family, genus, and species, with order fitting in between class and family...

 Cladocera
Cladocera
Cladocera is an order of small crustaceans commonly called water fleas. Around 620 species have been recognised so far, with many more undescribed. They are ubiquitous in inland aquatic habitats, but rare in the oceans. Most are long, with a down-turned head, and a carapace covering the apparently...

, and are one of the several small aquatic crustaceans commonly called water fleas because of their saltatory swimming style (although flea
Flea
Flea is the common name for insects of the order Siphonaptera which are wingless insects with mouthparts adapted for piercing skin and sucking blood...

s are insect
Insect
Insects are a class of living creatures within the arthropods that have a chitinous exoskeleton, a three-part body , three pairs of jointed legs, compound eyes, and two antennae...

s and thus only very distantly related). They live in various aquatic environments ranging from acid
Acid
An acid is a substance which reacts with a base. Commonly, acids can be identified as tasting sour, reacting with metals such as calcium, and bases like sodium carbonate. Aqueous acids have a pH of less than 7, where an acid of lower pH is typically stronger, and turn blue litmus paper red...

ic swamp
Swamp
A swamp is a wetland with some flooding of large areas of land by shallow bodies of water. A swamp generally has a large number of hammocks, or dry-land protrusions, covered by aquatic vegetation, or vegetation that tolerates periodical inundation. The two main types of swamp are "true" or swamp...

s to freshwater
Fresh Water
Fresh Water is the debut album by Australian rock and blues singer Alison McCallum, released in 1972. Rare for an Australian artist at the time, it came in a gatefold sleeve...

 lakes, ponds, streams and rivers.





The division of the body into segments
Segmentation (biology)
Segmentation in biology refers to either a type of gastrointestinal motility or the division of some animal and plant body plans into a series of repetitive segments. This article will focus on the segmentation of animal body plans, specifically using the examples of the phyla Arthropoda,...

 is not visible.
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Encyclopedia
Daphnia are small, plankton
Plankton
Plankton are any drifting organisms that inhabit the pelagic zone of oceans, seas, or bodies of fresh water. That is, plankton are defined by their ecological niche rather than phylogenetic or taxonomic classification...

ic crustacean
Crustacean
Crustaceans form a very large group of arthropods, usually treated as a subphylum, which includes such familiar animals as crabs, lobsters, crayfish, shrimp, krill and barnacles. The 50,000 described species range in size from Stygotantulus stocki at , to the Japanese spider crab with a leg span...

s, between 0.2 and 5 mm in length. Daphnia are members of the order
Order (biology)
In scientific classification used in biology, the order is# a taxonomic rank used in the classification of organisms. Other well-known ranks are life, domain, kingdom, phylum, class, family, genus, and species, with order fitting in between class and family...

 Cladocera
Cladocera
Cladocera is an order of small crustaceans commonly called water fleas. Around 620 species have been recognised so far, with many more undescribed. They are ubiquitous in inland aquatic habitats, but rare in the oceans. Most are long, with a down-turned head, and a carapace covering the apparently...

, and are one of the several small aquatic crustaceans commonly called water fleas because of their saltatory swimming style (although flea
Flea
Flea is the common name for insects of the order Siphonaptera which are wingless insects with mouthparts adapted for piercing skin and sucking blood...

s are insect
Insect
Insects are a class of living creatures within the arthropods that have a chitinous exoskeleton, a three-part body , three pairs of jointed legs, compound eyes, and two antennae...

s and thus only very distantly related). They live in various aquatic environments ranging from acid
Acid
An acid is a substance which reacts with a base. Commonly, acids can be identified as tasting sour, reacting with metals such as calcium, and bases like sodium carbonate. Aqueous acids have a pH of less than 7, where an acid of lower pH is typically stronger, and turn blue litmus paper red...

ic swamp
Swamp
A swamp is a wetland with some flooding of large areas of land by shallow bodies of water. A swamp generally has a large number of hammocks, or dry-land protrusions, covered by aquatic vegetation, or vegetation that tolerates periodical inundation. The two main types of swamp are "true" or swamp...

s to freshwater
Fresh Water
Fresh Water is the debut album by Australian rock and blues singer Alison McCallum, released in 1972. Rare for an Australian artist at the time, it came in a gatefold sleeve...

 lakes, ponds, streams and rivers.

Appearance and characteristics






The division of the body into segments
Segmentation (biology)
Segmentation in biology refers to either a type of gastrointestinal motility or the division of some animal and plant body plans into a series of repetitive segments. This article will focus on the segmentation of animal body plans, specifically using the examples of the phyla Arthropoda,...

 is not visible. The head is fused, and is generally bent down towards the body with a visible notch separating the two. In most species the rest of the body is covered by a carapace
Carapace
A carapace is a dorsal 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 tortoises, the underside is called the plastron.-Crustaceans:In crustaceans, the...

, with a ventral gap in which the five or six pairs of legs lie. The most prominent features are the compound eyes, the second antennae
Antenna (biology)
Antennae in biology have historically been paired appendages used for sensing in arthropods. More recently, the term has also been applied to cilium structures present in most cell types of eukaryotes....

, and a pair of abdominal seta
Seta
Seta is a biological term derived from the Latin word for "bristle". It refers to a number of different bristle- or hair-like structures on living organisms.-Animal setae:In zoology, most "setae" occur in invertebrates....

e. In many species, the carapace is translucent or nearly so and as a result they make excellent subjects for the microscope
Microscope
A microscope is an instrument used to see objects that are too small for the naked eye. The science of investigating small objects using such an instrument is called microscopy...

 as one can observe the beating heart.

Even under relatively low power microscopy
Microscopy
Microscopy is the technical field of using microscopes to view samples and objects that cannot be seen with the unaided eye...

, the feeding mechanism can be observed, with immature young moving in the brood-pouch; moreover, the eye being moved by the ciliary muscle
Ciliary muscle
The ciliary muscle is a ring of striated smooth muscle in the eye's middle layer that controls accommodation for viewing objects at varying distances and regulates the flow of aqueous humour into Schlemm's canal. It changes the shape of the lens within the eye not the size of the pupil which is...

s can be seen, as well as blood corpuscle
Corpuscle
Corpuscle may refer to:*a small free floating biological cell, especially a blood cell, but not a fat cell*a nerve ending such as Meissner's corpuscle or a Pacinian corpuscle*any member of Corpus Christi College, Cambridge or Corpus Christi College, Oxford...

s being pumped around the circulatory system
Circulatory system
The circulatory system is an organ system that passes nutrients , gases, hormones, blood cells, etc...

 by the simple heart. The heart is at the top of the back, just behind the head, and the average heart rate is approximately 180 bpm under normal conditions. Daphnia, like many animals, are prone to alcohol
Ethanol
Ethanol, also called ethyl alcohol, pure alcohol, grain alcohol, or drinking alcohol, is a volatile, flammable, colorless liquid. It is a psychoactive drug and one of the oldest recreational drugs. Best known as the type of alcohol found in alcoholic beverages, it is also used in thermometers, as a...

 intoxication, and make excellent subjects for studying the effects of the depressant on the nervous system
Nervous system
The nervous system is an organ system containing a network of specialized cells called neurons that coordinate the actions of an animal and transmit signals between different parts of its body. In most animals the nervous system consists of two parts, central and peripheral. The central nervous...

 – due to the translucent exoskeleton, and the visibly altered heart rate
Heart rate
Heart rate is the number of heartbeats per unit of time, typically expressed as beats per minute . Heart rate can vary as the body's need to absorb oxygen and excrete carbon dioxide changes, such as during exercise or sleep....

. They are tolerant of being observed live under a cover slip and appear to suffer no harm when returned to open water. This experiment can also be done using caffeine
Caffeine
Caffeine is a bitter, white crystalline xanthine alkaloid that acts as a stimulant drug. Caffeine is found in varying quantities in the seeds, leaves, and fruit of some plants, where it acts as a natural pesticide that paralyzes and kills certain insects feeding on the plants...

, nicotine
Nicotine
Nicotine is an alkaloid found in the nightshade family of plants that constitutes approximately 0.6–3.0% of the dry weight of tobacco, with biosynthesis taking place in the roots and accumulation occurring in the leaves...

 or adrenaline
Epinephrine
Epinephrine is a hormone and a neurotransmitter. It increases heart rate, constricts blood vessels, dilates air passages and participates in the fight-or-flight response of the sympathetic nervous system. In chemical terms, adrenaline is one of a group of monoamines called the catecholamines...

 and observing an increase in heart rate.

Systematics and evolution


Daphnia is a large genus comprising about 150 species, belonging to the cladocera
Cladocera
Cladocera is an order of small crustaceans commonly called water fleas. Around 620 species have been recognised so far, with many more undescribed. They are ubiquitous in inland aquatic habitats, but rare in the oceans. Most are long, with a down-turned head, and a carapace covering the apparently...

n family Daphniidae
Daphniidae
Daphniidae is a family in the order Cladocera . Species within this family occur in several places within Africa including Lake Chad in northwest Africa and in the Makgadikgadi Pans in Botswana...

. It is subdivided into several subgenera (Daphnia, Hyalodaphnia, Ctenodaphnia), but the division has been controversial and is still in developmental phase. Each subgenus has been further divided into a number of species complexes. The understanding of species boundaries has been hindered by phenotypic plasticity, hybridization, intercontinental introductions and poor taxonomic descriptions.

Reproduction


Daphnia reproduce parthenogenetically
Parthenogenesis
Parthenogenesis is a form of asexual reproduction found in females, where growth and development of embryos occur without fertilization by a male...

 usually in the spring until the end of the summer. One or more juvenile animals are nurtured in the brood pouch inside the carapace. The newly hatched Daphnia must molt several times before they are fully grown into an adult usually after about two weeks. The young are small copies of the adult; there are no true nymph
Nymph (biology)
In biology, a nymph is the immature form of some invertebrates, particularly insects, which undergoes gradual metamorphosis before reaching its adult stage. Unlike a typical larva, a nymph's overall form already resembles that of the adult. In addition, while a nymph moults it never enters a...

al or instar
Instar
An instar is a developmental stage of arthropods, such as insects, between each molt , until sexual maturity is reached. Arthropods must shed the exoskeleton in order to grow or assume a new form. Differences between instars can often be seen in altered body proportions, colors, patterns, or...

 stages. The fully mature females are able to produce a new brood of young about every ten days under ideal conditions. The reproduction process continues while the environmental conditions continue to support their growth. When winter approaches or in drought conditions or at times of other harsh environmental conditions, production of new female generations cease and parthenogenic males are produced. However, even in harsh environmental conditions males may make up considerably less than half the population, in some species they are unknown entirely. Males are much smaller in size than the female and they typically possess a specialized abdominal appendage
Appendage
In invertebrate biology, an appendage is an external body part, or natural prolongation, that protrudes from an organism's body . It is a general term that covers any of the homologous body parts that may extend from a body segment...

 which is used in mating to grasp a female from behind, pries open her carapace
Carapace
A carapace is a dorsal 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 tortoises, the underside is called the plastron.-Crustaceans:In crustaceans, the...

, insert a spermatheca
Spermatheca
The spermatheca , also called receptaculum seminis , is an organ of the female reproductive tract in insects, some molluscs, oligochaeta worms and certain other invertebrates and vertebrates...

, and thus fertilize the eggs.

These fertilized eggs are termed winter eggs and are provided with extra shell layer called ephippium. The extra layer preserves and protects the egg inside from harsh environmental conditions until the more favourable times, such as spring, when the reproductive cycle is able to take place once again.

In addition to the production of eggs capable of overwintering, this switch to sexual reproduction has also been proposed to allow greater offspring variation (through genetic recombination) which may be useful in varied or unpredictable conditions, this idea is often proposed under the name of the lottery model.

Lifespan


The lifespan of a Daphnia does not exceed one year and is largely temperature dependent. For example, individual organisms can live up to 108 days at 3 °C (37.4 °F) while some organisms live for only 29 days at 28 °C (82.4 °F). A clear exception to this trend is during the winter time in which harsh conditions limit the population in which females have been recorded to live for over six months. These females generally grow at a slower rate but in the end are larger than ones under normal conditions.

Ecology


Several Daphnia species are considered threatened. The following are listed as vulnerable by IUCN
World Conservation Union
The International Union for Conservation of Nature and Natural Resources is an international organization dedicated to finding "pragmatic solutions to our most pressing environment and development challenges." The organization publishes the IUCN Red List, compiling information from a network of...

: Daphnia nivalis
Daphnia nivalis
Daphnia nivalis is a species of water flea in the family Daphniidae, closely related to Daphnia carinata. It is endemic to the Snowy Mountains of eastern Australia, where it lives only in water bodies that have existed for less than 20,000 years, including Lake Cootapatamba, Australia's highest lake...

, Daphnia coronata
Daphnia coronata
Daphnia coronata is a species of crustacean in the family Daphniidae. It is endemic to South Africa....

, Daphnia occidentalis
Daphnia occidentalis
Daphnia occidentalis is a species of crustacean in the family Daphniidae. It is endemic to Australia.-Source:* Benzie, J. 1996. . Downloaded on 9 August 2007....

, and Daphnia jollyi
Daphnia jollyi
Daphnia jollyi is a species of crustacean in the genus Daphnia. It is endemic to Western Australia, where it lives in shallow freshwater pools over granite bedrock. Daphnia jollyi is listed as a vulnerable species on the IUCN Red List....

. Some species are halophile
Halophile
Halophiles are extremophile organisms that thrive in environments with very high concentrations of salt. The name comes from the Greek for "salt-loving". While the term is perhaps most often applied to some halophiles classified into the Archaea domain, there are also bacterial halophiles and some...

s, and can be found in hypersaline lake
Hypersaline lake
A hypersaline lake is a landlocked body of water that contains significant concentrations of sodium chloride or other mineral salts, with saline levels surpassing that of ocean water . Specific microbial and crustacean species thrive in these high salinity environments that are otherwise...

 environments, an example of which is the Makgadikgadi Pan
Makgadikgadi Pan
The Makgadikgadi Pan is a large salt pan in the middle of the dry savanna of northeastern Botswana. It is one of the largest salt flats in the world...

.

Daphnia may sometimes ingest tiny crustacean
Crustacean
Crustaceans form a very large group of arthropods, usually treated as a subphylum, which includes such familiar animals as crabs, lobsters, crayfish, shrimp, krill and barnacles. The 50,000 described species range in size from Stygotantulus stocki at , to the Japanese spider crab with a leg span...

s and rotifer
Rotifer
The rotifers make up a phylum of microscopic and near-microscopic pseudocoelomate animals. They were first described by Rev. John Harris in 1696, and other forms were described by Anton van Leeuwenhoek in 1703...

s, but usually they are filter feeder
Filter feeder
Filter feeders are animals that feed by straining suspended matter and food particles from water, typically by passing the water over a specialized filtering structure. Some animals that use this method of feeding are clams, krill, sponges, baleen whales, and many fish and some sharks. Some birds,...

s, ingesting mainly unicellular algae
Algae
Algae are a large and diverse group of simple, typically autotrophic organisms, ranging from unicellular to multicellular forms, such as the giant kelps that grow to 65 meters in length. They are photosynthetic like plants, and "simple" because their tissues are not organized into the many...

 and various sorts of organic detritus including protist
Protist
Protists are a diverse group of eukaryotic microorganisms. Historically, protists were treated as the kingdom Protista, which includes mostly unicellular organisms that do not fit into the other kingdoms, but this group is contested in modern taxonomy...

s and bacteria
Bacteria
Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...

. They can gather detritus not only from water column, but also from the bottom of lakes (especially in winter) or laboratory enclosures. Daphnia can be kept easily on a diet of yeast
Yeast
Yeasts are eukaryotic micro-organisms classified in the kingdom Fungi, with 1,500 species currently described estimated to be only 1% of all fungal species. Most reproduce asexually by mitosis, and many do so by an asymmetric division process called budding...

, but this occurs mostly in laboratories or controlled environments. Beating of the legs produces a constant current through the carapace which brings such material into the digestive tract. The trapped food particles are formed into a food bolus which then moves down the digestive tract until voided through the anus located on the ventral surface of the terminal appendage. The first and second pair of legs are used in the organisms' filter feeding ensuring large un-absorbable particles are kept out while the other sets of legs create the stream of water rushing into the organism.

Swimming is powered mainly by the second set of antennae which are larger in size than the first set. The action of this second set of antennae is responsible for the jumping motion.

Uses


Sometimes Daphnia may be used in certain environments to test the effects of toxin
Toxin
A toxin is a poisonous substance produced within living cells or organisms; man-made substances created by artificial processes are thus excluded...

s on an ecosystem
Ecosystem
An ecosystem is a biological environment consisting of all the organisms living in a particular area, as well as all the nonliving , physical components of the environment with which the organisms interact, such as air, soil, water and sunlight....

. This makes Daphnia an indicator species
Indicator species
An indicator species is any biological species that defines a trait or characteristic of the environment. For example, a species may delineate an ecoregion or indicate an environmental condition such as a disease outbreak, pollution, species competition or climate change...

, particularly useful in that area because of its short lifespan and reproductive capabilities. Because they are nearly transparent, their internal organs are easy to study in live specimens (an example might be to study the effect of temperature on the heart rate of these ectothermic organisms). They are often fed to tadpoles or small species of amphibians such as the African Dwarf Frog
African dwarf frog
African dwarf frogs are small aquatic frogs native to parts of Africa, spreading from tropical to subtropical areas primarily in the Congo region.-Description:...

(Hymenochirus biettgeri). Daphnia are also a popular live food in tropical and marine fish keeping.

External links