Volvox

Volvox is a genus of freshwater green algae in the family Volvocaceae. It forms spherical colonies that range from about 0.5 to 1.0 millimeter in diameter, consisting of many thousands of biflagellate cells embedded in a gelatinous matrix. The colonies are often visible to the naked eye and exhibit coordinated movement as a single organism.

Each colonial cell bears two flagella and a single cup-shaped plastid; many cells function as somatic, providing locomotion and light sensing, while a smaller number act as reproductive cells, or gonidia, that give rise to new colonies. The cells are connected by cytoplasmic strands, enabling intercellular communication and synchronized beating that drives the colony's rotation and steering.

In asexual reproduction, gonidia divide by mitosis to produce daughter colonies inside the parent. As the daughter colonies mature, they become distinct, and the parent colony eventually ruptures to release them, allowing rapid population expansion in favorable conditions.

Under stressful or seasonal conditions Volvox can reproduce sexually. Some colonies differentiate as males, producing packets containing sperm; others develop eggs. Fertilization yields a zygote with a thick protective wall that can enter dormancy; when conditions improve, the zygospore germinates to produce a new haploid Volvox colony.

Volvox is widely distributed in freshwater habitats worldwide, especially in warm, nutrient-rich waters. It has been a model organism for studying the evolution of multicellularity, differentiation, and development, as well as phototaxis and colony-level behavior. The genome of Volvox carteri has been sequenced, providing insights into gene families involved in cell differentiation and reproductive development and enabling comparisons with unicellular relatives such as Chlamydomonas reinhardtii.