By skyforbes
Ulva
has a alternation of generations. Biflagellate isogametes are
formed by certain cells of the haploid, gametangial plant. These
are liberated and fuse in pairs to form a diploid zygote which
germinates to form a separate diploid plant called the
sporophyte; this resembles the haploid gametangial plant in
outward appearance. Certain cells of the sporophyte undergo
meiosis and form zoospores in sporangia; these zoospores are
quite different to the gametes in that they form
quadriflagellate zoospores (with 4 flagella). These are
released, swim around for a time, settle and germinate to form
the haploid gametangial thallus. Note that the haploid gametes
are capable of settling and germinating without fusion to form a
haploid thallus directly; most Ulva populations
reproduce by this form of parthenogenesis and sexual
reproduction appears not to be very common. There is evidence
that gamete production in Ulva has a lunar (monthly or
bi-monthly) cycle.
Ulva can be quite a nuisance in areas that are nutrient
enriched from sewage outfalls, such as Ireland (Clonakilty Bay,
Cork Harbour and Belfast Lough), the south coast of Enlgand
(Langstone Harbour), and some estuaries in Brittany, where
populations of Ulva may cover large areas of mudflats in
the summer. Famously, a massive bloom of
Ulva prolifera
occurred in the Yellow Sea during the Olympics in June 2008
(below, Quingdao; © Weijun Duan), which is said to have
cost over $100 million to remediate. Minor blooms occur in the
rocky intertidal of most populatied countries now.