Illustrated here are the
larval and adult stages of different species of bryozoa, Phylum
Bryozoans, along with phoronid
worms and brachiopods,
are lophophorates. Although superficially different in appearance,
they all share a common feeding structure, the lophophore. This
structure is a ciliated tentacular crown surrounding the mouth.
Other body plan similarities between these organisms include a
U-shaped gut, transient reproductive system and outer casings
such as tubes, compartments or shells.
Bryozoans, traditional phylum
name Bryozoa, new phylum name Ectoprocta, are often commonly known
as moss animals. They are sessile, colonial animals that can form
erect sturdy colonies or can encrust rocks and other organisms'
carapaces forming a uniform 'moss-like' covering. The colonies
are composed of individual zooids. These zooids are often polymorphic,
with individuals performing different functions and taking forms.
The autozooids are adapted for feeding, with a U-shaped gut and
mouth located in the centre of the lophophore, with the anus opening
outside the lophophore. When the lophophore is not in use it is
retracted within the horny calcified exoskeleton and covered with
an operculum 'lid'. Other zooids include the avicularia, where
the operculum has been modified into a jaw, and the vibracula
where the operculum has been modified into a bristle.
Bryozoan zooids are mostly
hermaphroditic. Gametes are released into the water through minute
pores around the tentacles. Fertilization occurs during discharge
of the ova, when another zooid's spermatozoa are wafted by water
currents to the first zooid's lophophore. The bryozoan zygote
develops rapidly into a minute, ciliated larva. In most species
these larvae are brooded in ovicells within the parent colony
before being released into the water to then fall, after approximately
one day, to the benthos. However, many non-brooding species have
a triangular laterally compressed feeding larva (cyphonautes)
(see above left) that is immediately released and swims freely
for several months before settling to the substrate where it metamorphoses
into a typical polyp-like first zooid (ancestrula). Repeated asexual
budding then produces many polyp-like zooids that form a single
Individual zooids are usually
less than 1 mm in size but colonies may be 0.5 m or more in diameter.
These animals are rapid colonziers and colourful additions to
reef habitats, forming calcareous encrustations to continental
shelf frameworks and inshore rocks. The phylum Ectoprocta has
both freshwater and marine species. The marine representatives,
one of the most poorly known groups of marine invertebrates, can
be found in great abundance and diversity in the correct environmental
conditions. They are thought to have a wide global distribution
within the topical and sub-tropical belts.
Bryozoans are moderately
tolerant of variations in environmental conditions. They prefer
well-oxygenated, clear, moderately agitated waters. As suspension
feeders they filter small organic particles from the water currents
with their lophophore (see above right). They are themselves predated
and are ocasionally parasitized by animals such as nematodes.
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