Lecture 2

Phylum CnidariaAnemones, corals, jellyfishes,

hydras and relativesby John R. Finnerty

Animal Phylogeny

PROTOSTOMIA

ChordataHemichordataEchinodermata

Arthropoda OnychophoraNematoda

Deuterostomia

AnnelidaMolluscaPlatyhelminthes

Ecdysozoa Lophotrochozoa

Sili

cisp

ongi

ae

Cal

cisp

ongi

a

Acoelomorpha

Porifera Ctenophora Cnidaria

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)

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.

Cnidocytes & Cnidae

(Barnes, Invertebrate Zoology, 1987)

Cnidae

(Pechenik, Biology of the Invertebates, 2000)

Cnidae Discharge

Ca2+

macromolecule(e.g., protein)

cytoplasm of cnidocyte

cnida

stimulationcalcium releaseincrease in osmotic pressurewater rushes in by osmosis

cnida

H2O

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

gastrodermis(endoderm)

epidermis(ectoderm)

mesoglea

mouth

pharynx

basal disc

enteron

Diploblasty

“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

Large Bowel

Mechanical processing

Protein hydrolysis

Protein hydrolysis

Water resorption

Initial carbohydrate digestion

Pepsin(pH < 6)

Trypsin(pH 7-9)

Carbohydrate digestion

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

One-way gut

Polyp and Medusa

mouthmouth mouth

gastrodermis(endoderm)

pharynx

basal disc (Oliver & Coates, in The Fossil Invertebrates, 1992)

epidermis(ectoderm)

enteron

enteronenteron mesoglea

Medusa = pelagic drifter

Jellyfish are mixing the oceans?

A theoretical model for the relative contributions of Darwinian mixingand turbulent wake mixing is created and validated by in situfield measurements of swimming jellyfish using a newly developedscuba-based laser velocimetry device. Extrapolation of theseresults to other animals is straightforward given knowledge ofthe animal shape and orientation during vertical migration. Onthe basis of calculations of a broad range of aquatic animal species,we conclude that biogenic mixing via Darwin’s mechanism can be asignificant 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

(Blanquet and Riordan, 1981)

Scale bar = 1.0 µm

Cnidarian Diversity & Evolution

Class Anthozoa: sea anemones, corals, sea pens, etc.

Class Cubozoa: box jellyfishesClass Scyphozoa: true jellyfishes

ANTHOZOA — polyp body form only; simpler life histories; bilateral and biradial symmetry

Class Hydrozoa: hydras, hydroids, hydromedusae

MEDUSOZOA — most have both polyp and medusa; radial and tetraradial symmetry

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

_Trachymedusae

no yes yes no no

_Hydra yes no no no no

_Siphonophora yes yes yes yes no

_Buddenbrockia no yes no yes yes

_Myxobolus no yes no yes yes

Cnidarian Phylogeny

+

Anthozoa

-

Cubozoa ScyphozoaTrachylinehydrozoa

Otherhydrozoa

(Bridge et al. 1997)

hydras

-

MEDUSOZOA

Sexual Reproduction (Anthozoa)

Planula larvaBroadcast spawning

Polyps

Sexual Reproduction (Medusozoa)

Planula larva

Broadcast spawning

Medusae

Polyps

Zygote Planula Polyp

NematostellaEmbryogenesis & Metamorphosis

Nematostella

Asexual Reproduction in Cnidaria

Hydra

MetridiumAurelia

Strobilation

Budding

Transverse fission

Pedal laceration

“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

mesoglea

colenteron(gut)

BILATERIACBA

oral

aboral

ant. post.

dorsal

ventral

BILATERAL SYMMETRYprimary body axis (A-P)& secondary body axis (D-V)

RADIAL SYMMETRYprimary body axis (oral-aboral)

Head

Foot

Column

oral

aboral

Nematostella — the starlet sea anemone

footcolumnhead

tentacle

mouthmesentery

gutpharynx

retractormuscle

pharynx

*

after Stephenson, 1926

directive axis

siphonoglyph

mesentery

gut cavityendodermmesogleaectoderm

SYMMETRY IN CNIDARIA

Radial TetraradialPorpita (Hydrozoa) Aurelia (Scyphozoa)

BiradialCerianthus (Anthozoa)

Bilateral

Nematostella (Anthozoa)

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…...

pharynxsiphonoglyph

mesentery

coelenteron

ciliary filaments on asulcal septum

Modified from Kaestner, 1984

Alcyonaria polyp

Why is a sessile organism bilateral?

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

Scyphozoa

Hydrozoa

biradial or bilateral

tetraradial

radial biradial

Can this selective explanation be extrapolated back to the Cnidarian-Bilaterian 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).

Hox genes, dpp

Ancestral Bilaterian Benthic Crawling Bilateral symmetry

(manifest internally & externally)

Cnidarian-Bilaterian Ancestor

An alternate scenario….

gut

Bilateral symmetry (developmentally plastic?)

gutgut

Ancestral Cnidarian Benthic Sessile Bilateral symmetry

(manifest primarily internally)

Finnerty, BioEssays 2005

! Pronounced external manifestations of bilateral symmetry! Centralized nervous system! Directed locomotion.

Predictions and Implications…The location of ciliary tracts was under the control of “dorsal-ventral 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.