Colony growth in both marine and freshwater bryozoans
occurs through asexual budding of zooids. A colony is founded
when a sexually produced larva attaches to a substrate and metamorphoses
into a single founding zooid, or ancestrula. This
immediately commences budding and the colony develops as a
single genetic entity, comprising a few to many contiguous zooids;
colony form may be indeterminate, as in the sheet-encrusting
Membranipora (Figure C), or have an ordered structure, as in
the fan-shaped or spiraled branching colonies of Bugula species.
All bryozoan species are hermaphroditic; zooids are most often
protandric hermaphrodites, a given zooid forming sperm and
then eggs in sequence, but in many species separate male and
female zooids are found in a single colony. Gonads lack ducts;
sperm discharged into the body cavity leave through pores in the
lophophore and are entrained in the ciliary currents of female
zooids, and fertilization occurs within the body cavity of the
female. Embryos may develop within the body cavity, the polypide
degenerating as the embryo increases in size, or within a brood
chamber, or ovicell. Fertilized eggs pass from the female body cavity
through a pore (coelomopore) at the base of the lophophore
and into the associated ovicell. In Bugula ovicells are single modified
zooids, each attached to the maternal zooid; in other species
the ovicell may be a part of the female zooid, or a compound
polymorphic structure. Cleavage is radial, and in most marine
bryozoans the egg develops as a nonfeeding coronate larva that
settles within hours of release. In a minority of species, the larva is
a type termed a cyphonautes, which is enclosed within a bivalved
chitinous shell, has a functional gut, and a planktonic existence
of several weeks. Coronate larvae have a girdle or crown of cilia
used for swimming, an anterior tuft of long cilia, and a posterior
adhesive sac. At first, larvae are positively phototactic and swim
toward light, but soon become negatively phototactic and tend to
settle in shade. Settlement is induced by biological stimuli, microbial
films, and many species settle on only a narrow range of substrata;
in particular, epiphytic species may be restricted to just one
or two algal species.
Colony growth in both marine and freshwater bryozoans
occurs through asexual budding of zooids. A colony is founded
when a sexually produced larva attaches to a substrate and metamorphoses
into a single founding zooid, or ancestrula. This
immediately commences budding and the colony develops as a
single genetic entity, comprising a few to many contiguous zooids;
colony form may be indeterminate, as in the sheet-encrusting
Membranipora (Figure C), or have an ordered structure, as in
the fan-shaped or spiraled branching colonies of Bugula species.
All bryozoan species are hermaphroditic; zooids are most often
protandric hermaphrodites, a given zooid forming sperm and
then eggs in sequence, but in many species separate male and
female zooids are found in a single colony. Gonads lack ducts;
sperm discharged into the body cavity leave through pores in the
lophophore and are entrained in the ciliary currents of female
zooids, and fertilization occurs within the body cavity of the
female. Embryos may develop within the body cavity, the polypide
degenerating as the embryo increases in size, or within a brood
chamber, or ovicell. Fertilized eggs pass from the female body cavity
through a pore (coelomopore) at the base of the lophophore
and into the associated ovicell. In Bugula ovicells are single modified
zooids, each attached to the maternal zooid; in other species
the ovicell may be a part of the female zooid, or a compound
polymorphic structure. Cleavage is radial, and in most marine
bryozoans the egg develops as a nonfeeding coronate larva that
settles within hours of release. In a minority of species, the larva is
a type termed a cyphonautes, which is enclosed within a bivalved
chitinous shell, has a functional gut, and a planktonic existence
of several weeks. Coronate larvae have a girdle or crown of cilia
used for swimming, an anterior tuft of long cilia, and a posterior
adhesive sac. At first, larvae are positively phototactic and swim
toward light, but soon become negatively phototactic and tend to
settle in shade. Settlement is induced by biological stimuli, microbial
films, and many species settle on only a narrow range of substrata;
in particular, epiphytic species may be restricted to just one
or two algal species.
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