Download LOPHOTROCHOZOA: LOPHOPHORA AND ANNELIDA

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Biology 1B – Animal Diversity and Evolutionary Principles
Spring 2011
Figure source: Brusca and Brusca, 2003. Invertebrates, second edition. Sinauer Associates,
Inc.
LOPHOTROCHOZOA: LOPHOPHORA AND ANNELIDA
Let's take a closer look at the word
"Lophotrochozoa." The first part "lopho-" refers to
the circular or horseshoe-shaped ring of coelomate,
ciliated feeding tentacles, the lophophore, possessed
by a clade of animals called
the Lophophora. We've
already seen how the
"trocho-" part of the word
refers to the trochophore larva of molluscs and annelids. This
larva bears one or more bands of cilia, called trochs, which it uses to
swim and to feed.
Lophophoran subclades (formerly phyla):
Phoronida – unsegmented worms, live in tubes in sand
Brachiopoda – lamp shells
Bryozoa – colonial animals
Trochozoan subclades (formerly phyla and classes):
Phylum Annelida
Polychaeta – marine segmented worms
Oligochaeta – earthworms
Hirudinea – leeches
Pogonophorans – deep-sea beard worms
Echiurans – spoon worms
Sipunculans – peanut worms
Phylum Mollusca (covered in a separate lab)
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Lophophora: Bryozoans
The Bryozoa are the only entirely
colonial phylum of animals. You will
(hopefully!) recall that we have
already encountered the phenomenon
of coloniality when we studied the
cnidarian hydrozoans. Is this an
example of convergence or
divergence? As in the hydrozoans,
bryozoan colonies come in a variety of
shapes and colors, from encrusting
sheets to upright foliaceous forms.
Some grow only on specific surfaces,
such as kelp blades, and others grow
just about everywhere. Although most
of the ~4000 bryozoan species are
marine, there are some freshwater
species.
A bryozoan colony is made up of interconnected units called zooids. Each zooid lives in its
own tiny exoskeleton and has its own lophophore, which it extends for suspension feeding.
As in the hydrozoans, some bryozoan colonies exhibit polymorphism, where the zooids
become specialized for separate functions such as feeding, reproduction, and defense.
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Lophophora: Brachiopods
The extant brachiopods are a mere
fraction of the brachiopod diversity that
flourished in Paleozoic seas. The endPermian mass extinction (~250 mya)
slammed the brachiopods, which had
until then been the dominant bivalved
invertebrates.
So, how would you distinguish between
a bivalved mollusc and a brachiopod?
The easiest difference is in the shells.
Molluscan bivalves have two shells, left
and right, and generally look the same.
Brachiopods have two shells but they
are dorsal and ventral, and are unequal
in size, with the ventral shell generally
being smaller.
The entire body is enclosed within the dorsal and ventral shells, which open just a tiny bit to
allow water to flow over the lophophore. The lophophore itself is curled up and may be
supported by an armlike structure called the brachium.
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LOPHOTROCHOZOA: Phylum Annelida (the segmented worms)
The Phylum Annelida has traditionally been the phylum of segmented worms, best
represented in the sea by members of the Class Polychaeta and on land by the Classes
Oligochaeta (earthworms) and Hirudinea (leeches). However, recent molecular studies
indicate that the group of (mostly) wormlike creatures called annelids might be more properly
categorized into two other subclades, the Errantia and the Sedentaria. In this scheme, the
Sedentaria comprises the oligochaetes, hirudineans, echiurans, and some polychaetes, while
the remaining polychaetes make up the Errantia.
The good thing about the names Errantia and Sedentaria is that for the most part they
correctly describe the animals: the errant worms generally crawl around, and sedentary ones
don't. In errant polychaetes segmentation is usually visible externally as a series of
annulations in the body wall, each of which bears a pair of lateral parapodia with setae.
Sedentary polychaetes, which don't move around as much as their errant relatives, often have
reduced parapodia and external segmentation. Some tube-dwelling polychaetes, namely
Chaetopterus, demonstrate what appears to be a form of serial homology, having a
heteronomous body that is divided into distinct functional regions. We will revisit the
concept of serial homology when we discuss arthropods.
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Here are some photos of tube-dwelling polychaetes. Are any of them familiar to you?
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Errant polychaetes tend to be predators or scavengers (or both). Their roaming lifestyle
allows them many opportunities to encounter prey, and some of them are armed with
elaborate weapons to subdue prey. Some polychaetes have an eversible pharynx that can be
shot out of the mouth at prey and is armed with either one or two pairs of chitinous jaws that
can be large enough to take a chunk of flesh out of your hand.
Living in a tube severely limits an animal's ability to forage actively for live prey, so
tube-dwelling polychaetes tend to be suspension feeders or deposit feeders. Suspension
feeders feed on particles in the water column, either passively (catching particles that fall on
them) or actively (using metabolic energy to move water through a filtering apparatus of some
sort); the latter are called filter feeders. The figure below diagrams suspension feeding in
two polychaetes.
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Deposit feeding polychaetes collect organic matter that settles onto the substrate around
them, and most of them live in burrows. Active deposit feeders actually ingest the
surrounding substratum and digest the associated organic materials; much of their fecal matter
consists of sediment that passes unprocessed through the digestive system. Selective deposit
feeders are able to sort the desired organic matter from the indigestible sediments before
ingesting anything.
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LOPHOTROCHOZOA: Annelida (Subclade Oligochaeta)
The oligochaetes (earthworms and kin) live mostly on land or in fresh water. Earthworms are
important members of the underground community. They aerate soils, and thus prevent
compaction, and drive the decomposition of buried plant material. Their castings (worm
poop) make great organic fertilizers.
Australia is a laboratory of
natural selection, and
some mighty odd animals
have evolved (and been
driven extinct by humans)
there. One example is the
giant Australian
earthworm, Megascolides
australis. It routinely
grows to lengths of 2 to 3
meters!
Earthworms are hermaphrodites, possessing functional ovaries and testes at the same time.
When the worms mate, they line up anti-parallel (what does that mean?) and exchange sperm
to fertilize their own eggs. Then each worm goes off to deposit its fertilized eggs in a cocoon
in the ground, where they hatch into little wormlets.
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LOPHOTROCHOZOA: Annelida (Subclade Hirudinea)
Leeches are highly derived annelids, lacking both external segmentation and bristles. They
are a sister group to the Oligochaeta. Although many of them are free-living predators, the
more notorious ones are parasites that feed on the blood of vertebrates. They live in
freshwater ponds and streams.
The parasitic leeches can detect a warm-blooded animal and home in on it. Their mouth
contains a set of three jaws to pierce the skin of their prey. They also have a very complex
saliva that contains: (1) an anaesthetic, to keep the prey from reacting to the bite; (2) a blood
vessel dilator, and (3) an anticoagulant to keep the blood flowing.
Although they seem gross to us, leeches have been used medicinally for several centuries, and
have even been incorporated into modern medicine. They can help reduce swelling after
injury or surgery, and are most helpful when surgeons are attaching lost limbs or digits.