Volvox

Volvox is a remarkable colonial green alga that consists of many individual cells living together in a spherical colony. 

1.0Systematic Position of Classification 

This Volvox classification places within the green algae division Chlorophyta, specifically within the class Chlorophyceae. The order Chlamydomonadales encompasses a variety of organisms with colonial structures, while the family Volvocaceae contains species related to Volvox. The genus Volvox represents a group of colonial green algae characterized by their distinctive spherical colonies.

2.0Characteristics of Volvox 

Colonial Structure: Volvox forms a colony composed of numerous individual cells that collectively create a spherical or globular structure known as a coenobium.

Coenobium Structure: The colony of Volvox comprises numerous vegetative cells forming a coenobium, a globular structure and fuctions held together by a gelatinous sheath. The number of cells in a colony ranges from 500 to 50,000, each enclosed in its mucilaginous envelope. This signifies Volvox as an association of independent cells, not a single organism.

Cell Morphology: Resembling Chlamydomonas cells, individual Volvox cells possess two anterior flagella emerging from basal bodies, extending beyond the colony's surface. They feature cup-shaped chloroplasts with pyrenoids and multiple contractile vacuoles.

Cooperative Flagellar Movement: Volvox cells demonstrate cooperation in flagellar movement but operate independently in nutrition, respiration, and excretion functions.

Photosynthesis in Volvox: As a photosynthetic organism, Volvox performs photosynthesis within individual cells, specifically in the chloroplasts. This process converts light energy into chemical energy, producing glucose from carbon dioxide and water while releasing oxygen as a byproduct.

Also Read Photosynthesis in Higher Plants

Asexual Reproduction in Volvox

• Some cells of the posterior region withdraw the flagella, become larger in size and differentiate into asexual reproductive cells called gonidium or parthenogonidia. In a coenobium 2- 50 gonidia may be produced. 
• The gonidium divides mitotically several times mostly longitudinally. The group of 8 daughter cells form a curved plate-like structure called Plakea stage.
• After the 16-celled stage, the cells are arranged at the periphery of a hollow sphere having an opening called phialopore. 
• When the cell division stops, the cells of the sphere turn inside out passing through the phialopore by the inversion process. Now the individual cells develop flagella and become part of a new colony. This new colony may or may not escape from the parent colony.

Sexual Reproduction in Volvox 

• In Volvox, sexual reproduction takes place during unfavorable seasons. It is a strictly oogamous type. Depending on the species, the coenobia may be monoecious or dioecious.
• Monoecious species are protandrous. In dioecious species like, the male and female sex organs produce in two different colonies. Certain special cells in the posterior region of a mature colony enlarge, lose its flagella to become gametangia. The male gametangia are called antheridia and female oogonia. 
• The cell antheridium divides successively forming many spindle- shaped biflagellate antherozoids similar in manner to the development of zoospore, passing through Plakea-stage before inversion and by inversion process through the phialopore. Usually only a few cells in the coenobium develop into the antheridium. The cell which has fate to form the oogonium enlarges considerably many times as compared to the normal vegetative cell.
• During fertilization, the entire mass of the antherozoids are liberated from the colony which reaches to the egg cell of another coenobium. Their individual antherozoids are set free and one of them fuses with the egg cell and forms zygote which is called oospore. 
• The oospore subsequently secretes the three-layered wall which may be smooth or spiny depending upon the species. It also accumulates the haematochrome which imparts its orange-red color. Oospore is released by the rupture of the colony. It is the perennial body of the Volvox. 
• After a period of rest, During germination, the oospore divide meiotically to form four nuclei, out of which three degenerate while remaining one divides and re-divides mitotically to form the new coenobium.

Also Read Sexual Reproduction in Flowering Plants