Volvox

Volvox is one of the best-known genera of green algae (chlorophytes) and is the most developed in a series of genera that form spherical colonies. Each Volvox is compsed of numerous biflagellate cells similar to Chlamydomonas, on the order of 1000 in total, interconnected and arranged in a hollow sphere (coenobium). The cells swim in coordinated fashion, with a distinct anterior and posterior – or since Volvox resembles a little planet, a ‘north and south’ pole. The cells have eyespots, more developed near the anterior, which enables the colony to swim towards light.

An asexual colony includes both somatic or vegetative cells, which do not reproduce, and gonidia near the posterior, which produce new colonies through repeated division. These daughter colonies are initially held within the parent and have their flagella directed inwards. Later, the parent disintegrates and the daughters invert. In sexual reproduction two types of gametes are produced. Male colonies release numerous microgametes or sperm, while in female colonies single cells enlarge to become oogametes or eggs.