General Description of Daphnia
Daphnia belongs to the genus of small planktonic crustaceans that measures around 0.2 to 6.0 mm (0.01 to 0.24 in) as its total length. It is a well-known water flea referred to as daphnia water flea that belongs to the order cladocera and they are classified in the family of Daphniidae. It is so because they belong to the group of the water crustaceans that precede in the water with leaping and not by a gradual transition that gives an illusion of a discontinuous movement. They are adapted to a wide range of aquatic environments from acidic swamps to sweet waters like pools or ponds.
There are two major species of daphnia water flea namely, Daphnia Plurex and Daphnia Magna. They are classified in the genus Mona of the order Cladocera. That means they belong to the family of Monidae rather than Daphniidae. The daphnia water flea is usually of much smaller size than that of D. plurex, that is, almost half the size of the latter.
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Image: Illustration of a water daphnia
Appearance and Characteristics
An average length of water daphnia that ranges from 1 to 5mm or 0.039 to 0.197 inches. They are divided into two micro-segments and are not visible. The head is fused with the body and it is tilted downwards to the body but is separated from the body by a visible notch. Usually, many of the crustaceans possess a dorsal section like a shell of the exoskeleton with a gap where its five or six legs lie. They have compound eyes that is one of the prominent features followed by the second antennae and a pair of abdominal setae.
Their heart lies behind the head and at the top of the back with an average heart rate of 180bpm at normal conditions. Their entire nervous system can be clearly studied under the microscope due to its translucent exoskeleton system with altered heart rate that is also quite visible. Their blood circulation system consists of only the heart and blood with no blood vessels in the entire body. As the dorsal section of the exoskeleton appears as a shell covering both the body and the head of the daphnia. So when the heart pumps the blood it sprouts and transmits it to all the body through the shell along with the water that enters the body section from the gap. They have a fast heart rate because they are quite tiny in size.
Due to their small size, they need to use both circulatory motions as well as diffusion in order to produce haemoglobin, especially in low-oxygen environments.
Ecology and Behaviour
As daphnia invests in early reproduction due to their short lifespan they are considered as r-selected. Their lifespan usually depends on two major factors, namely, the temperature of the water and the population of its predator present in the location. Normally it has a lifespan of about 13-14 months in cold, fish free and oligotrophic lakes or water bodies. However, in most of the typical conditions, they do not live more than 5 to 6 months.
They are basically the subgroup of the suspension feeders as they usually feed themselves by straining out the food particles or the feeding matter from the water using their special water filter structure. They mostly intake unicellular algae, which includes bacteria as well as protists. With constant flapping of their legs, they produce a constant current that passes through the hard shell that helps bring the food to the digestive tract. The food in the digestive system then transforms into a small rounded mass of the substance and moves down the digestive tract until emptied through the anus that is situated at the end of the terminal appendage on a ventral surface. Their second and the third pair of legs are used as filter-feeders keeping the big particles from entering the body while the other pair of legs rushes a stream of water carrying the organisms.
They swim with the help of the second set of antennae as they are much larger in size than the first set. The second set of antennae actually enables the jumping motion of the water daphnia.
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Image: Five trunk limbs for filter-feeding
Daphnia usually shows typical characteristics of altering its behavioural structure and morphological changes in the presence of the predators’ chemical signals. The morphological changes may include bulkiness in size, larger size hatching or appearance of the neck-teeth. For instance, in the presence of the Chaoborus kairomones or chemical signals the D. pulrex have larger hatchings and the juveniles develop neck teeth. These morphological changes in the daphnia have increased their lifespan reducing the degree of threats from such predators. Deacetylases which are chitin-related genes are observed to have played a major role throughout the behavioural as well as morphological changes in the daphnia. Chitin-related enzymes actually initiate and catalyze the N-deacetylation that increases the affinity of protein binding with the chitin filaments.
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Image: Illustration of the anatomy of D. plurex
Lifecycle of Water Daphnia
The lifecycle of Daphnia water fleas primarily depends on the cyclical-parthenogenesis that is altering between sexual as well as the asexual mode of reproduction. Females reproduce asexually in most of the growing seasons. Now every time the female molts they tend to produce diploid eggs in their brood segment. Now the brood may contain as many as one or two eggs for small species like D. cucullata but ranges over 100 eggs in the brood chamber for the larger members of the daphnia like D. Magna. Usually, the egg hatches in a day but stays inside the brood of the female for almost three days at about 20 degrees celsius. They are then passed into the water currents where they pass through 4 to 5 development stages finally before they become adult daphnia to again reproduce asexually. Asexual reproduction typically produces female offspring.
At the end of the growing season, however, the females start producing tough resting eggs that are also known as winter eggs as the mode of reproduction changes. When the environmental condition deteriorates and is not any more favourable for females to produce asexually some of the asexually produced larvae develop into male daphnia. The females then produce the sexual haploid eggs that are fertilized by the males. In some cases, the hard resting eggs are also undergoing reproduction asexually and are dipole. In both cases, the eggs are protected by ephippium which is a hard covering that is cast off when the females undergo the next stage of moltation. The ephippia are conditioned in a way that they can easily withstand extreme cold, drought or scanty food availability and then finally hatches into females as the conditions again become favourable.
Types of Pond Daphnia
The two major types of daphnia for ponds that are mostly studied are as follows:
Daphnia Plurex
Description
They are the most common species are known as water fleas which are found in almost all the eutopic and permanent water bodies. Some of the species prefer to dwell in marine conditions but most of them belong to freshwaters like ponds or pools. They are small species with a total body length ranging from 0.2 to 3.0 mm. Although their bodies are not segmented like other marine species, they have a folded shell-like structure that has an opening in both ventral as well as posterior locations. Under the microscopic study of the specimen, it is clear that it has a transparent outer covering of the exoskeleton structure that makes the study of the internal organs very clear. Their head is flexibly attached to the body but bent inwards facing the body and has dark coloured compounded eyes as well as several sized antennas that help them in feeding as well as swimming.
They have a build-in specialized light-sensing organ called the ocellus that is the size of a tiny eye. At the junction of the head, there are really tiny mouthparts at the posterior junction that are hard to be seen. They only possess mandibles that are in a continuous motion to help grind the food which then passes through the intestine and terminates at the anus at the post abdomen. The post abdomen terminates itself into hook-like structures that are cuticle claws that are used to clear debris from the carapace.
Habitat
They are mostly found in the permanent, nutrition-rich water bodies, sometimes in the rainy water deposits in tires and often several meters from the ground on the moss of the rainforests. They are mainly freshwater species and their highest concentration is found in the dense vegetation of the ponds or lakes. They are usually the most abundant species that are found in the water bodies that are suitable for their reproduction. They either live like plankton in the freshwater or attach themselves to the vegetation grown in the pond or the lake.
They cannot survive in the strong currents of water in any water bodies as they are tiny and weak to withstand or swim in heavy currents of water. Thus it prefers the water bodies that are stagnant for them to swim or jump with the help of their legs and antennae. They can stay suspended in mild currents of water. They mostly prefer to live near the algae rich portion of the water body that is near to the upper surface of the water. But they are often caught with diel vertical movement, that is, they move up or down the surface of water depending upon the condition and the approaching predator. They often put a large amount of energy to stay down in the water during the morning to avoid easy predation from the predators but move up to the surface at night. Their location and movement depend on the seasonal availability of the planktonic food diet they follow.
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Image: Illustration of Daphnia Plurex
Daphnia Magna
Description
Their shape is similar to that of the kidney bean and has a total body length of 2 to 5 mm long which is usually very small in size. Like that of the other daphnia, they also possess a carcase, which is like a hard shell-like structure that runs along the body and is made up of chitin. The special characteristic is that it also runs overhead as a strong protection shield and is transparent.
As their body is transparent their colour becomes similar to that of the organism they are feeding on. They have two antennas where the second pair is larger and helps them to swim with long thoracic legs out of which two pairs help in digestion and the other pairs help to induce the current motion for the planktons to reach them faster. In this species the males are smaller than females with measurement of a male length is 2 mm while that of a female is 3 to 5 mm. But males have long antennae and modified long legs for holding the females during mating.
Habitat
They are the prime dwellers of freshwater but can also sustain brackish water that has a maximum of 8 ppt salinity. Thus their habitats are usually lakes, pools or rivers. They mostly prefer to stay in water within the temperature range of 18 to 22 degrees but they can tolerate a much broader range of temperature. Thus they are mostly found in Western Europe that includes England, Belgium, Netherlands, Finlands and the areas along the border of the black sea of Ukraine and some Baltic islands.
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Image: Illustration of Daphnia Magna
Conservation status
Daphnia is declared as a threatened species and listed as vulnerable by IUCN. This list consists of the following species of Daphnia: Daphnia coronata, Daphnia occidentalis and Daphnia jollyi. Some of the species are found in hypersaline lake content that is difficult for them to survive due to various habitat threats.
Fun Facts of Daphnia Water Flea
D. pulrex makes the majority of the Daphnia species that are found today mostly in freshwater ecosystems.
It has the ability to filter all the algae in a lake at the rate of 4ml/hr.
As they are transparent some of the D.pulrex are studied in the laboratory as their entire nervous system including the digestive tract are easily visible through their skin.
These are often used as indicators of the degree of wellness of an ecosystem because of their place in the food chain.
They can taxonomically change with the requirements of the environment.
FAQs on Daphnia
1. What Are the Uses of Daphnia?
Answer: They make a great source of diet for the sea fish and have been a great indicator of the condition of the ecosystem as they can barely survive in the brackish water and are sensitive to the change of water chemistry like its pH. They also feed on a number of planktons like algae and bacteria thus balance the condition of the water. They are very easy and inexpensive to breed in an aquarium.
2. What is the Lifespan of a Daphnia?
Answer: Daphnia usually lives in an actual situation for up to 9 to 13 days. They have a short lifespan but they can live up to a hundred days if there are no predators in their dwelling water body with the very ideal temperature maintained of the water. Throughout its lifespan, daphnia undergoes ten to twenty development stages known as instars.
