Morphology image of
R. viridis showing
mitochondrion (M),
Golgi body (G) and 16 pellicle strips (arrows). Scale bar: 1 μm
Rapaza viridis is a
unicellular flagellate, a type of
protist that is capable of swimming by using two
flagella that differ in length and in movement. The cells are slender with a tapered
posterior end, measuring approximately 10–38 μm long and 3–15 μm wide. Both flagella arise from a pocket located at the anterior end of the cell, one twice as long as the other but with the same thickness. The longer flagellum, about 1.25 times the length of the cell, is always directed forward. The shorter flagellum, about 0.65 times the cell length, is directed backward, but sometimes moves forward in an oar-like motion. Like other
euglenids, cells are surrounded by a
pellicle composed of 16 protein strips arranged helically below the
cell membrane, and contain
mitochondria with discoidal
cristae. As in other
spirocutes (i.e. flexible euglenids), cells of
R. viridis are capable of '
metaboly' or 'euglenoid movement', which allows for active
peristaltic deformation of the cell shape. Its feeding apparatus consists of one rod built of four rows of
microtubules and a feeding pocket. There is a
stigma composed of 1 to over 10 pigmented particles. In the same sample where the species was discovered, the microorganism consumed native
Tetraselmis algae and grew to distinctly larger and brighter cells in their presence, digesting them completely in the course of around 12 hours. When starved from the algae, cells of
R. viridis became smaller and colorless, retaining at least one healthy
chloroplast within its
cytoplasm. During growth experiments, cells of
R. viridis were exposed to a variety of different algae (e.g.,
Navicula,
Pycnococcus,
Dunaliella,
Scrippsiella and non-native strains of
Tetraselmis) while starved from the
Tetraselmis strain that the species was found with. However, the mixotroph rejected all other preys, and could not survive for longer than 35 days without being exposed to that specific algal strain. Even under constant supply of that strain, the species could not survive for more than a week in the absence of a light source for photosynthesis. However, subsequent studies revealed that
R. viridis does not have canonical plastids. Instead, it extracts and temporarily retains the chloroplasts of its prey for its own use, a process known as
kleptoplasty ('stealing of plastids'). After
phagocytosis of the algal prey, its
cell membrane is digested and the plastids are separated from the other cellular components, which are later excreted from the host cell. Then, the stolen plastids ('kleptoplasts') are transformed until they resemble canonical plastids: they are divided into smaller fragments by fission, the green algal pyrenoid surrounded by starch disappears, smaller pyrenoids penetrated by thylakoids are formed, the starch grains gradually disappear, and a three-membrane envelope is displayed (two membranes from the original chloroplast and one membrane belonging to the
food vacuole).
Rapaza viridis needs a regular influx of kleptoplasts, obtained through the phagocytosis of its prey. Without acquiring new kleptoplasts, the cells cannot survive for more than 35 days. During starvation, the remaining kleptoplasts are gradually degraded, and
vacuoles are formed to recycle intracellular substances. == Distribution and habitat ==