Phacus longicauda

Phacus longicauda is a species of flattened, pointed single-celled protist microalgae belonging to the phylum Euglenozoa. It is a solitary and free-swimming algae.

Phacus longicauda has a very striking and rigid periplasm, with spiral bands, and numerous small disc-shaped chloroplasts. The cell is flat and rigid and powered by a flagellum at the front of the cell. There is a highly visible red eye spot near the flagellar base. These red eyespots are present in most Euglenoïds.

Phacus longicauda is found in freshwater habitats around the world. These habitats include ponds, pools and standing water. An organically enriched freshwater environment is essential for the development of these species.

Phacus species sometimes have a peculiar twisted shape. They are photosynthetic unicellular organisms, which means that they are able to produce their own food. Although the genus receives their nutrients primarily through photosynthesis, they are also able to feed on certain types of algae and bacteria.

Most of these organisms also have a semi-rectangular eyespot, often reddish in color, and a single flagellum, although some species have two that emerge. The flagellum is responsible for movement of the cell by rotating in the direction of travel, which allows the cell to slide and swim in the water.

At the base of the flagellum, adjacent to the eyespot, lies the paraxial swelling that functions in photoreception. It is the intricate coordinated interactions of the eyespot, paraxial swelling and some other parts of the cell that enable the cell to swim to and from the light. The ‘eyespot’ designation is therefore somewhat misplaced because it is only part of the photosensitive device and is not the photoreceptor itself, which is the paraxial swelling.

For detailed descriptions, see the standard work "Freshwater Algae of North America, Ecology and Classification".