ZLY 303: Biology of Free-living Non-Arthropods

PHYLUM CNIDARIA

Classification• Domain: Eucaryota• Kingdom: Animalia• Subkingdom: Eumetazoa• Phylum: CnidariaCnidarians were classified into the following Subphyla viz:

• Anthozoa (Sessile) e.g. Sea anemones,

Corals, Sea pens– Ceriantharia (tube anemones),– Hexacorallia,– Octocorallia,

Classification• Medusozoa– Scyphozoa (Swimming) e.g. true jellyfish

– Cubozoa e.g. box jellyfish, Sea wasps– Hydozoa (Sessile) e.g. Hydra and colonial

swimmers e.g. Portuguese Man o’ War (A diverse group that include all freshwater cnidarians as well as many marine forms).

Recently, the following group has been recognized as a class rather than a sub-group of Scyphozoa; they include:

• Staurozoa (Stalked Jellies) • Myxozoa (Parasites)• Polypodiozoa (Parasites)

There is still debate whether they are Cnidarians or Bilaterians

Phylogeny of Cnidaria

Four main classes (even though most sources consider 3) stem group - possess basic structural organization of other more advanced metazoa

• have radiated into many habitats – yet body plan has been retained.

Classes

For study purpose, the following classes shall be considered:-•HYDROZOA e.g. Hydra, Obelia•SCYPHOZOA e.g. Aurelia aurita (Jelly fish)•CLASS ANTHOZOA e.g. Sea anemones, Corals•CLASS CUBOZOA e.g. Tripedalia cystophora, Sea wasp

General Characteristics

• Possess the simplest degree of structural and functional organization

• Exclusively marine, few sp. in freshwater

• They are acoelomates (lack coelom)• Radially symmetrical• There is diversity of body types found

among the members of this phylum.• Has 2 distinct layers i.e. Ectoderm and

Endoderm and a thin layer of non-living jelly-like mesoglea

General Characteristics

• Tentacles around the mouth

• Tissues level of organization

• No choanocytes (flagellated collar cells of the sponges)

• Alternation of generation with 2 morphologically distinct stages:

• Polyp (sessile, remain attached, asexually reproducing form)

• Medusa(motile, sexually reproducing form)

General Characteristics

• No excretory or respiratory organs• Nerve net with symmetrical &

asymmetrical synapses• Gastrovascular cavity used for both

digestion and excretion. • Mesoglea (jelly-like non-living

substance between the two tissue layers (epidermis & gastrodermis) for support).

• Hydrostatic Skeleton - chitinous, calcerous, or protein.

General Characteristics • Radiata have only two embryonic tissue

layers.• Specifically, no mesoderm (the tissue that

gives rise to structures, including muscles, in triploblastic organisms).

• Lack sophisticated movement seen in triploblastic organisms.

• Mouth: both ingestion & egestion- no anus• Asexual reproduction by budding in polyps • Sexually by gametes in all medusa & some

polyps

Body Cavity Diversity

Why Cnidarians are ACOELOMATES

Cnidarians do not possess the following features, hence, they are Acoelomates:-– Coelom (also called the body cavity)

cavity within the body of all animals higher than the coelenterates and certain primitive worms.

– Formed by the splitting of the embryonic mesoderm into two layers. 

– In mammals it forms the peritoneal, pleural, and pericardial cavities. 

Dimorphism & Polymorphism in Cnidarians

• A striking & puzzling feature

• Dimorphism: existence in polyp & medusa

• Polymorphism: colonies of some species have morphologically differing individuals each specialized for certain roles e.g. feeding, reproduction & defense.

Task differences in HydrozoaIn Hydrozoans, colonial individuals arising from individual zooids will take on separate tasks. For instance, in Obelia there are:•Feeding individuals referred to as the Gastrozooids; •Asexual reproduction individuals only, referred to as Gonozooids;•Sexually reproducing individuals (medusae) and are free-living, referred to as the Blastostyles •Defensive individuals referred to as Dactylozooids

Polymorphism in Cnidaria

Differences between Medusa & Polyp

Polyp• Sedentary or sessile

life• Non free swimming• Has tubular body• Simple body structure• Velum absent• Mouth is circular• Mesoglea poorly

developed

Medusa• Adapted for a floating

life• Free swimming• Bell or umbrella-

shaped• Complex structure• Velum present• Mouth rectangular• Mesoglea highly

developed

Attack & Defence Feature

•Cnidarians also have cnidocytes (specialized cells that function in defense & capture of prey)•Cnidocytes contain organelles called cnida, which are able to evert (extend or retract). •Cnida that sting are called nematocysts. •These nematocysts can immobilize fish for capture, and they can also be used for defence.

Coiled & Uncoiled Nematocysts

Types of Cnidocytes

1. Nematocysts: They have barbs which are used to inject venom into prey. This is common to almost all Cnidarians.

2. Spirocysts: They entangle the prey and do not penetrate and inject the prey with venom.

3. Ptychocysts: They have tiny threads used for building protective tubes for the organism to habitate; they are not used for prey capture, this feature is common to tube anemones.

Features of a Cnidocyte• Cilium: A fine hair which projects above the

surface and acts as a trigger. They are absent in Spirocysts.

• Cnida: A tough capsule which houses the thread (payload) and contains mixture of chemicals like venom, adhesives etc.

• Finger-like-extensible tube: Found on the wall of the cnida so that when a cnidocyte fires, the finger pops out and a set of barbs anchor the prey to release venom.

Features of a Cnidocyte

• The thread: An extension of the "finger" and coils round the prey until the cnidocyte fires. (The thread is usually hollow and delivers chemicals from the cnida to the target).

• Operculum: A lid covering the end of the cnida. (The lid may be a single hinged flap or three flaps arranged like slices of pie).

• Cell body: Produces all the other parts.

Firing Mechanism of CnidocytesFour theories were postulated by Hinde (1998):•Rapid contraction of fibers round the cnida may increase its internal pressure.•The thread may be like a coiled spring that extends rapidly when released.•Chemical changes in the cnida's contents may cause them to expand rapidly by polymerization e.g. Chironex (the "sea wasp").•Chemical changes in liquid of the cnida increases the concentration, so that osmotic pressure forces water in very rapidly to dilute it e.g. Hydrozoa,

Figure Showing Firing Sequence

Stimulus Required to Avoid Wastage in Firing

Cnidocytes1. Their cilia detect contact, and nearby

sensory cells "smell" chemicals in the water. This combination prevents them from firing at distant or non-living objects.

2. Groups of cnidocytes are usually connected by nerves and, if one fires, the rest of the group requires a weaker minimum stimulus than the cells that fire first.

The Hydrozoa e.g. Hydra vulgaris

• Class Hydrozoa includes hydras and the infamous Portuguese Man-of-War, an organism noted for its potent sting, in the medusa form.

• Most Hydrozoans exist in the polyp and medusa forms.

• For example, some hydrozoans e.g. Obelia exist as an asexually reproducing polyp that alternates with a sexually reproducing medusa form (see next slide).

Body plan of Hydra

Body plan of Hydra• Epidermis consist:

• Epitheliomuscular cells (can self-reproduce) for covering & muscular contraction

• Interstitial cells: differentiate to form the cnidoblast, sex cells, buds, nerve cells.

• Gland cells- located around the basal disc and mouth, secrete adhesive substance for attachment

• Cnidocytes- contain nematocyst for catching prey

• Sensory cells found around the mouth for detection of stiimuli

• Nerve cells- synapse with sensory cells & other nerve cells

Body plan of Hydrozoa: Hydra

Transverse Section through body wall of Hydra

Body plan of Hydra

• Gastrodermis consist:• Nutritive-muscular cells all tall,

columnar, large and vacuolated with flagella at one end & microvilli at the other end

• Interstitial cells: can transform to form other kind of cells when the need arises.

• Gland cells- surround the mouth to aid feeding & digestion.

• Mesoglea lies btw the epidermis & dermis and act like a type of elastic skeleton

Biology of Hydra: Hydrozoans

• May be solitary or colonial• Found in freshwater• Hydra, unlike other hydrozoa exist as

solitary(no medusa or colonial forms).• Cylindrical body shape• Body size depends on level of

contraction and extension.

Movement in Hydra• Somersaulting- Can be found hanging

downward• Walking• Looping• Floating• Climbing• Gliding• Swimming

Feeding & Nutrition In Hydra

• Use Tentacles to capture & direct the food• Exclusively carnivorous feeds on Cyclops,

annelids, crustaceans, insect larvae.• Digestion in enteron (gastrovascular

cavity)& is both • Extracellularly- the prey is killed by the

action of digestive enzymes from gland cells of gastrodermis

• Intracellular- occur in the gastrovascular cavity

Feeding & Nutrition In Hydra

• Tentacles have many cnidoblasts in them for capturing prey

• Nematocysts: Immobilize & paralyze the prey

• Inside the gastrovascular cavity, gland cells secrete enzyme to digest the food.

• Egestion via the mouth.

Feeding process in Hydra

Reproduction in Hydrozoa: Hydra

• Mostly dioecious(male & female parts separate

• Reproduce both sexually & asexually by budding

• Zygote undergo holoblastic(complete) cleavage to form the blastula.

• Cleavage is indeterminate(regulative)• Fertilization: Cyst formed around the

embryo which break loose from parent to form the young hydra.

Reproduction in Hydrozoa: Hydra

Biology of Obelia

• Unlike Hydra, has both colonial & medusa form

• Sedentary• It’s a plant-like Hydrozoa• Whitish in colour under microscope• Largely carnivorous• Use tentacles to capture prey• Digestion both extra & intracellular

Biology of Obelia• Dioecious• Fertilization is external• Mouth open into manubrium• Zygote forms planular larva• Statocysts are gravity receptor for

maintenance of balance

Obelia life cycle

The life cycle of Obelia, (Hydrozoan)

Alternation of Generation in Hydromedusoid forms

Cnidaria Larva Types A. Planula of Renilla sp.

B. Planula of Anemone Lebruina

C. Brooded Planula of Urticinia filina (anemone)

D. Planula of Zoanthidea

E. Planktotropic planula M. senile

F. Early & Late planula of stone coral

G. Actinula larva Ceriantheria

Body plan of Scyphozoa: Aurelia aurita

Life Cycle of Jelly fish

Class Scyphozoa e.g. Jellyfish

• Cup-animals with umbrella or bowl-shape

• May attain a bell diameter of more than 2m

• Most scyphozoans range from 2 to 40cm in diameter

• May be colourless, orange or blue• Jellyfish exists predominantly in the

medusa form• Mesoglea unusually thick giving the

bell a fairly firmed consistency

Biology of Class Scyphozoa e.g. Jellyfish

• Predominantly Medusa, polyp form occurs only as small larva during life cycle stage.

• Thaumatosyphus hexaradiatus are unusual member because medusa are sessile, attached to seaweed

• Movement is by rhythmical pulsation of the umbrella

• Tentacles may be few or short as in Aurelia aurita or long as in Cyanea

Biology of Class Scyphozoa e.g. Jellyfish

• Aurelia aurita is a common member• No velum or nerve rings• Oral arms bears the nematocysts• Have sense organ called rhopalium• Nervous system is made of a nerve net

with subumbrella net that control bell pulsations

•

Biology of Class Scyphozoa e.g. Jellyfish

• Sexes separate

• Sperm carried by ciliary current into gastric pouch of female

• Internal fertilization

• Reproduction: The medusa sperm + egg = Zygote

• Ciliated planular larva emerge (Scyphystoma) (hydra-like form), strobila ephyra

Class: AnthozoaSea anemone

Class Anthozoa

• E.g. sea anemones, corals , and sea fans. • Anthozoans are all polyps, no medusa

i.e. Not Dimophic• Symbioses between reef-building corals

and the dinoflagellates. • In these relationships, corals provide

housing and protection for the protists, and the dinoflagellates provide food for the corals.

• Members of all of the Cnidaria classes can respond to external stimuli and can use stinging nematocysts for prey capture and defense. 

Anthozoa characteristics

• Flower-shaped, with body far more complex than Hydrozoans

• Exclusively polyploidy, no medusa• The most advanced of all marine

Cnidarian• Mouth lead to pharynx• Gastrovascular cavity subdivided by

septa, or mesenteries into chamber• Mesoglea stout with cells and fibres• Skeleton secreted in the form of coral • Nematoblast found in the body layers

Class Anthozoa e.g. The Corals

Biology of Anthozoans

Feeding•Similar to Hydra polyp•Aided with tentacles, cnidoblast and nematocyst.•Pharynx lined with lateral ciliated grooves•Movement by rhythmic contraction of muscles, mesoglea and water in gastric cavity

Biology of Anthozoans e.g. Sea anemones

• Move by gliding slowly along the substrate on their pedal disc

• Are carnivorous• Can expand & stretch their tentacles to

catch the prey• Nematocysts immobilizes the prey• Pharynx with ciliated grooves:

siphonoglyps• When disturbed, can contract & withdraw

their tentacles and oral discs.

Biology of Anthozoans e.g. Sea anemones

Sexual Reproduction•Sexes are separate•Gonads borne on the margin of septa/mesenteries•Fertilization occurs in enteron or externally:•Planular larva emergeAsexual Reproduction•Occur by pedal laceration or at times budding

Diversity in forms

HYDROZOA1.Life history features both Polyp and Medusa stages

2. Mesoglea non cellular

SCYPHOZOATrue Jellyfishes have conspicuous medusoid form, Polyploid larva stage2. Mesoglea thickened with fibres and cells

ANTHOZOASea anemones, coral are all polyps

2. Mesoglea stout with fibres and cells

CUBOZOAMedusa predominant

Class Cubozoa

• Class Cubozoa are the box jellies, which have a box-shaped medusa form.

• Some species in this group are among the most venomous organisms in the world and their stings can be fatal to humans

Class Cubozoa

• Most recently classified• Exist as both Polyp & Medusa• Medusa is the predominant form of this

class• Cubozoa medusa may be about 25cm

tall• Medusa are strong swimmer and

voracious predators• Base of each tentacle form pedalium• Tentacle(s) present in each corner of

umbrella margin

Class Cubozoa

• Medusa dominant

• Polyp state develop directly to medusa

• Bell margin with velarium

• Tetramerous (four part radial symmetry)

• Bell may be cube or square-shaped with tentacles arising from each corner.

• Gonads are endodermal (found in the gatrodermis)

• Specialized sense organ called rhopalium

Class Cubozoa