Volvox

Volvox, commonly known as green algae, is a group of freshwater free-floating organisms, divided into about twenty species. They are primitive, multi-celled organisms belonging to the plant kingdom, forming a direct link between the lower single-celled organisms, and the higher complex organisms.

It forms spherical or oval colonies, as small as the size of a pinhead with a hollow mucilaginous mass, with each colony containing almost 50,000 cells. Dutch microscopist Antonie van Leeuwenhoek first observed Volvox in 1700.

Where do they Live

Volvox are found throughout the world, inhabiting various temporary and permanent freshwater-bodies such as tanks, ponds, lakes, ditches, and puddles. They exist by forming colonies known as coenobium. They multiply fast in nutrient-rich environments, especially during the rainy season, often causing their habitat water-bodies to turn green.

Domain	Eukaryota
Kingdom	Plantae
Phylum	Chlorophyta
Class	Chlorophyceae
Order	Chlamydomonodales
Family	Volvocaceae
Genus	Volvox
Common Species	V. aureus, V. carteri, V. globator, V. barberi, V. rouseletti, V. dissipatrix, and V. tertius

Size

The individual cells of Volvox are 16.25 µm in length, which may vary according to species. The diameter of their colony ranges between 100 to 6000 µm.

Vegetative Body

The colony of Volvox forms a hollow ball-like structure in which the cells appear green. Every motile colony can swim freely in their environment. The individual pear-shaped cells are arranged in a single layer joined by delicate strands of cytoplasm. Each colony develops three types of cells:

Vegetative Cells: These contain flagella, and are capable of locomotion, and are capable of food production. But they are unable to form new colonies.

Asexual Reproductive Cells: These are larger compared to the vegetative cells. They produce a motile asexual spore called zoospore.

Sexual Reproductive Cells: These cells produce sex cells, namely sperms and eggs.

Parts

1. Cell Wall: Forms the outermost boundary of the cell and protects it from outside shock.
2. Plasma Membrane: Forms the inner boundary of the cell with the protoplast. It is semi-permeable in nature that protects the inner cellular contents from the outside environment.
3. Cytoplasm: Has a central cytoplasm that forms the inner mass of the cell, containing all other organelles such as mitochondria, ribosomes, endoplasmic reticulum, and others. 
4. Chloroplast: Has a cup-shaped chloroplast present at the broader end of the cell. It is responsible for performing photosynthesis. Within the chloroplast are found a single or multiple pyrenoids, structures that help the chloroplast fix carbon required during photosynthesis.
5. Eyespot: A photoreceptive organ that senses light and thus helps the cell to survive by performing photosynthesis. Eyespot also helps the cell to move with the help of flagella.
6. Nucleus: Contains a single central nucleus that helps in cell division.
7. Contractile Vacuoles: They are the organs present at the base of the flagella through which Volvox get rid of their wastes. There are two to three vacuoles present per cell.
8. Flagella (Cilia): Each cell has two equal-sized whiplash flagella (cilia) that projects outwards.

Most species can be viewed only under a microscope due to their small size, except a few that can be as big as 1 mm in diameter. Common behaviors studied in Volvox are given below:

What do Volvox Eat

Each species of Volvox obtains their food through photosynthesis due to the presence of the pigment chlorophyll inside their chloroplast, and thus are autotrophic in nature. Their food is stored in the form of carbohydrate, which provides them the energy required for their growth and reproduction.

How do they Move

Movement in Volvox is achieved by the coordinated action of the flagella present in all cells of the colony. The entire colony rolls over the water surface, and hence Volvox is also called the rolling algae. The eyespot also controls the movement of the flagella by sensing light with the help of their photoreceptive organs.

How do they Reproduce

Reproduction in Volvox can be both sexual and asexual. Under favorable growth conditions, during the summer season, they perform asexual reproduction, whereas sexual reproduction occurs at the end of the season. The presence of sex-inducing chemicals called pheromones also determines the type of reproduction they perform.

Asexual Reproduction

1. A few cells at the posterior portion of the colony or coenobium enlarge in size by storing food. These fully mature cells are called gonidia (singular, gonidium). The number of such asexual cells varies between 2 to 50 in each colony.
2. The cells then lose their flagella and become rounded with the presence of dense cytoplasm.
3. Next, they enter the phase of division. The first division of the gonidium is longitudinal to the plane of the colony, forming two cells.
4. The first division is followed by a second, which is perpendicular to the plane of the first division, forming four cells.
5. During the third longitudinal division, the four cells divide further to produce eight cells. This division continues until the number of cells reaches the desired number found in a species.
6. Next is the colony inversion, in which the cells face in the opposite direction with their anterior ends facing the periphery of the colony.
7. Finally, the cells develop flagella, and the daughter colony comes out by rupturing the mother colony. During all this period, the cells exist in their haploid, gametophyte (n) stage.

Sexual Reproduction

It involves a male sex cell called antheridia and a female sex cell known as oogonia, together called gametangia.

1. Some cells of the posterior portion of the colonybecome enlarged and develop into antheridia or oogonia.
2. During the growth of gametangia, the cells become rounded and lose their flagella.
3. One male sex cell fuses with one female sex cell during fertilization, thus forming a zygote or oospore. This stage marks the beginning of the diploid (2n), sporophyte stage. After the completion of fertilization, the oospore develops a thick wall around it.
4. The zygote then breaks open the parent colony, sinks to the bottom of the water, and enters a resting period.
5. Under favorable conditions of growth, the diploid nucleus of the zygote divides by meiosis to produce four haploid nuclei, of which three degenerates and one survives.
6. The single nucleus escapes the parent colony by swimming and forms a zoospore that develops into a new colony. The cells of the colony perform asexual reproduction for the next few generations.

Thus, the lifecycle in Volvox mainly consists of the dominant free-living haploid (n) gametophyte stage and a less dominant sporophyte stage, represented by the diploid (2n) zygote.

• Being a primary food producer in the freshwater ecosystem, it serves as a food source for several organisms living in that ecosystem, especially for the microscopic invertebrates called rotifers.
• It produces oxygen through photosynthesis that helps to sustain life in freshwater-bodies.
• Provides a cheap source of food in fish cultivation (pisciculture).
• Being easyto cultivate, having relatively few cells, and a fast multiplication rate, it is commonly used as a model organism that helps understand the complex life processes in higher animals, including humans.