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cellular organisms -
Fungi/Metazoa group -
Daphnia taxonomy explained
- Bilateria: the organisms possess bilateral symmetry. Bilateral symmetry probably arose first in simple animals consisting of flattened masses of cells.
- Coelomata: the animals have a coelom. Coelom is a body cavity that develops within the embryonic mesoderm. It has muscles on both sides and is lined with a special structure derived from mesoderm, called the peritoneum.
- Protostomia: the animals are characterized by spiral and determinate cleavage. Blastopore becomes mouth. Mesoderm derives from cells on lip of blastopore. Mesoderm splits to form coelom.
- Arthropoda: the organisms are characterized by segmented external exoskeleton containing strong, flexible, water-proof polysaccharide chitin. Arthropods grow by molting. Among arthropods, the coelom evolved into a hemocoel - a cavity that is filled with blood that directly bathes the animal's organs and allows hydraulic extension of limbs.
- Crustacea: most of the 40,000 species of crustaceans have a body that is divided into three regions: head, thorax, and abdomen. The segments of the head are fused together, and the head bears five pairs of appendages. Each of the multiple thoratic and abdominal segments usually bears one pair of appendages. In many species, a fold of the exoskeleton, the carapace, covers the head and protects some other segments.
- Branchiopoda: the organisms have branched legs.
- Phyllopoda: the organisms have flattened leaf-like legs, often numerous, which they use in swimming.
- Diplostraca: compressed carapace usually encloses trunk and limbs; large biramous antennae; four to twenty-seven pairs of trunk limbs.
- Cladocera: the organisms with 4-6 pairs of limbs; carapace does not cover the head; reproduction occur without nauplius larva.
- Daphnia: fresh water zooplanktonic organisms that reproduce parthenogenetically under favorable conditions.
Daphnia is one of the several small aquatic crustaceans commonly called water fleas because of their jerky swimming movements. The most prominent external features of Daphnia are a single large compound eye and two pairs of slightly branched antennae.
Daphnia spp. can occupy a broad range of habitats. They are often found in ephemeral pools as well as in lakes ranging from ultra oligotrophic (clear bodied with little pigmentation) to eutrophic, where they may be bright red because of high hemoglobin production.
One of most prominent characteristics of typical Daphnia habitat is its instability. The habitat instability may have both detrimental as well as beneficial aspects for Daphnia populations. For example, dessiccation event can effectively kill all predators and most parasites, whereas rock pool Daphnia species will persist in form of resting eggs and be ready to re-populate the pool once it is filled with water. Exposure of resting stages to winds and birds during drought may have a further benefit by increasing dispersal of the species to other habitats.
Ecologically Daphnia spp. are considered zooplankton (microscopic animals that feed on other plankton). Daphnia feed on phytoplanktonic organisms such as microscopic algae (Chlamydomonas reinhardi, Scenedesmus sp., Ankistrodesmus falcatus and others). Daphnia also play an important role in aquatic food webs as a resource for consumers on higher trophic levels, including young breams (Abramis brama), roaches (Rutilus rutilus), and perches (Perca fluviatilis) as well as planktivorous fish (for example, Leucaspius delineatus), and invertebrate predators (for example, larvae of Chaoborus spp.).
Adaptive polyphenism produces alternative phenotypes depending on environmental stimuli such as changes in temperature, photoperiod, and food conditions.
Daphnia spp. have been shown to exhibit a variety of changes in phenotype in response to predator-produced kairomones. The following phenotypic changes were studied extensively: changes in life history traits (size and age at maturity, size and number of eggs produced, production of sexual eggs), in morphology (helmet development, neck spines) and in behavior (diel vertical migration, phototaxis, swarming and escape reactions). According to one study, different clones of same species may produce a subset of the changes in their phenotype rather than all of them, phototactic behavior and size at maturity being the most responsive traits in Daphnia magna clones.
Daphnia spp. are commonly used to detect various water pollutants because they are very sensitive to chlorine and various heavy metals. Also, in recent years, zooplankton research (limnology) conducted on Daphnia has developed new ideas in community, population and evolutionary ecology.
Daphnia exhibit heterogonic reproduction by alternating asexual (parthenogenic) and sexual stages. Cyclical partenogenesis seems to combine the best aspects of the two modes of reproduction.
Parthenogenic reproduction in Daphnia is apomictic (apomixis):
female produces unreduced eggs that hatch into exact clonal copies of their mother with
exception of rare accidental mutations.
Daphina molt to increase in size.
following each molt, the the mature females put 2 to 20 eggs in their brood chamber;
without fertilization, these eggs develop into females.
Advantages of asexual reproduction
- preservation of favorable genotypes;
- fast maturation;
- high fecundity.
Under certain conditions Daphnia switches to bisexual reproduction, which results
in two resting eggs encased in a robust structure carried on back of the female.
This epiphium is shed with next molt, and can survive dormant for many years
resisting freeze and desiccation.
Multiple environmental cues are required to induce resting-egg production, among them
food availability, photoperiod and population density.
In one study, in a low-food environment, offspring of short-day mothers produced significantly more resting
eggs than the offspring of long-day mother; whereas, none of the offspring produced resting eggs in a high-food
In nature, the strongest ephippia production is mostly observed in late fall. These resting eggs, having survived
winter condition in diapause, provide hatchlings for the start of the population in the next spring,
in addition to a few overwintering parthenogenetic females.
Advantages of sexual reproduction
- creation of variation (fodder for natural selection and evolution);
- DNA repair during recombination;
- avoidance of Muller's ratchet (accumulation of deleterious mutations);
- resistance to parasites (both ecological and genetic).
The eggs are formed in ovaries, which
look as a pair of elongated sacs
one on each side of the alimentary canal.
- Stage I:
newly laid egg The formed elongated ovum is squeezed out into the brood chamber within hours after molting. Later, it becomes more spherical. The egg is surrounded by thin membrane.
- Stage I:
Numbering of stages can vary between studies.
Opinions differ regarding the existence of number of embryonic
membranes in different species of cladocerans.
Timing of stages is approximate because
the time taken for eggs to hatch depends greatly
on temperature, nourishment and species.
- Stage II:
cleavage The egg remains spherical. Cell divisions not accompanied by growth of the embryo start in about one hour after laying and continue for about 2 hours.
- Stage III:
gastrula The embryo elongates. The cells surrounding the fat globule in the center of the embryo start growing. This stage lasts for about 6 hours.
- Stage IV:
outer egg membrane The embryo continues to elongate anteroposteriorly. The head is not yet formed. The egg membrane is cast off leaving the naupliar membrane as the boundary. This stage lasts for about 3 hours.
- Stage V:
organogenesis A distinct head lobe was formed at the end of the stage. Two, not yet fused, eyes are visible. Segmentation occurs in the antennary rudiment. This stage lasts for about 5 hours.
- Stage VI:
naupliar membrane An advanced development of the maxillae. Two membranes burst at the ventral part of the body. Head and body are distinct. This stage has a duration of about 2 hours.
- Stage VII The embryo starts to move and heart to pulsate. The paired eyes darken and fuse. The abdominal appendages are developed. Alimentary canal (foregut) and the postabdomen are visible. The rostrum elongates slightly. Embryo before birth is about 20 µm in size in Daphnia pulex.
- Stage VIII:
hatching When the brood pouch is packed with embryos, their release occurs due to the pressure inside the brood pouch after which they straighten their spine.
- Stage II:
- juvenile Fully developed neonate (1st instar) is about 100 µm in size in Daphnia pulex. Before achieving adult size, individuals undergo 4 moltings, gradually growing in size.
- adult Fifth instar, sexually matured female Daphnia. Age at first reproduction varies greatly from 5 to 40 days of age. Size of mature individuals highly depends on environmental conditions and species. For example, D. parvula ranges from 0.4 to 1.55 mm, whereas, D. magna reaches from 0.8 to 5 mm in size. The lifespan of a Daphnia does not exceed one year and depends on temperature, food availability and habitat. For example, individual organisms can live up to 108 days at 3°C while some organisms live for only 29 days at 28°C. Short lifespan appears advantageous in frequently changing environments, while long lifespan with late or prolonged reproduction is fit for safe permanent habitats or locations where environmental conditions remain unsuitable for young individuals.
Youtube: Daphnia magna video (06/15/10): mixed developmental stages in near natural habitat
Youtube (embedded below): Daphnia magna video (06/17/10): more dense population, growth of microalgae is supported by broad spectrum lamp
Olmstead AW et al. The Environmental-Endocrine Basis of Gynandromorphism (Intersex) in a Crustacean Int J Biol Sci. 2007; 3(2): 77–84.
Female (panel A), male (panel B), and gynandromorphic (panel C) D. magna. Differentiating sex characteristics include the pair of minute first antennae (FA) of the females, the elongated FA of the males. Gynandromorphic individuals possessed an elongated FA partnered with a diminutive antenna. The female-like diminutive FA is obscured by the male-like elongated FA in the micrograph. The bivalved-like carapace of the female has two uniform, symmetrical edges (CE). Both CEs of the male are asymmetrical and are edged by setae. The gynandromorphic daphnid has one female-like symmetrical CE and one male-like asymmetrical CE.
Altermatt F, Pajunen VI, Ebert D.
Desiccation of rock pool habitats and its influence on population persistence in a Daphnia metacommunity.
PLoS One. 2009;4(3):e4703.
Typical rock pools on islands in the Tvärminne archipelago in southern Finland. These pools are part of the herein studied Daphnia metacommunity.
Rock pools in panel A, B and C are depicted while filled with water.
Rock pools in panel D and E are depicted while halfway desiccated.
The rock pool in panel F is depicted while completely dry.
Note that the rock pools are almost free of sediments. The water colour depends on the amount of dissolved humic acid. In the background of A and B, the Baltic Sea is visible. Maximal diameter of each pool: A) 9.2 m, B) 6.9 m, C) 4.3 m, D) 1.3 m, E) 1.2 m, and F) 3 m.
Photos B, D and E courtesy of Thomas Zumbrunn.
Fröhlich T et al.
LC-MS/MS-based proteome profiling in Daphnia pulex and Daphnia longicephala: the Daphnia pulex genome database as a key for high throughput proteomics in Daphnia.
BMC Genomics. 2009 Apr 21;10:171.
Scanning electron micrograph from Daphnia longicephala (l) and Daphnia pulex (r).
Are We Underestimating Species Extinction Risk?
PLoS Biol. 2005 July; 3(7): e253.
Experiments with Daphnia magna, the water flea, show that traditional extinction models may be underestimating extinction risk.
Shaw JR et al.
Gene response profiles for Daphnia pulex exposed to the environmental stressor cadmium reveals novel crustacean metallothioneins.
BMC Genomics. 2007 Dec 21;8:477.
Cadmium induced phenotype.
Representative micrographs of 21-d Daphnia pulex maintained under
A) control conditions or B) exposed to cadmium (2.5 μg Cd/L).
Images were collected at the same scale and are presented as raw image files.
Differences were observed in a) body length and b) number of eggs in the brood chamber. The control animal also has a larger c) ovary and more pronounced lipid stores (represented by arrows).
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- Major topic "Daphnia": free full-text articles in PubMed