Are physalia sessile?
No, unlike many other Cnidarins, they never attach to a surface
at any stage. Mature Physalia reproduce sexually by releasing
either male or female gametes into the water where the actual
fertilization takes place. This usually is triggered when a large
group of mature individuals happen to gather together, increasing
the fertility rate.
The free-swimming, larval offspring then starts to grow and
differentiate into specialized individuals both by mitotic cell
division and by asexual 'budding'. Budding results in individuals
that share the same genetic information with other individuals in
that colony, however they remain attached to each other to form a
colony. Further mitotic cellular replication, under the influence
of genetically controlled mechanisms, allows individuals to
differentiate into one of four specialized types of zooids, much
like the differentiation in multi-cellular animals of their various
types of tissues and organs. The difference, of course, is that
most multi-cellular animals are only one individual instead a
collection of attached individuals. Asexual budding continues
throughout the life cycle of the colony, constantly replacing or
adding individuals as the need arises.
The Physalia physalis species near Florida has been studied
intensely. They grow about 32mm in float length per month, and
reach sexual maturity in nine months. While individuals can live
hardly longer than a year, the entire population reproduces in an
annual cycle. The asexual, nutritional phase of budding allows a
fast growth cycle, while the sexual reproductive phase allows for
genetic variation.