Daphnia pulicaria
Daphnia pulicaria is a species of freshwater crustaceans found within the genus of "Daphnia," which are often called "water fleas," and they are commonly used as model organisms for scientific research Like other species of "Daphnia", they reproduce via cyclic parthenogenesis. "D. pulicaria" are filter-feeders with a diet primarily consisting of algae, including "Ankistrodesmus falcatus," and they can be found in deep lakes located in temperate climates. Furthermore, "D. pulicaria" are ecologically important herbivorous zooplankton, which help control algal populations and are a source of food for some fish. "D. pulicaria" are closely related to "Daphnia pulex , and numerous studies have investigated the nature and strength of this relationship because these species can produce Daphnia pulex-pulicaria" hybrids. In recent years, "D. pulicaria" along with other "Daphnia" species have been negatively affected by invasive predators, such as "Bythotrephes longimanus." Habitat and life history. "Daphnia pulicaria" generally live in deep, permanent lakes. These lakes provide a more stable environment than temporary ponds, which eventually dry up, so populations of "D. pulicaria" tend to have lower mortality rates than "D. pulex" populations living in ponds. Furthermore, "D. pulicaria" have a relatively long lifespan of 60–65 days. The populations of  D. pulicaria in the Great Lakes in the United States have been negatively affected by the invasive species "Bythotrephes longimanus". This invasive predator of "D. pulicaria" has also contributed to a decline of other zooplankton species in the Great Lakes. Reproduction. Cyclic parthenogenesis is the primary mode of reproduction in "D. pulicaria" and other species within the genus "Daphnia". Therefore, "D. pulicaria" are capable of switching between sexual and asexual reproduction based on environmental conditions. Typically, "Daphnia" undergo asexual reproduction when living in favorable conditions, such as in environments with abundant food or with negligible crowding. In contrast, they produce  ephippia, which are dormant eggs, and reproduce sexually if environmental conditions worsen. Some studies suggest that populations of "D. pulicaria" in lakes in North America reproduce using the expected pattern of cyclic parthenogenesis while other populations in smaller ponds have shifted toward obligate parthenogenesis. The number of offspring produced through asexual reproduction is heavily influenced by the environmental conditions experienced by an individual. For instance, females in a high-food environment with a longer photoperiod tend to have more offspring. Environmental cues, such as food level, photoperiod, and temperature, significantly influence the reproduction of "D. pulicaria." Morphology. "Daphnia pulicaria" have  a translucent carapace and two prominent second antennae, which they use to move. The carapace, composed primarily of chitin, helps protect the feeding apparatus, and it is periodically shed during an individual's life. "Daphnia" have a compound eye, and they are known to have an optomotor response. "D. pulicaria" generally have a body length less than 3mm. Because of the clear carapace of "Daphnia", it is possible to see the heart and digestive tract, which often appears to be green due to the consumption of algae. The abdominal claw is also visible toward the end of the abdomen , and it can be used to dislodge any algae from the feeding apparatus if some begins to stick. Closely related species. "D. pulicaria" are considered to be part of the "Daphnia pulex" species complex and can produce hybrids with "D. pulex". While it is difficult to distinguish between these two species using morphological traits, "D. pulicaria" and "D. pulex" have significant genomic differences. Phylogenetic studies, using mitochondrial DNA analysis, have identified genetic divergence between "D. pulicaria" and "D. pulex". For instance, variations in the "Lactate dehydrogenase" gene can help identify "D. pulicaria" from others in the "D. pulex" species complex . Model organisms. Species of "Daphnia", including "D. pulicaria," are commonly used as model organisms for studying life-history traits and phenotypic plasticity. For example, "D. pulicaria" can detect and respond to kairomones produced by predatory fish. Their sensitivity to environmental cues contributes to the observed seasonal trends in population sizes of "D. pulicaria." Moreover, because "D. pulicaria" reproduce using cyclic parthenogenesis, they are ideal models for genetic studies, including ones concerning spontaneous mutations.