As the foregoing quotation so colorfully indicates, Volvox often appears in great abundance within days after the warm rains of early summer accumulate in depressions in the ground, and it is often joined by several of its colonial relatives. The ability of these volvocaceans to appear as soon as vernal pools are formed (or as soon as temperate lakes have thawed and stabilized in the spring) depends on a feature that they share with Chlamydomonas and many other green algae: When conditions begin to deteriorate toward the end of the growing season, they switch from asexual to sexual reproduction and produce dormant zygotes, or “zygospores,” that are resistant to desiccation and freezing (Coleman 1983). These zygospores settle into the mud to wait out the adverse times; then, once favorable conditions return, they quickly germinate, and the germlings swim toward the surface and begin proliferating asexually.

Many volvocaceans (like many other algae) are astonishingly cosmopolitan, being found in similar environments around the world. For example, isolates of Gonium pectorale from Europe, Asia, and all parts of North America have been shown to be members of an interfertile population that is capable of sharing a single gene pool and is accordingly quite homogeneous at the DNA-sequence level (Stein 1966a,b; Coleman et al. 1994). Even “species” such as Pandorina morum that are extensively subdivided into reproductively isolated units, or syngens, are actually cosmopolitan, because members of a single syngen can be found on several different continents (see Chapter 2).