Daphnia (genus)

Daphnia are a genus of small water organisms commonly known as water fleas. These planktonic crustaceans are aquatic arthropods that belong to the Phyllopoda group of organisms, which have leaf-like legs that are used to swim through water. Daphnia are an important member of the watery ecosystems in which they live, serving near the bottom of the food chain. While humans do not eat them, they are food for the fish that humans do eat, such as sockeye salmon. Daphnia are also an indicator species, which means that they show the conditions or health of their environment. A change in their population serves as an early warning sign that other species are threatened due to disease, pollution, or climate change.

Background

Daphnia are called water fleas because they are very small organisms, and when they are swimming in water, they look like fleas jumping around. These organisms can be found throughout the world in nearly all bodies of fresh water, such as swamps, ponds, or lakes. Some can survive in low concentrations of saltwater, but most species live only in fresh water. They are so common that species of Daphnia are found in nearly all freshwater sources, except those with harsh conditions, such as hot sulfur springs. More than two hundred species of Daphnia exist, but the most common species are the smaller Daphnia pulex and the larger Daphnia magna.

Some species of Daphnia are considered threatened or vulnerable by the International Union for Conservation of Nature (IUCN), such as Daphnia coronata, Daphnia jollyi, Daphnia nivalis, and Daphnia occidentalis. These species reside in delicate ecosystems that are affected by pollution or climate change. Other Daphnia species, such as Daphnia lumholtz, are considered an invasive species. Daphnia lumholtz has developed a spiny defense against predator fish, but the long tail spines cause them to become entangled together along fishing lines or other structures in water in large numbers. Likewise, if any species of Daphnia with strong protective mechanisms is introduced into a non-native environment, it may not be able to be eaten by predators. Then it might clog and cloud the water.

Overview

Daphnia are very small aquatic creatures, only .2 to 6 millimeters in length. Some are barely visible to the human eye without magnification. They have a body that is divided into segments, although a microscope is needed to see them. Daphnia have either five or six pairs of paddle-like legs, depending on the species. Daphnia also have two sets of branched antennae, and the larger is used for swimming. Daphnia have compound eyes and abdominal setae, or hair-like bristles. Daphnia do not have lungs or a complex respiratory system, so they get oxygen through diffusion from water molecules that travel through their exoskeleton.

Their simple invertebrate body is covered by a double-walled carapace, which is an exoskeleton shell that is found in crustaceans. Because the carapace is translucent in most species, internal organs are visible with a microscope. It is possible to see Daphnia feed, digest, reproduce, and move its eyes and appendages around. The simple heart is located behind its head on the back. It is possible to observe it beating and see the blood move around the open circulatory system of its body.

Daphnia are often used for scientific study using simple experiments because their organs can be seen using a low-powered microscope. They can be used to examine the effect of drugs, toxins, or even climate change in an environment. These organisms are resilient and seem to survive well while being observed under a microscope and then released.

Daphnia usually have short lifespans. Although the individual lifespan depends on the species, the average environmental temperature, and the density of predators, usually a Daphnia will live for about six months. However, in colder climates without fish to eat them, a Daphnia may survive for about a year. Most species of Daphnia have a cyclical parthenogenesis lifecycle. This means they alternate between asexual and sexual reproduction. Asexual reproduction usually results in females. When conditions are harsh, however, the eggs are protected by a tough coating, and females can develop into males.

Males are smaller than females and have hooks on their first legs that are used to clasp eggs during fertilization. Every time a female Daphnia molts its exoskeleton, it produces a clutch of eggs. Smaller species have just a few eggs, while larger species may have one hundred or more eggs in a brood. The eggs hatch after about a day and remain in the brood pouch of a female for about three days before she releases them into the water. In about a week, under ideal conditions, the young Daphnia are sexually mature and able to reproduce.

Daphnia are filter feeders that usually eat single-cell green algae, although they also eat bacteria and organic debris suspended in water. This makes them a vital part of the food chain in any body of water. They beat their flat legs to produce a current through their carapace to move and filter food into their digestive tract, while keeping out unsuitably large particles.

Daphnia can also be farmed and used as food for tropical fish, tadpoles, or frogs. They are not difficult to cultivate. Home aquarists are able to grow them using a starter culture from an aquarium supply company. A tank can be set up with an ecosystem that mimics their natural environment, and they can be fed active dry yeast and spirulina algae powder. Since Daphnia grow and reproduce quickly, they can be harvested frequently. This also keeps the existing population healthy.

In a study published in 2024, researchers used Daphnia to measure the impact of microplastics in aquatic environments. Microplastics are tiny pieces of plastic that are difficult or impossible to remove from many environments. The study showed that when combined with Per- and Polyfluorakyl Substances (PFAS), better known as forever chemicals, the negative impact on Daphnia was significantly greater than expected. Scientists observed that many of the exposed Daphnia displayed delayed sexual maturity and stunted growth. This study provided scientists with a concerning image of what many populations of Daphnia may be experiencing in the wild.

Bibliography

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“Dapnia pulex.” Animal Diversity Web: University of Michigan Museum of Zoology, 2020, animaldiversity.org/accounts/Daphnia‗pulex/. Accessed 17 Feb. 2022.

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"Microplastics and PFAS: Daphnia Study Finds Combined Impact Results in Greater Environmental Harm." Phys.org, 22 Oct. 2024, phys.org/news/2024-10-microplastics-pfas-daphnia-combined-impact.html. Accessed 15 Nov. 2024.

Stollewerk, Angelika. “The Water Flea Daphnia—A ‘New’ Model System for Ecology and Evolution?” Journal of Biology, vol. 9, no. 21, 2010, jbiol.biomedcentral.com/articles/10.1186/jbiol212#citeas. Accessed 17 Feb. 2022.