Flagellale

Flagellale, also called flagellates in English, denotes a broad, informal group of mostly unicellular eukaryotes defined by the presence of one or more flagella used for locomotion. The term is descriptive rather than a formal taxonomic rank and encompasses organisms across multiple supergroups.

Flagellates display great diversity in form and ecology. Their cells may be bare or possess a strengthening cell covering, and flagella can vary in number, arrangement, and surface structures. Movement is primarily driven by whip-like flagella, though some species use two or more flagella in coordinated beating. Many flagellates are free-living in freshwater, marine, and soil environments; others are symbionts or parasites.

Nutritional modes are likewise diverse. Some flagellates are photoautotrophic, possessing chloroplasts and contributing to primary production (for example, certain euglenids). Others are strictly heterotrophic, ingesting bacteria or other small organisms, while many are mixotrophic, combining photosynthesis with phagocytosis depending on environmental conditions.

Reproduction is usually asexual, most commonly by binary fission. In several lineages, sexual processes such as meiosis or syngamy occur under stress or in particular life stages, contributing to genetic diversity. Life cycles can be simple or involve complex stages and cyst formation.

Taxonomy and classification have shifted with molecular data. Flagellates are spread across multiple major eukaryotic groups, including Excavata, SAR, and others, and are not a single coherent clade. Consequently, flagellate is best used as a functional or ecological descriptor rather than a strict taxonomic category.

Ecological and practical importance varies by lineage. Flagellates contribute to nutrient cycling and carbon flow in ecosystems, include important model organisms in biology, and contain several notable parasites and pathogens.