Phacus is a genus of unicellular excavates, of the phylum Euglenozoa (also known as Euglenophyta), characterized by its flat, leaf-shaped structure, and rigid cytoskeleton known as a pellicle. These eukaryotes are mostly green in colour, and have a single flagellum that extends the length of their body. They are morphologically very flat, rigid, leaf-shaped, and contain many small discoid chloroplasts.
Phacus are commonly found in freshwater habitats around the globe and include several hundred species that continue to be discovered to this day. Currently, there are 564 species of Phacus in the database, but only 171 have been accepted taxonomically. The genus was established in 1841 and since then major discoveries have led it to become an extremely large group containing hundreds of species with varied physiological characteristics. Contemporary studies agree that Phacus is not monophyletic or holophyletic, but is actually polyphyletic. Unfortunately, the phylogenetic relationships within the genus are currently poorly understood.
Etymology
The genus name is believed to have originated from the Greek word Phakos, meaning lentil or lens. This may be due to the general round or oval shape of the many species that are part of this genus. Its origins date back to the nineteenth century, in France, where it first received publication and establishment by Dujardin. The genus name is currently treated in literature as masculine.
History of knowledge
The genus was first established by the French biologist Félix Dujardin in 1841. Dujardin first published collective findings of the genus in the scientific journal Histoire naturelle des Zoophytes, Infusoires in 1841, separating them from the genus Euglena. The reason behind the separation was in order to create a group that correctly organizes their established morphological characteristics such as their rigid, flat, leaf-shape and small discoid chloroplasts with absent pyrenoids.
Christian Ehrenberg was one of the first scientists to discover and classify members of the genus; his discovery of Phacus longicauda (Ehrenberg) was one of the first species of the genus to be discovered (1830) and was used as the holotype species when describing the genus. Ehrenberg, Hubner, Dujardin, and Klebs were some of the earliest researchers of Phacus, but they all had different diagnostic criteria when classifying these organisms. Originally, Ehrenberg tried to classify three species he discovered as Euglena because of their color, but ultimately were moved to the genus Phacus by Dujardin due to the similarity they exhibited in other more prominent morphological characteristics such as shape and strip arrangement.
Since the establishment of the genus, there has been difficulty in classifying organisms due to morphology because many species of Phacus have physiological traits that closely resemble other related genera. Even the most minor difference in certain physiological properties caused great confusion in determining the proper classification for many species. This occurred more prominently during the time of Dujardin because the methodology and technology used at the time in studying these organisms were far more simplistic compared to today, leading to greater difficulty in determining critical differences between organisms. Since its inception, Phacus has undergone constant reclassification and considerable re-evaluation of different taxa in accordance with new morphological and molecular information.