Megan Van Der Plank - University of the Western Cape
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Transcript Megan Van Der Plank - University of the Western Cape
Annelids: The first segmented
bodies
Megan Van Der Bank
Department of Biodiversity and Conservation
Biology, University of the Western Cape
[email protected]
Available at http://planet.uwc.ac.za/nisl/Eco_people/Presentations/
Contents
1 The major groups of annelids living today
2 Conservation status of annelids
3 Why are annelids so successful
4 Medicinal use of Hirudo medicinalis
5 The ecological role of earthworms
6 The first appearance of annelids
7 The environmental conditions during periods of diversification
8 Evolutionary advantage of segmentation
9 Myzostomida as the link between flatworms and polychaetes
10 The relationship between annelids and arthropods
Major annelid groups living today
Three major classes can be distinguished, namely
Polychaeta, Hirudinea, Oligochaeta (Branch and Branch,
1981)
These groups vary significantly in the habitat and niche
that they occupy
The annelids are highly successful and ubiquitous,
occupying mostly moist environments
All members show true segmentation and have chaeta,
are protostome and triploblastic (Hickman et al, 2004)
Class: Polychaeta
Also known as bristle worms
The largest annelid group, containing as many as
10 000 species (Hickman et al, 2004)
Traditionally, free living forms (planktonic) are
called Errantaria, while sedentary forms (tubedwelling) are called Sedentaria (Branch and
Branch, 1981)
Paddle-like parapodia with chaeta, trochophore
larval stage, definite head (Hickman et al, 2004)
http://images.google.co.za/imgres?im
gurl=http://www.artbrush.net/itp/fall20
03/spatial/images/andy/nature/anneli
da.jpg&imgrefurl=http://www.artbrush
.net/itp/fall2003/spatial/nature.htm&h
=162&w=200&sz=14&tbnid=breYabe
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n&start=2&prev=/images%3Fq%3Da
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Fig 2. Free living polychaete
with parapodia
Fig 1. Ciliated trochophore larval
stage
http://www.ucmp.berkeley.edu/phyl
a/trochophore.gif
http://www.reefseekers.com/PIXPAGES/Bristle_worm.jpg
Fig 3. Free living polychaete with bristle containing parapodia
Class: Oligochaeta
Earthworms are predominantly detritus feeders that are
mainly terrestrial, but can be freshwater or occupy the
interstices of marine sediments (Branch and Branch,
1981).
They are hermaphroditic and secretes a cocoon into
which eggs and sperm are deposited, namely a clitellum
(http://en.wikipedia.org/wiki/Clitella).
Oligochaetes lack the cilliated trochophore larvae
present in polychaetes (Hickman et al, 2004).
http://www.geocraft.com/WVFossils/Carboniferous_climate.html
Fig 4.The morphology and anatomy of the oligochaete
Class: Hirudinea
Also known as leeches
The organisms contain a posterior and anterior
sucker used to attach to the exterior surface of
vertebrates such as amphibians and even humans.
However most are free-living, preying on small
invertebrates and tend to lack appendages such as
parapodia and chaeta.
Species such as Hirudo medicinalis supply heparin,
a natural anticoagulant
(http://en.wikipedia.org/wiki/Clitella/Hirudinea).
Fig 6. The leech
Zeldia.cap.ed.ac.uk/teacheng/
odl/odl6/leech.gif
http://faculty.clintoncc.suny.edu/faculty/Michael.Gregory/f
iles/Bio%20102/Bio%20102%20lectures/animal%20diver
sity/protostomes/leech_showing_suckers.jpg
Fig 5. The leech with visible anterior and posterior
suckers
Conservation status of the annelids
The annelids are highly successful, however some
vulnerable species such as Driloleirus americanus have
been identified
(http://www.redlist.org/search/detail.php?species=6828).
Extinction can mainly be contributed to habitat loss due
to development and industrialization.
Hypolimnus pedderensis as an example of an extinct
annelid species
(http://www.redlist.org/search/details.php?species=4125
4).
Why are the annelids so successful?
The success of the annelids can mainly be contributed to
their mode of reproduction.
Sexual reproduction allows better adaptation to the
environment.
Asexual reproduction via fission and regeneration allows
a fast rate of reproduction
(http://en.wikipedia.org/wiki/Annelids#Reproduction)
Segments have their own autonomy but unite to form a
common body function.
Coelomic compartments serve as a supportive
hydrostatic skeleton.
Annelids have a wide range of adaptive features.
Medical use of Hirudo medicinalis
The therapeutic use Hirudo medicinalis dates back to
ancient Egypt where it was used in bloodletting.
The medical use of leeches lost its popularity by the end
of the 19th century.
In 1884 it was discovered that the leech saliva contains a
natural anticoagulant, heparin.
With the advent of genetic engineering in 1986 heparin
could be produced in relatively large quantities.
Recently it has been used to relieve blood congestion in
compromised tissue
Researchers are currently developing a mechanical
leech (Whitaker et al, 2004)
Ecological role of earthworms
Increases soil fertility
Plays an important role in the cycling of soil organic
matter
Plays a role in soil mixing, porosity, aeration and water
holding capacity
Affects the overall soil structure (Edwards and Lofty,
1972)
The fossil record: When did
annelids first appear?
The annelids, like many other soft bodied animals, are
sparsely represented in the fossil record.
Some polychaetes leave a calcareous cement to their
tube walls allowing these tubes to be preserved in
marine sediment
(http://www.palaeos.com/Mesozoic/Cretaceous/AptianAl
ban.htm#Annelida)
The polychaete Canadia is the oldest fossil found in
Burgess shale, dating back as far as the Late
Precambrian, Early Cambrian
(http://tolweb.org/Annelida).
Fig 8. Ichnofossil of a segmented worm found during the late Cambrian
http://gpc.edu/~pgore/geology/geo102/cambrian.htm#camb.
http://www.palaeos.com/Mesozoic/Cretaceous/Im
ages/SerpulaHamulus.jpg
Fig 7. Calcareous tube fossils of polychaetes
Members of the Sepulidae, Spionidae, and Eunicida
were recovered, dating back to the Ordovician
(http://tolweb.org/Annelida).
By the end of the Carboniferous most polychaete
lineages had appeared.
Archarenicola, a member of the group Scolecida dates
back to the Triassic.
Oligochaetes evolved during the Jurassic and diversified
during the Cretaceous
Conditions during periods of
diversification.
Cambrian 542 million years ago
(http://en.wiki.org/wiki/Cambrian_explosion.htm)
Explosive adaptive radiation of most metazoan phyla
Warmer climate and higher oxygen levels.
Four major continents, Laurentia, West Eurasia, East
Eurasia and Gondwanaland were concentrated around
the equator
(http://www.ucmp.berkeley.edu/cambrian/camblife.html).
http://www.ucmp.berkeley.edu/cambrian/camblife.html
Fig 9. The major four continents around the equator during the
cambrain period
http://www.geocraft.com/WVFossils/Carboniferous_climate.html
Fig 10. Global temperatures and CO2 over geologic time
Mesozoic Cretaceous 142-65 million years ago
Period of extensive sea floor spreading along the
oceanic ridges and Gondwana fragmentation
Increased carbon dioxide levels leading to increased
global temperatures caused by the greenhouse effect
Rise in sea levels
Early Cretaceous was dominated with conifers, ferns and
cycads
Appearance of the first angiosperms leads to
diversification of oligochaetes (Hickman et al, 2004).
The evolutionary significance of
segmentation.
True metamerism is shared by annelids, arthropods and
chordates (Davis et al, 1999)
The advent of segmentation allowed the development of
greater complexity in structure of function.
Segments are a repetition of body units and are able to
function independently.
Segmentation allow better flexibility and increased the
efficiency of burrowing in annelids (Hickman et al, 2004).
Myzostomida as the link between
polychaetes and flatworms.
Myzostomida is frequently classified within annelida but
are actually more closely related to flatworms.
Ultrastructural evidence suggests that the segmentation,
chaeta and trochophore larvae of Myzostomida are
homologous to those of annelids.
The ancestor of myzostomids, flatworms and
trochozoans is segmented, worm-like with chaeta and a
trochophore larval stage (Eeckhaut et al, 2000)
The relationship between annelids
and arthropods
Arthropods and annelids evolved segmentation
separately.
The last common ancestor of arthropods and annelids
was unsegmented but possibly had repeating organ
systems resembling that of some large flatworms or
nemerteans.
The arthropods became segmented and evolved jointed
appendages while annelids evolved segmentation and
retained their flexible epidermal cuticle (Valentine, 1990).
References
Branch G, Branch M (1981) Living Shores of Southern Africa. Struik
Publishers, Cape Town.ISBN 0869771159, pp272.
Davis G and Patel N (1999) The origin and evolution of segmentation.
Trends in Genetics 15(12)M68-M72
Edwards C, Lofty J (1972) Biology of earthworms. Chapman and Hall
LTD, London. ISBN 412110601,pp 283
Eeckhaut I, McHugh D, Mardulyn P, Tiedemann R, Monteyne D,
Jangoux M, Milinkovitch C (2000) Myzostomida: A link between
Trochozoans and Flatworms. Biological Sciences 267(1451)1383-1392
Hickman C, Roberts L, Larson A, I’Anson H (2004) Integrated principles
of Zoology. McGraw Hill, New York. ISBN 0072439408, pp 872
Valentine J (1990) Molecules and the Early Fossil Record. Paleobiology
16(1)94-95
Whitaker J (2004) Historical Article:Hirudo medicinalis : ancient origins
of, and trends in the use of medicinal leeches throughout history. British
Journal of Oral and Maxiillofacial surgery 42(2)133-137