Download Phylum Cnidaria

Lecture 2
Phylum Cnidaria
Anemones, corals, jellyfishes,
hydras and relatives
by John R. Finnerty
Animal Phylogeny
Calcispongia
Silicispongiae
Porifera
Ctenophora Cnidaria
Deuterostomia
Ecdysozoa
Chordata
Arthropoda
Hemichordata Onychophora
Echinodermata Nematoda
Acoelomorpha
Lophotrochozoa
Annelida
Mollusca
Platyhelminthes
PROTOSTOMIA
Phylum Cnidaria
Cnidaria (Greek: “stinging thread”)
distinguished by the possession of
cnidae
10,000 described species—sea
anemones, corals, jellyfishes and
hydras
diploblast = 2 germ layers
(ectoderm & endoderm)
blind gut (single opening)
nerve net & muscle cells
radial symmetry???
sexual & asexual reproduction
Cnidocytes & Cnidae
Diagnostic of
cnidarians.
However,
both
ctenophores
(Haeckelia
[=Euchlora])
and aeolid
nudibranchs
may re-deploy
cnidae that they
have obtained
from their
cnidarian prey.
(Barnes, Invertebrate Zoology, 1987)
Cnidocytes & Cnidae
The cnidocyte is a sensory-effector cell containing a cnida.
Each cnida is a rounded proteinaceous capsule, with an
opening on the apical surface that is often covered by a hinged
operculum.
At the surface, where the cnida opens, there are generally
found a number of modified cilia, called cnidocil, which assist
in the perception of tactile stimulation and chemical stimulation.
In this sac, there is a long hollow thread. Upon mechanical
contact and receipt of appropriate chemical stimuli, this
thread is explosively everted from the sac.
The cnidae may serve to deliver venom, like a hypodermic
needle. Many “nematocysts” function like this.
The cnidae may also serve to anchor the animal to a
substrate or to adhere to a prey item. This adherent role can be
performed by various subtypes of nematocysts but also by
spirocysts and ptychocysts.
Cnidae
(Barnes, Invertebrate Zoology, 1987)
(Pechenik, Biology of the Invertebates, 2000)
Cnidae Discharge
cytoplasm of
cnidocyte
cnida
macromolecule
(e.g., protein)
Ca2+
cnida
H2O
stimulation
calcium release
increase in osmotic
pressure
water rushes in by osmosis
cnida
eversion of tubule
Diploblasty
Cnidaria diverged from Bilateria prior to the evolution
of Mesoderm. So, Cnidaria lack mesoderm.
Cnidaria are diploblasts, having only two germ layers,
the primary germ layers: ectoderm and endoderm.
Outer ectodermal epithelium (ectoderm)
Inner gastrodermal epithelium (endoderm)
Central layer of mesoglea of varying thickness.
Mesoglea is a gelatinous, largely acellular
substance. It may have a few living cells within it—
often mobile amoeboid cells. However, the cells are
not organized into a tissue like true mesoderm.
The mesoglea can act as a hydrostatic skeleton,
providing support to the rest of the cnidarian body
which is really just two thin layers of epithelium
Diploblasty
mouth
gastrodermis
(endoderm)
pharynx
enteron
epidermis
(ectoderm)
mesoglea
basal disc
“For those contemplating reincarnation, a major
drawback to life as a cnidarian would seem to be
the absence of an anus. All undigested food
material passes through the same opening through
which the food enters: the mouth. This is not
particularly appetizing from the human point of
view, but the shortcomings of life without an anus
are not merely aesthetic. The sequential
disassembly of particulate food material that
occurs in an open-ended tubular gut is not
possible in the cnidarian digestive system and,
indeed, the animal must expel the undigested
remains of one meal before it can ingest more
food.”
— Pechenik, 2001
In the through gut, different functions are localized to
different sections of a linear tube.
For example, consider your own digestive tract.
Mouth
Stomach
Small
Bowel
Mechanical
processing
Protein
hydrolysis
Protein
hydrolysis
Initial
carbohydrate
digestion
Pepsin
(pH < 6)
Trypsin
(pH 7-9)
Large
Bowel
Water
resorption
Carbohydrate
digestion
One-way gut
In the one way gut, it is widely thought that you
cannot have specialized regions of extracellular
digestion.
In other words, all extracellular digestion would
have to occur in the same physio-chemical
environment.
In animals with one-way guts, there tends to be
a greater emphasis on intracellular digestion,
where undigested food particles are phagocytosed
into the cells lining the gut.
However, in both Cnidaria and Ctenophora,
there is evidence for distinct gut regions with
distinct extracellular environments
Polyp and Medusa
enteron
mouth
mouth
mouth
gastrodermis
(endoderm)
pharynx
enteron
basal disc
epidermis
(ectoderm)
enteron
mesoglea
(Oliver & Coates, in The Fossil Invertebrates, 1992)
Medusa = pelagic drifter
Jellyfish are mixing the oceans?
A theoretical model for the relative contributions of Darwinian mixing
and turbulent wake mixing is created and validated by in situ
field measurements of swimming jellyfish using a newly developed
scuba-based laser velocimetry device. Extrapolation of these
results to other animals is straightforward given knowledge of
the animal shape and orientation during vertical migration. On
the basis of calculations of a broad range of aquatic animal species,
we conclude that biogenic mixing via Darwin’s mechanism can be a
significant contributor to ocean mixing and nutrient transport.
Polyp = sessile benthic
(but capable of some movement)
Cnidarian Nerve Net
Cnidarian Nerve Net
Spread of Excitation in
Cnidarian Nerve Net
Cellular Composition
Cnidarian Muscle Histology
Scale bar = 1.0 µm
(Blanquet and Riordan, 1981)
Cnidarian Diversity & Evolution
ANTHOZOA — polyp body form only; simpler life histories;
bilateral and biradial symmetry
Class Anthozoa: sea anemones, corals, sea pens, etc.
MEDUSOZOA — most have both polyp and medusa;
radial and tetraradial symmetry
Class Cubozoa: box jellyfishes
Class Scyphozoa: true jellyfishes
Class Hydrozoa: hydras, hydroids, hydromedusae
Hydrozoan Life-History & Bodyplan Diversity
Polyp
Planula Medusa Colony Worm
_Anthomedusae yes
yes
yes
yes
no
_Leptomedusae yes
yes
yes
yes
no
_Limnomedusae reduced yes
yes
no
no
_Trachymedusa no
e
_Hydra
yes
yes
yes
no
no
no
no
no
no
_Siphonophora
yes
yes
yes
no
_Buddenbrockia no
yes
no
yes
yes
_Myxobolus
yes
no
yes
yes
yes
no
Cnidarian Phylogeny
Anthozoa
Cubozoa
Scyphozoa
Other
Trachyline
hydrozoa hydras hydrozoa
-
+
-
MEDUSOZOA
(Bridge et al. 1997)
Sexual Reproduction (Anthozoa)
Broadcast
spawning
Planula larva
Polyps
Sexual Reproduction (Medusozoa)
Medusae
Broadcast
spawning
Planula larva
Polyps
Nematostella
Embryogenesis & Metamorphosis
Zygote
Planula
Polyp
Asexual Reproduction in Cnidaria
Transverse
fission
Budding
Nematostella
Metridium
Pedal laceration
Hydra
Strobilation
Aurelia
“Cnidarians are radially
symmetrical animals.”
-Audesirk et al., 2001
-Barnes et al., 2001
-Brusca & Brusca, 1990
-Campbell et al., 2002
-Enger & Ross, 2003
-Lewis et al., 2004
-Mader, 2004
Hydra is radially symmetrical
ectoderm
endoderm
colenteron
(gut)
mesoglea
CBA
RADIAL SYMMETRY
primary body axis (oral-aboral)
oral
BILATERIA
BILATERAL SYMMETRY
primary body axis (A-P)
& secondary body axis (D-V)
dorsal
ant.
post.
ventral
aboral
Nematostella — the starlet sea anemone
oral
Head
Column
Foot
aboral
head
column
foot
mesentery
mouth
pharynx
gut
tentacle
gut cavity
endoderm
mesoglea
ectoderm
siphonoglyph
directive axis
after Stephenson, 1926
pharynx
mesentery
*
retractor
muscle
SYMMETRY IN CNIDARIA
Porpita (Hydrozoa)
Radial
Aurelia (Scyphozoa)
Tetraradial
Nematostella (Anthozoa)
Cerianthus (Anthozoa)
Biradial
Bilateral
Why did bilateral symmetry
originate?
What was its original selective
advantage?
The Standard Explanation:
Directed Locomotion
An alternate scenario….
In the Cnidaria, locomotion is not correlated with symmetry.
Modern Cnidaria are either sessile, or they locomote in a manner
that is random with respect to their secondary axis.
The bilaterally symmetrical corals and anemones are essentially
sessile.
The ancestral Cnidarian was a sessile polypoid animal.
(Bridge et al., 1992, 1995, Collins 2003, and others)
Therefore, bilateral symmetry did not evolve under selection
for directed locomotion in the Cnidaria.
In the Cnidaria, symmetry IS correlated with internal ciliary
circulation…...
Why is a sessile organism bilateral?
pharynx
mesentery
siphonoglyph
ciliary filaments
on asulcal septum
coelenteron
Alcyonaria polyp
Modified from Kaestner, 1984
An alternate scenario….
The correlation of symmetry with internal circulation
holds for bilaterally symmetrical forms, bi-radially
symmetrical forms, and tetradially symmetrical forms.
Among polyps, true radiality characterizes the smallest
hydrozoan polyps.
Size interacts with symmetry and the location of ciliary
tracts to affect the efficiency of internal circulation.
Size dependency of internal polyp anatomy
Anthozoa
biradial or bilateral
Scyphozoa
tetraradial
Hydrozoa
radial
biradial
Can this selective explanation be
extrapolated back to the CnidarianBilaterian Ancestor?
If we assume:
Homology of bilateral symmetry in Bilateria and Cnidaria
(Finnerty et al., 2004)
That the Cnidarian-Bilaterian ancestor was a sessile
benthic animal (e.g., Collins, 2004).
An alternate scenario….
Ancestral Bilaterian
Ancestral Cnidarian
Benthic
Sessile
Bilateral symmetry
(manifest primarily
internally)
gut
gut
gut
Benthic
Crawling
Bilateral symmetry
(manifest internally &
externally)
! Pronounced external manifestations of
bilateral symmetry
! Centralized nervous system
! Directed locomotion.
Cnidarian-Bilaterian Ancestor
Bilateral symmetry
(developmentally plastic?)
Hox genes, dpp
Finnerty, BioEssays 2005
Predictions and Implications…
The location of ciliary tracts was under the control of “dorsalventral patterning genes” in the Cnidarian-Bilaterian
ancestor. Perhaps this aspect of developmental gene
regulation is conserved among modern Cnidaria and
Bilateria.
Variation in the arrangement of ciliary tracts within Cnidaria
may be attributable to variation in the expression of dpp and
other genes that pattern the “directive” axis.