The Cladocerans, or water fleas, are common freshwater crustaceans. There are over 11,000 described species found around the world. Most of these important zooplankton are found in freshwater, though a few do live in marine waters. Cladocerans are abundant in Vermont lakes and ponds, regardless of waterbody size. They are also common in rivers and streams. Most are free-swimming and found in the water column. Some are benthic, living in the sediments near the bottom.
Chydorus spp, image: Proyecto Agua, Eol.org.
Most Cladocera are small, less than 0.1 inches in length., so you can’t see them with the naked eye. Their bodies are often in the form of a flat disk, covered with a firm transparent shell called a carapace. They have a head with a visible eye, while the rest of the body is covered by the carapace. There are usually 2 larger swimming appendages near the head and several pairs of smaller legs inside the carapace. The swimming legs do just that, move the cladoceran through the water. The other legs are used in feeding. Their movement keeps a steady stream of water flowing toward the mouth, carrying food particles. Bristles on the legs filter out larger bits and assist with moving the smaller ones into the mouth.
Cladocerans eat a variety of foods. Some scrape food from plants and rocks. Many are filter feeders, using the current created by their legs to gather phytoplankton, bacteria, and other organic materials from the water. Species may be strictly herbivorous and eat most phytoplankton. A few, like Leptodora kindtii, are predators and feed on other zooplankton. Cladocerans, in particular the Daphnia spp., are often the most abundant zooplankton group in individual lakes in early summer and they are a hungry bunch! In northern lakes, they can eat so many phytoplankton that the water can become clear in a very short time (see limnology.wisc.edu).
Leptodora kindtii. Image: Hanexna, iNaturalist, Eol.org.
Fish love to eat cladocerans. Cladocerans have several ways to avoid been seen by their predators. Many are transparent, with little color in their internal structures or external carapace. Individuals can be hard to see unless the light is just right. In littoral areas, they will use rocks, plants, and other structures as cover. In deeper areas of the lake, they may migrate vertically, staying in the darker deep waters while the sun is up and moving up to the surface at night. Some species of Daphnia change their physical appearance to avoid predation. They grow larger tail spines or change the shape of their heads to become more difficult to eat. See the deeper dive resources to learn more.
Most populations of cladocerans are dominated by females much of the year. The females reproduce asexually and the resulting eggs are clones of their mothers. Most produce multiple eggs in each brood and develop new broods every few days. Eggs typically remain inside the protective carapace until they are fully developed, so it is not unusual to see an adult Daphnia swimming around with a carapace full of babies. The youngsters are then released from the carapace to start life on their own. In some cases, the mother sheds her carapace to release the young. At the end of the summer or under stressful environmental conditions, the females will produce males as well as females. Mating results in an overwintering egg case called an ephippium that contains 1 or 2 eggs. These can survive for long periods of time before growing into new adults.
Daphnia magna with an overwintering egg (epipphium). Image: Dieter Ebert, Eol.org.
Cladocerans, especially Daphnia spp. are important members of a healthy lake system. In general, a robust population of large-bodied cladocerans is a good indication of a healthy lake. There are invasive cladocerans too- Bythotrephes longimanus (spiny water flea), Cercopagis pengoi (fishhook water flea), and Daphnia lumholtzi. These invasive species compete with the native cladoceran population for food. Fish however, don’t find them an easy meal because of their very large spines. Reproducing rapidly and not requiring males to mate means the introduction of a single individual can lead to an infestation very quickly. Eradication is impossible in large lakes. Cleaning equipment that moves between lakes becomes critically important to halting the spread of invasive cladocerans. Make sure you don’t forget to clean, drain, and dry!
Deeper Dive Resources