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Amphionides reynaudii is the sole representative of the order Amphionidacea, and is a small (less than one inch long) planktonic crustacean found throughout the world's tropical oceans, the larvae mostly in shallow waters, and the adults at greater depth.

Description

Amphionides grows to a length of 25 millimetres (1 in). Morphologically, Amphionides is somewhat unusual, with many body parts being reduced or absent. For example, it has only one pair of mouthparts – the maxillae – the mandibles and maxillules being vestigial.

Males and females differ in the form of the antennae, and also by the presence in males of the eighth thoracic appendage, albeit in a reduced form. This is the site of the male gonopore (the female's gonopore is on the sixth thoracic appendage). The first pleopod of the female is greatly enlarged and almost encloses the enlarged carapace. This is assumed to be a chamber in which the eggs are fertilised and retained until hatching. The more streamlined carapace and pleopods of the male make it more hydrodynamic, and so fewer males are caught than females.

Distribution and ecology

Amphionides reynaudii has a cosmopolitan distribution in the world's tropical oceans. It is planktonic, inhabiting waters less than 100 metres (330 ft) deep as a larva, but more often at depths of 700–1,700 m (2,300–5,600 ft) as an adult.

Classification

Originally described from its larvae, Amphionides was originally thought to be a shrimp. It was not until 1969 that the adult form was observed to be that described by Carl Wilhelm Erich Zimmer in 1904, and only in 1973 was Amphionides placed in its own order by Donald I. Williamson. The specific epithet reynaudii was given by Henri Milne-Edwards in honour of a friend of his, possibly Count François Dominique Reynaud de Montlosier. The generic name used by Milne-Edwards, Amphion, was invalid as a junior homonym of Amphion Hübner, 1819, a genus of hawk moths.


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Asellus aquaticus is a freshwater crustacean resembling a woodlouse. It is known by common names including "waterlouse", "aquatic sowbug" and "water hoglouse".

Ecology

"Scud fly" is common throughout the temperate zone including Europe, Russia, and North America. Asellus aquaticus is found in rivers, streams and standing water particularly where there are plenty of stones under which it hides although not where the water is strongly acidic. It is omnivorous.

Asellus aquaticus is relatively tolerant of a range of pollutants and has been used as an indicator of water quality.

Life Cycle and Reproduction

Asellus aquaticus can breed throughout the year, if the temperature is high enough; they do not breed under cold temperatures. Maturity can be reached in few months under warm summer temperatures, but maturation may take as much as two years in permanently cold water bodies (e.g., high-latitude or mountain waters). Life span varies similarly: from 9 months in South Africa to 20 months in Northern Europe. Females carry eggs in brood pouches underneath their body.

Conservation


A number of subspecies are considered to be endangered including:

Asellus aquaticus carniolicus, endemic to Slovenia.
Asellus aquaticus cavernicolus found only in Italy and Slovenia.
Asellus aquaticus cyclobranchialis, endemic to Slovenia.
^ Boris Sket (1996). "Asellus aquaticus ssp. carniolicus". IUCN Red List of Threatened Species. Version 2009.2. International Union for Conservation of Nature. Retrieved May 15, 2010.
^ Boris Sket (1996). "Asellus aquaticus ssp. cavernicolus". IUCN Red List of Threatened Species. Version 2009.2. International Union for Conservation of Nature. Retrieved May 15, 2010.
^ Boris Sket (1996). "Asellus aquaticus ssp. cyclobranchialis". IUCN Red List of Threatened Species. Version 2009.2. International Union for Conservation of Nature. Retrieved May 15, 2010.


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Daphnia magna is a small planktonic crustacean (adult length 1.5–5 mm) that belongs to the subclass Phyllopoda. Daphnia magna inhabits a variety of freshwater environments and is broadly distributed throughout the northern hemisphere, and in South Africa.

Daphnia magna has been subject of biological research since the 18th century. It is widely used in ecological and evolutionary studies, and in ecotoxicology. Daphnia magna is also a popular fish food in aquaculture and aquariums.

Description

Daphnia magna displays the general morphological features of the genus Daphnia. The females reach up to 5 mm in size, the males about 2 mm, and thus being among the largest Daphnia species. The body is protected by a translucent carapace, made of chitin, a transparent polysaccharide. It also presents a ventral opening, and has five pairs of thoracic limbs, which are used to help the filtering process. Spike rows run along the back of the carapace. The intestine is hook-shaped and possesses two digestive cecae. The head is equipped with 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 males are smaller than the females and have larger first antennas, a diagnostic feature that distinguishes them from small females.


Abdomen with abdominal claw of a female Daphnia magna. The two combs with a gap on the margin and the deeply nicked posterior margin are diagnostic features of the species Daphnia magna.

Adult male of the cladoceran Daphnia magna, highlighting the first antenna
Ecology

D. magna is a key species in many lentic habitats. It can be found in lakes and in shallow ponds rich in organic matter sediment. Numerous natural predators are known and can lead to plastic phenotypic responses. In the presence of kairomones Daphnia develop conspicuous protective structures as an elongated spine and a large body size. It is an important primary consumer and, also, prey of many planktivorous fishes. Other invertebrate predators are the larvae of the phantom midge Chaoborus and hemipterans (Notonecta) and Triops. The large size of the adults protects them from predation from some planktivorous invertebrates.

D. magna is widespread in the northern hemisphere and in particular in the holarctic. It can be found in fresh and brackish water bodies of different sizes, from lakes to ponds and ephemeral rook pools near the sea. D. magna tolerates higher levels of salinity (up to 20%) 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 (mainly algae but also bacteria and detritus). 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 and sediment.

The main feeding strategy of D. magna is the filtering of suspended particles. A specialized filtering apparatus, formed by the thoracic appendixes, generates a water current within the thoracic opening of the carapace which permits the collection and the ingestion of unicellular algae, bacteria and detritus. 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


Female Daphnia magna carrying an ephippuim

Schematic representation of cyclic parthenogenesis in the cladoceran Daphnia magna (water flea).
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 one day after being laid and are released after three days. Juveniles go through four to about six moults before becoming mature over a period of five to ten days. An adult female produces one clutch with up to one hundred eggs every three to four days until her death. It can live over three months in the laboratory at 20 °C.


Dead Daphnia magna with ephippia in a dried pond
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 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 may 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.

Behaviour

File:Daphnia slow motion.ogvPlay media
The characteristic swimming behaviour of Daphnia magna
The name "water fleas" might come from the typical swimming behaviour of Daphnia species which reminds 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 behaviour 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 behaviour (movements of entire organisms to, or away from, a light source). In D. magna phototactic behaviour 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 behaviour, termed browsing behaviour, 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.


Female Daphnia magna infected with Hamiltosporidium magnivora

Female Daphnia magna non-infected (left) and infected (right) by the bacteria Pasteuria ramosa
Table 1: Parasites of Daphnia magna
Taxa Parasite Tissue or infection site Transmission
Bacteria Pasteuria ramosa Blood, extracellular Horizontal, from dead host
Spirobacillus cienkowskii Blood, extracellular Horizontal, from dead host
Fungi Metschnikowia bicuspidata Body cavity, extracellular Horizontal, from dead host
Microsporidia Hamiltosporidium tvärminnensis Intracellular Horizontal and vertical
H. magnivora Intracellular Vertical, Horizontal (?)
Glucoides intestinalis Gut wall, intracellular Horizontal, from living host
Ordospora colligata Gut wall, intracellular Horizontal, from living host
Amoeba Pansporella perplexa Gut wall, extracellular Horizontal, from living host
Chytrid fungus Caullerya mesnili Gut wall, intracellular Horizontal, from living host
Nematoda Echinuria uncinata Body cavity, extracellular Horizontal, to a second host
Cestoda Cysticercus mirabilis Body cavity, extracellular Horizontal, to a second host
Microbiota

D. magna can be looked at as a complex ecosystem, colonized by a community of commensal, symbiotic and pathogenic microorganisms called microbiota. The close proximity of the microbiota to its host allows for a tight interaction, capable of influencing development, 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.

Daphnia magna as model organism for ecosystem health and ecotoxicology

D. magna presents numerous advantages when used as experimental organism. Its transparency allows for the observations of its inner anatomical structures at the microscope, while its reproduction through cyclical parthenogenesis 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, limited storage usage, easy and cheap feeding and simple maintenance.

D. magna is used in different field of research, such as ecotoxicology, population genetics, the evolution of sex, phenotypic plasticity, ecophysiology (including global change biology) and host-parasite interactions.

Historically, D. magna allowed researchers to test some interesting theories and conduct pioneering studies:

Macrophages attack invading parasites as a part of cellular immunity (Metchnikoff – Nobel prize in 1908)
Ecotoxicological studies (Einar Naumann)
Other recent experiments used the resting eggs of Daphnia present in a pond sediment to reconstruct the evolutionary history of that population in relation to one of its parasites (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, 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 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. Test No. 211 is a 21-day chronic toxicity 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.

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Hemilepistus reaumuri is a species of woodlouse that lives in and around the deserts of North Africa and the Middle East, "the driest habitat conquered by any species of crustacean". It reaches a length of 22 mm (0.87 in) and a width of up to 12 mm (0.47 in), and has seven pairs of legs which hold its body unusually high off the ground. The species was described in the Description de l'Égypte after the French Campaign in Egypt and Syria of 1798–1801, but was first formally named by Henri Milne-Edwards in 1840 as Porcellio reaumuri. It reached its current scientific name in 1930 after the former subgenus Hemilepistus was raised to the rank of genus.

H. reaumuri occurs at great population densities and fills an important niche in the desert ecosystem. It feeds on plant leaves, obtains most of its water from moisture in the air and sand, and is in turn an important prey item for the scorpion Scorpio maurus. H. reaumuri is only able to survive in such arid conditions because it has developed parental care of its offspring. Adults dig burrows which are inhabited by family groups, which are recognised using pheromones. The burrows are 40–50 cm (16–20 in) deep, and the woodlice retreat to the relatively cool and moist conditions of the burrow when surface conditions are unfavourable. The territorial limit of each colony is marked with a faecal embankment.

Description

A woodlouse stands on the soil surface, looking into a narrow burrow entrance; the trunk of a small shrub can be seen rising near the burrow entrance.
H. reaumuri at a burrow entrance
Hemilepistus reaumuri is approximately 22 millimetres (0.87 in) long, and 9–12 millimetres (0.35–0.47 in) wide. In common with other woodlice, it has seven pairs of legs and a pair of conspicuous antennae. It is classified in the family Trachelipodidae; within that family, it is placed in the genus Hemilepistus because of the presence of tubercles only on the head and the nearby parts of the thorax.

H. reaumuri differs from other desert woodlice in a number of respects. It is crepuscular, while other species are nocturnal. Apart from at the highest temperatures, it is also positively phototactic (is attracted to sunlight), while other species are negatively phototactic (move away from bright light). Compared to other woodlice, H. reaumuri walks in an unusual manner, with its body held high off the ground.

Distribution

Hemilepistus reaumuri is found in the steppes, semideserts and deserts of North Africa, and the Middle East, and occasionally on the margins of salt lakes. This has been described as "the driest habitat conquered by any species of crustacean". H. reaumuri is most closely associated with loess soils in the Sahara Desert and Negev Desert, although its range extends from eastern Algeria to western Syria.

Ecology

A grey-brown scorpion stands on a rock in combative posture; its orange-tinted claws are opened wide, and its barbed tail is raised.
Scorpio maurus is a major predator of Hemilepistus reaumuri.
Hemilepistus reaumuri is an important part of the desert ecosystem. It has been found at population densities of up to 480,000 individuals per hectare, which is equivalent to a biomass of 19.2 kg/ha; in comparison, desert mammals are estimated to have a combined biomass of 39.9 kg/ha. The main predator of H. reaumuri is the scorpion Scorpio maurus, and it may compose up to 70% of the scorpion's diet. It appears to be vulnerable to attack only on the surface; no predators are known to attack H. reaumuri in its underground burrows.

Hemilepistus reaumuri can only escape the heat of the desert by constructing a burrow, which is time-consuming and energetically costly. One parent must therefore guard the burrow while the other forages for food. H. reaumuri can spend up to ten months of the year returning to the surface to forage, which is far longer than species which do not dig burrows, such as Armadillidium vulgare or Armadillo officinalis. Although they will forage at temperatures as high as 35 °C (95 °F), these woodlice retreat to their burrow when the temperature is too high. They are also unable to tolerate air with a relative humidity below 6%, which often occurs at depths of up to 30 centimetres (12 in) in the desert soil in the hottest months, and the burrows are therefore dug at least 40–50 cm (16–20 in) deep. The burrows are vertical, with a single entrance 9–12 mm (0.35–0.47 in) in diameter, and there may be up to 20 burrows per square metre (nearly 2 per square foot) in favourable areas. If a foraging woodlouse cannot find the burrow entrance on its return, it employs a complex and efficient strategy to find it again. It begins with a spirally widening search, and develops into a more meandering approach the longer it is unable to find the burrow.

Clouds tumble over the cliff edges that surround a patch of desert; a few small shrubs sprout from the dry earth.
Mist rolls over the edge of HaMakhtesh HaGadol in the Negev Desert: water vapour is an important source of water for Hemilepistus reaumuri.
Hemilepistus reaumuri has a significantly higher biomass than other herbivores in the Negev Desert, making it an important part of herbivore–omnivore food chains. They spend the day provisioning their burrows with leaf material from the surface of the desert, sometimes resting under stones or in crevices of rocks. Their faeces accumulates on the surface, and forms a "faecal embankment", similar to a levee, which demarcates the extent of the home territory of the colony in the burrow. The plants Artemisia herba-alba and Haloxylon scoparium are the most abundant bushes in the Negev desert, and seem to make up most of the diet of H. reaumuri.

The bulk of the water intake of Hemilepistus reaumuri is by taking up water vapour from saturated air and by eating damp sand. Water loss is minimised by the rectal epithelium, which absorbs water, ensuring that the faeces is drier than the food the animal consumed. Evaporation of water through the permeable exoskeleton may, however, provide a valuable cooling effect.

Life cycle

Hemilepistus reaumuri is the only species of Hemilepistus to have developed parental care of its offspring, and it is only because of this development that the species can survive in the desert. It is monogamous, and, unusually, both parents tend the young.

The colonies are quiescent during the winter, and young individuals emerge in February and March to establish new burrows. Sheltered sites below bushes are chosen, although larger males will often try to pair with a female who has already established a burrow, sometimes ousting her male partner. The anatomy of H. reaumuri is not specialised for digging, and the excavation is a slow process, taking place only in early spring. The first 3–5 centimetres (1.2–2.0 in) are dug by a single woodlouse, which then stops to guard the new burrow. Eventually, it will allow one other woodlouse of the opposite sex to enter, and they then engage in a ritual which often lasts for hours, before copulating.

The female bears 50–100 live young, typically in May. The young remain in the burrow for 10–20 days, being provided with food by their parents. On leaving the burrow, they are wary of other families, and adults may catch other adults' offspring and feed them to their own, but do not normally attack their own children. Members of each social group recognise each other using pheromones. Each pair only produces one brood, and the life of an individual of H. reaumuri is typically around 15 months long, considerably shorter than the 2–4 year lifespans of woodlice from more mesic habitats, such as Armadillidium vulgare, Porcellio scaber or Philoscia muscorum.

Taxonomic history

Hemilepistus reaumuri was illustrated in volume 21 of the Description de l'Égypte, researched during Napoleon's campaign in Egypt and Syria of 1798–1801, and dedicated to René Antoine Ferchault de Réaumur. The section on Crustacea was begun by Marie Jules César Savigny but finished by Jean Victoire Audouin after Savigny's health deteriorated. The species was not given a formal scientific name, however, until Henri Milne-Edwards did so in 1840, calling it Porcellio reaumuri. Although initially placed in the genus Porcellio, it was later moved by G. H. A. Budde-Lund in 1879 to his new subgenus Hemilepistus, which was raised from a subgenus of Porcellio to the rank of genus by Karl Wilhelm Verhoeff in 1930. Several species names that were previously thought to be synonyms of H. reaumuri have been re-examined, and found to refer to a species in a different family, now known as Porcellio brevicaudatus.


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Trachelipus ater is a species of woodlouse in the genus Trachelipus belonging to the family Trachelipodidae that is endemic to Romania.


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