Flagellates

Flagellates are a diverse group of unicellular eukaryotes that move primarily by one or more whip-like flagella. They occur in a wide range of environments, including freshwater, marine systems, soil, sediments, and the bodies of animals and plants. Most flagellates are motile, but the number and arrangement of flagella can vary; some have a single flagellum, others multiple, and some possess additional structures such as undulating membranes that aid movement.

Morphology among flagellates is highly varied. Flagella arise from basal bodies and are powered by a core axoneme, typically organized in a 9+2 microtubule pattern. Some flagellates are photosynthetic and contain chloroplasts (often from secondary endosymbiosis), while many are heterotrophic or mixotrophic. Reproduction is usually asexual by binary fission, though some species can engage in sexual processes such as meiosis or genetic recombination during specific life stages.

Ecologically, flagellates occupy important niches as primary producers, decomposers, and parasites. Photosynthetic flagellates contribute to carbon fixation in aquatic ecosystems, whereas non-photosynthetic taxa participate in various ecological interactions and can form symbiotic relationships. Several parasitic flagellates impact human and animal health, including Trypanosoma and Leishmania (causing diseases such as sleeping sickness and leishmaniasis), Giardia intestinalis (giardiasis), and Trichomonas vaginalis (trichomoniasis). Transmission typically involves insect vectors, contaminated water, or ingestion of cysts.

Notes on classification: flagellates are not a single natural group but a practical, informal collection that spans multiple evolutionary lineages within the broader eukaryotes. Taxonomic placement relies on combinations of flagellar apparatus, chloroplast presence, and developmental life cycles rather than a single defining character.