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Introduction

Euglenids (sometimes referred to as “euglenoids”) are a prominent group of free-living, aquatic flagellates with diverse modes of nutrition, including bacterivory (generally consumers of small prey cells like bacteria), eukaryovory (generally consumers of large prey cells like other microeukaryotes), osmotrophy (absorbers of organic molecules) and photoautotrophy (referred to as "euglenophytes"). The presence of both phagotrophic and photautotrophic species within the same group has led to redundant classification schemes under the International Code of Botanical Nomenclature and the International Code of Zoological Nomenclature (so-called “ambiregnal taxa”). Euglenids are neither plant nor animal, so the group does not fall neatly within the archaic plant-animal dichotomy that still pervades biological thinking.

Approximately 1,400 species of euglenids have been described so far, and it is possible that at least twice that many await discovery. The amount of morphological and behavioral diversity present in this group is exceptionally high (e.g. see title illustrations and Fig. 1) and provides compelling evidence for major events in eukaryote evolution, such as the punctuated effects of secondary endosymbiosis and mutations in underlying developmental mechanisms (Leander et al. 2007). Several photosynthetic and osmotrophic species are bloom-formers in nutrient-rich conditions and are useful indicators of environmental pollution. Phagotrophic species (i.e. bacterivores and eukaryovores) are ubiquitous primary consumers and are important components of microbial food webs across the globe. Knowledge of euglenids extends back to the invention of the first microscope in the late 1600’s by Leeuwenhoek, and a few of these species have subsequently been used as model systems for addressing a wide variety of questions in basic cell biology. Euglena gracilis, for instance, is familiar to nearly every student who has ever taken a general biology course in high school, college or university.


Fig. 1. Scanning electron micrographs showing the relative number of pellicle strips and the size differences between a small bacterivore (left, with 4 pellicle strips) and a large euglenophyte (right, with 48 pellicle strips). © Brian S. Leander

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