Water Flea (Daphnia sp.), family Daphniidae, order Anomopoda, Poland

Giant Daphnia Laboratory by Hiné Mizushima.

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It's all right to be itty-bitty and this week's Wet Beast Wednesday topic knows that well. Daphnia are a group of tiny crustaceans found all over the world who play a vital role in controlling algae populations in their habitats while also being a vital food source for larger animals. These creatures are great at survival in rough conditions, allowing them to survive drought, heat, salinity, and other conditions that would kill other freshwater species. This means that Daphnia are used as a model organism for ecological studies.

(Image: a Daphnia viewed by a microscope. It is a round, flat crustacean that looks a bit like a shrimp inside of a transparent pea. The head is distinguished from the body by a small notch. Two pairs of antennae extend from the head and have small branches extending from them. They eyes are large and black. At the rear end of the body is a spike. The digestive tract is visible presumably due to a recent meal. A clutch of round, dark eggs is being brooded inside of the body. End ID)

Daphnia is a genus of crustacean in the class Branchipoda. They are one of a large and loosely-related group of animals commonly called water fleas due to moving in sudden bursts. There are over 200 known species of Daphnia and it is suspected that up to 80% of the genus remains undescribed or poorly understood. Daphnia are small, the majority being around a millimeter long. The species Daphnia magna is considered large for reaching 5 mm. Like other arthropods, Daphnia have multiple body segments, though theirs are hard to spot due to being fused. Unlike most crustaceans, who are flexible where their body segments meet, the bodies of Daphnia are mostly fused into a single, inflexible structure. The head contains the mouth, eyes, and two pairs of antennae and is stuck looking down toward the body. The thorax and abdomen are enclosed in a large, round carapace that protects the body. This carapace is typically transparent. While legs remain, they are small and mostly used to circulate water within the carapace as they do not extend out of it. There is a spike at the end of the carapace. Some species grow additional spikes around the body as protection from predators. Some of these species will only grow the spikes in response to chemical signals released by predators, allowing them to use that energy for growth instead if they don't need to worry about getting eaten.

(Image: Daphnia magna with a clutch of eggs inside of it. End ID)

Daphnia are filter-feeders who consume algae, bacteria, protists, and organic detritus. Their legs are used to create a current that pulls water into the carapace. The second and third pair of legs have fan-like structures on them that filter out particles too large to eat. The rest is carried to the mouth, where particles are covered in mucus to form a bolus that is swallowed. After being carried through the digestive tract, feces are voided out the anus. Some species release feces into the water, while others contain the feces in boluses that are kept internally and discarded all at once during molting. Since the legs aren't useful for locomotion, Daphnia had adapted another method. The second pair of antennae have become enlarged and are used as paddles to force the animal through the water. This is the cause of the characteristic water flea movement.

(gif: a video of a Daphnia. The legs can be seen moving back and fourth within the body and the antennae are flicking back and fourth. End ID)

Daphnia are primarily freshwater animals, though some have adapted to live in hypersaline lakes. While many species are found in permanent water features like lakes, ponds, and rivers, a large number of species have adapted to live in vernal pools. These are temporary water features that fill during spring but are dry the rest of the year. Daphnia cannot survive on land and will die if dried out. To survive this kind of stasis, they have adapted an interesting form of reproduction. Daphnia tractice a form of reproduction called cyclical parthenogenesis. Parthenogenesis is reproduction without mating, which produces a clone of the mother. Daphnia spend most of their lives using parthenogenesis to reproduce asexually. They will produce a brood of eggs for every molt, which are stored in a brood pouch within the carapace. The eggs will hatch and gorw into larvae within the brood pouch. After the next molt, while the mother's body is still soft, the larvae will be released into the water. This ensures that the young Daphnia are more mature than the larvae of other plankton that are not brooded. After 5 instar stages, each of which ends with a molt, the larva will be mature and ready to start producing her own eggs. Only female Daphnia exist during this time. Changing conditions that signal hard times ahead, such as increasing water salinity, decreasing oxygen, overcrowding, and increasing temperatures, signal the Daphnia to switch reproductive strategies. The females will start producing eggs which will develop into male Daphnia. They will then switch to sexual reproduction, with the males fertilizing haploid eggs. These eggs will be stored in structures called ephippia and released after molting. Unlike eggs stored in the brood pouch, eggs stored in ephippia will reach a certain level of larval development before entering a state of cryptobiosis. Cyptobiosis is a state where almost all metabolic activity stops and the organism can essentially exist in stasis for a long (but not indefinite) time. Ephippia can survive drought, freezing, and other rough conditions. Once conditions improve, the ephippia will continue developing into the next generation. Not all ephippia will hatch each season. Some will stay in stasis through good seasons. This ensures that a bad season can't wipe out the whole population. Ephippia can be carried by wind and animals to be spread to distant and isolated bodies of water.

(image: a Daphnia giving birth. The larvae look like smaller and greenish versions of the adult and are stored within the body. One is emerging from the mother, exiting through an opening in the carapace. End ID)

Daphnia are found on every continent and islands around the world. Some species, like Daphnia pulex and Daphnia magna have spread all across the world while other species are more localized. Daphnia can be threatened by invasive species. In the Great Lakes region of North America, the invasive water fleas Bythotrephes longimanus and Cercopagis pengoi are damaging ecosystems by excessively preying on Daphnia, thus harming other planktivorous species. Daphnia provide an important service in their ecosystems by feedin on algae and detritus and acting as food for larger species. The loss of Daphnia can trigger a trophic cascade that affects the whole ecosystem. Daphnia can be used as an indicator species, meaning their health reflects the health of their ecosystem. If the local Daphnia population is suffering, the rest of the ecosystem will be in danger. Daphnia are also used as model lab organisms. Many species are transparent and small enough to be unharmed while under a microscope coverslip, allowing scientists to observe their organs and internal workings while the animals are alive. This makes them valuable when studying the effects of alcohol, nicotine, and adrenaline by directly observing the effects they have on the nervous system and heart.

sure. yeah i'll believe that

Daphnia pulex (Water flea)

Photo credit: Paul Hebert