8 Euglenoids

The euglenoids, also known as euglenids, euglenophytes, or euglenoid flagellates, are among the most ancient lineages of eukaryotic algae. Many euglenoids lack plastids, while others possess secondary green or colorless plastids (Whatley 1993) and thus vary in nutritional type. Non-photosynthetic euglenoids feed upon organic particles such as bacteria or unicellular eukaryotes or absorb dissolved organic compounds, and all require vitamins B₁ and B₁₂. Such nutritional variability explains why diverse euglenoids often occur in habitats rich in decaying organic matter, where dissolved organic compounds and bacteria are abundant. Euglenoids can be abundant in farm ponds, marshes, swamps, fens, bogs, oceanside beaches, and mud flats, even populating the hindguts of dipteran larvae and tadpole rectums. Their nutritional diversity also explains how euglenoids can ecologically link components of aquatic microbial food webs. In this chapter we look at how comparative studies of euglenoid nutrition, cellular structure, behavior, and molecular gene sequences have revealed the relationships and evolutionary history of euglenoids.