|Ectoprocts and other animals fouling a
When a plant grows somewhere that humans do not want it
to grow, we call it a “weed.” Sessile organisms that settle
and grow on pilings, boat hulls, pipes, cables, and other
structures placed there by humans are referred to as “fouling
organisms.” Since we do not want them there, we might call
them the marine equivalent of terrestrial weeds. And like
terrestrial weeds, they are very persistent.
Members of one phylum covered in this section, Ectoprocta,
are among the most important fouling organisms,
especially on ship and boat hulls. Fouling of boat hulls
causes turbulence as the vessel proceeds through water, and
increased resistance decreases speed of the vessel and
increases its fuel consumption. It is costly to scrape the
organisms from a boat hull either in dry dock or in water.
Consequently, boat hulls have often been painted with
paints containing a toxic antifouling agent. One of the most
effective of these is a substance called tributyl tin (TBT).
After application in paint, TBT is released at a low rate over
a long period of time, making scraping and repainting necessary
less frequently. Unfortunately, release of the toxin
into seawater, especially in harbors and basins where many
boats are concentrated, has catastrophic effects on many
organisms, particularly bivalves, which concentrate the compound
in their tissues.
In 1988 the U.S. Congress passed a law severely restricting
use of TBT in antifouling paints, and the problem should
be considerably alleviated by this law.
Ironically, eighteenth-century naturalists included ectoprocts
(along with cnidarians and some others) in a group
designated “zoophytes,” meaning “animal plants.” These
early workers thought that zoophytes were akin to both animals
and plants. Comparing ectoprocts with plant weeds
gives a new meaning to the term zoophytes.
Position in the Animal
- Members of lophophorate phyla
possess a true coelom, a body cavity
lined with a layer of mesodermal
epithelium called peritoneum.
- They belong to the protostome branch of the bilateral animals,
but they have some characteristics
typical of deuterostomes.
- The three phyla are usually grouped
together because they all possess a
crown of tentacles called a lophophore, which is specialized
for sedentary filter feeding. The
lophophore surrounds the mouth
but not the anus, thus differing from
the tentacular crown of Entoprocta.
- The lophophore is a unique ridge
that bears hollow, ciliated tentacles,
and it is an efficient, specialized filterfeeding
device that forms a ciliated
route, or trough, for trapping and
directing food particles to the mouth.
- Brachiopods and phoronids possess
vascular systems for circulation
of food nutrients and other
- Blood in phoronids possesses red
blood corpuscles that contain
hemoglobin for carrying oxygen.
Phoronida are wormlike marine forms
that live in secreted tubes in sand or
mud or attached to rocks or shells.
Ectoprocta are minute forms, mostly
colonial, whose protective cases often
form encrusting masses on rocks,
shells, or plants. Brachiopoda are
bottom-dwelling marine forms that
superficially resemble molluscs because
of their bivalved shells.
One might wonder why these
three apparently different types of animals
are lumped together in a group
called lophophorates. Actually they
have more in common than first
appears. They are all coelomate; they
have some deuterostome and some
protostome characteristics; and none
has a distinct head. But other phyla
share these characteristics. What really
sets this group apart from other phyla
is the common possession of a ciliary
feeding device called a lophophore
, crest or tuft, + phorein
A lophophore is a unique arrangement
of ciliated tentacles borne on a
ridge (a fold of the body wall), which
surrounds the mouth but not the anus.
The lophophore with its crown of tentacles
contains within it an extension
of the coelom, and the thin, ciliated
walls of the tentacles are not only
an efficient feeding device but also
serve as a respiratory surface for exchange
of gases between the environmental
water and the coelomic fluid.
The lophophore can usually be extended
for feeding or withdrawn for
In addition, all three phyla have a U
-shaped alimentary canal, with the
anus placed near the mouth but outside
the lophophore. The coelom is
primitively divided into three compartments, protocoel
, and the mesocoel
into the hollow tentacles of the lophophore.
The protocoel, where present,
forms a cavity in a flap over the mouth,
. The portion of the body
containing the mesocoel
is known as
, and that containing
is the metasome. Members
of all three phyla have a freeswimming
larval stage but are sessile