Daphnia pulex
Encyclopedia
Daphnia pulex is the most common species
Species
In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...

 of water flea. It has a cosmopolitan distribution
Cosmopolitan distribution
In biogeography, a taxon is said to have a cosmopolitan distribution if its range extends across all or most of the world in appropriate habitats. For instance, the killer whale has a cosmopolitan distribution, extending over most of the world's oceans. Other examples include humans, the lichen...

: the species is found throughout the Americas, Europe
Europe
Europe is, by convention, one of the world's seven continents. Comprising the westernmost peninsula of Eurasia, Europe is generally 'divided' from Asia to its east by the watershed divides of the Ural and Caucasus Mountains, the Ural River, the Caspian and Black Seas, and the waterways connecting...

 and Australia
Australia
Australia , officially the Commonwealth of Australia, is a country in the Southern Hemisphere comprising the mainland of the Australian continent, the island of Tasmania, and numerous smaller islands in the Indian and Pacific Oceans. It is the world's sixth-largest country by total area...

. It is a model species, and was the first crustacean to have its genome
Genome
In modern molecular biology and genetics, the genome is the entirety of an organism's hereditary information. It is encoded either in DNA or, for many types of virus, in RNA. The genome includes both the genes and the non-coding sequences of the DNA/RNA....

 sequenced.

Description

Daphnia pulex is an arthropod
Arthropod
An arthropod is an invertebrate animal having an exoskeleton , a segmented body, and jointed appendages. Arthropods are members of the phylum Arthropoda , and include the insects, arachnids, crustaceans, and others...

 whose body segments are difficult to distinguish. They can only be recognised by the 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...

s they bear (only ever one pair per segment), and by studying the internal anatomy. The head is distinct and is made up of six segments which are fused together even as an embryo
Embryo
An embryo is a multicellular diploid eukaryote in its earliest stage of development, from the time of first cell division until birth, hatching, or germination...

. It bears the mouthparts, and two pairs of 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....

, the second of which are enlarged into powerful organs used for swimming. There is no clear division between thorax
Thorax
The thorax is a division of an animal's body that lies between the head and the abdomen.-In tetrapods:...

 and abdomen
Abdomen
In vertebrates such as mammals the abdomen constitutes the part of the body between the thorax and pelvis. The region enclosed by the abdomen is termed the abdominal cavity...

, which collectively bear five pairs of appendages. The shell surrounding the animal extends posteriorly into a long or short spine
Spine (zoology)
A spine is a hard, thorny or needle-like structure which occurs on various animals. Animals such as porcupines and sea urchins grow spines as a self-defense mechanism. Spines are often formed of keratin...

.

Ecology

Daphnia pulex occurs in a wide range of aquatic habitats, although it is most closely associated with small, shaded pools. In oligotrophic lakes, D. pulex has little pigmentation, while it may become bright red in eutrophic waters, due to the production of haemoglobin.

Predation

Daphnia
Daphnia
Daphnia are small, planktonic crustaceans, between 0.2 and 5 mm in length. Daphnia are members of the order Cladocera, and are one of the several small aquatic crustaceans commonly called water fleas because of their saltatory swimming style...

are prey for a variety of both vertebrate and invertebrate predators
Predation
In ecology, predation describes a biological interaction where a predator feeds on its prey . Predators may or may not kill their prey prior to feeding on them, but the act of predation always results in the death of its prey and the eventual absorption of the prey's tissue through consumption...

. The role of predation on D. pulex population ecology extensively studied, and has been shown to be a major axis of variation in shaping population dynamics and landscape-level distribution. In addition to the direct population ecological
Population ecology
Population ecology is a sub-field of ecology that deals with the dynamics of species populations and how these populations interact with the environment. It is the study of how the population sizes of species living together in groups change over time and space....

 effects of predation, the process contributes to phenotypic evolution in contrasting ways: larger D. pulex are more visible to vertebrate predators, but invertebrate predators are unable to handle larger D. pulex. As a result, larger D. pulex tend to be found with invertebrate predators while smaller size is associated with vertebrate predators.

Similar to some other Daphnia
Daphnia
Daphnia are small, planktonic crustaceans, between 0.2 and 5 mm in length. Daphnia are members of the order Cladocera, and are one of the several small aquatic crustaceans commonly called water fleas because of their saltatory swimming style...

species, the morphology
Morphology (biology)
In biology, morphology is a branch of bioscience dealing with the study of the form and structure of organisms and their specific structural features....

 of D. pulex exhibits a plastic response
Phenotypic plasticity
Phenotypic plasticity is the ability of an organism to change its phenotype in response to changes in the environment. Such plasticity in some cases expresses as several highly morphologically distinct results; in other cases, a continuous norm of reaction describes the functional interrelationship...

 to the presence of predators. Phantom midge larvae (Chaoborus) release kairomones – chemical cues – that induce the development of small jagged protrusions on the head, known as "neck-teeth". Neck-teeth increase survivorship in the presence of the invertebrate predator, but there are costs – longer development time, for example – when those predators are not present.

Ecological stoichiometry

Daphnia pulex ecology is shaped by nutrient availability and balance
Ecological stoichiometry
Ecological stoichiometry considers how the balance of energy and elements affect and are affected by organisms and their interactions in ecosystems. Ecological stoichiometry has a long history in ecology with early references to the constraints of mass balance made by Liebig, Lotka, and Redfield...

, which affects traits that mediate intra- and interspecific interactions. Because nutrients are required for an array of biological processes – for example, amino acid synthesis – the environmental availability of these nutrients regulates downstream organismal characteristics. Low nutrient availability reduces both body size and growth rate, which, as noted above, regulates Daphnia relationships to predators. Daphnia pulex in particular has been an important model species for investigating ecological stoichiometry
Ecological stoichiometry
Ecological stoichiometry considers how the balance of energy and elements affect and are affected by organisms and their interactions in ecosystems. Ecological stoichiometry has a long history in ecology with early references to the constraints of mass balance made by Liebig, Lotka, and Redfield...

, demonstrating that pond shading by trees increases nutrient concentrations relative to carbon in 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...

, which increases D. pulex body size, and therefore competitive ability and susceptibility to predation by vertebrates.

Genomics

Daphnia pulex was the first crustacean to have its genome
Genome
In modern molecular biology and genetics, the genome is the entirety of an organism's hereditary information. It is encoded either in DNA or, for many types of virus, in RNA. The genome includes both the genes and the non-coding sequences of the DNA/RNA....

 sequenced. Its genome contains 31,000 gene
Gene
A gene is a molecular unit of heredity of a living organism. It is a name given to some stretches of DNA and RNA that code for a type of protein or for an RNA chain that has a function in the organism. Living beings depend on genes, as they specify all proteins and functional RNA chains...

s – 8,000 more than are present in the human genome
Human genome
The human genome is the genome of Homo sapiens, which is stored on 23 chromosome pairs plus the small mitochondrial DNA. 22 of the 23 chromosomes are autosomal chromosome pairs, while the remaining pair is sex-determining...

 – as a result of extensive gene duplication
Gene duplication
Gene duplication is any duplication of a region of DNA that contains a gene; it may occur as an error in homologous recombination, a retrotransposition event, or duplication of an entire chromosome.The second copy of the gene is often free from selective pressure — that is, mutations of it have no...

.
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