Daphnia |
![Daphnia magna with eggs](./modules/Fish-MM/images/225px-Daphnia_magna01.jpg)
Daphnia magna with eggs
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Scientific classification |
Kingdom: |
Animalia
|
Phylum: |
Arthropoda
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Subphylum: |
Crustacea
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Class: |
Branchiopoda
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Order: |
Cladocera
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Family: |
Daphniidae
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Genus: |
Daphnia
Müller, 1785 |
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Species |
-
- D. ambigua
- D. arenata
- D. catawba
- D. cheraphila
- D. latispina
- D. melanica
- D. middendorffiana
- D. minnehaha
- D. neo-obtusa
- D. obtusa
- D. oregonensis
- D. parvula
- D. pileata
- D. prolata
- D. pulex
- D. pulicaria
- D. retrocurva
- D. tenebrosa
- D. villosa
-
- D. curvirostris
- D. dentifera
- D. dubia
- D. laevis
- D. longiremis
- D. mendotae
- D. thorata
- D. umbra
-
- D. brooksi
- D. ephemeralis
- D. exilis
- D. lumholtzi
- D. magna
- D. salina
- D. similis
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Daphnia are small, mostly
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 (although
fleas are insects and thus only very distantly related).
They live in various aquatic environments ranging from
acidic swamps to freshwater lakes, ponds, streams and rivers.
Biology
The division of the body into
segments is nearly invisible. 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, 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, and a pair of
abdominal setae. In many species, the carapace is
translucent or nearly so and as a result they make excellent
subjects for the microscope.
Even under relatively low power, it is possible to
observe the feeding mechanism working, watch immature young
moving in the brood-pouch, observe the eye being moved by
the
ciliary muscles and even watch blood corpuscles being pumped
round the circulatory system by the simple heart. The heart
is at the top of the back, just behind the head. and their
average heart rate is approximately 180bpm under normal
conditions. Daphnia, like many animals, are prone to alcohol
intoxication, and make excellent subjects for studying the
effects of the depressant on the nervous system - due to the
translucent exoskeleton, and the visibly altered heart rate.
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 and watching an increase in heart rate.
A few Daphnia prey on tiny crustaceans and
rotifers, but most are filter feeders, ingesting mainly
unicellular algae and various sorts of organic detritus
including protists and bacteria. Daphnia can be kept easily
on a diet of yeast. 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 unabsorbable particles are
kept out while the other sets of legs create the stream of
water rushing into the organism. Swimming, on the other
hand, 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.
Reproduction
Daphnia reproduce
parthenogenetically 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 moult 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 nymphal or instar
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. Winter or
drought conditions brings an end to the production of new
female generations. At this time, the reproduction method
changes. Parthenogenic males are produced, followed by
mating and fertilisation of the eggs. Fertilised 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.
Males are only found at times of harsh environmental
conditions, typically during portions of the year of scarce
resources due to population overgrowth or winter conditions,
and even then may make up considerably less than half the
population, in some species being unknown entirely. They are
much smaller in size than the female and they typically
possess a specialised abdominal appendage which is used to
grasp a female from behind and prise open her carapace and
insert a spermatheca. Their appearance is for the creation of
resting or winter eggs, allowing for the survival of the
population through harsh conditions.
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
while some organisms live for only 29 days at 28°C. 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 slower rate but in the end are larger than
ones under normal conditions.
Uses
Daphnia provide an important source of food for
many larger aquatic organisms including various fish species
(e.g. lake trout) and the immature stages of many insects
including the Odonata- dragonflies and dameselflies.
They are easy to culture in the laboratory, and
D.magna has been a model species for developing the
Dynamic Energy Budget theory. They are frequently used to
test the effects of toxicants on reproduction and survival.
Daphnia are sold by aquatic retailers in both live and
freeze-dried form as food for aquarium fish. Their tiny size
renders them edible in live form even for fish as small as
the
neon tetra
Daphnia may also be used to clear unwanted algae from
fish tanks, provided they are not eaten by the fish.
Many speciea of Daphnia are used in aquatic
toxicology. As experimental animals they have many
advantages being simple to produce in large numbers and
exhibiting consistent responses to toxins.
Daphnia is considered an
indicator species or sentinel species which is an
indicator of ecosystem health.
Ecology
The populations of several water flea species are
considered threatened. The following are listed as
vulnerable by
IUCN: Daphnia nivalis, Daphnia coronata,
Daphnia occidentalis, and Daphnia jollyi.
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