Cnidaria-chapter 13

When do you use the term fishes?

When there is more than one species of fish present

Cnidaria Characteristics

Mainly marine, a few freshwater

Mostly shallow-water animals

All are predators

Solitary and colonial forms-multiple organisms create one large colony

Slow moving/swimming;  largely passive movers-drifters usually. Allow the water currents to move them.

Significance of Phylum

-Contains the most ancestorial (opposite of derived) members on the main line of evolution of the Eumztaeoa (animals) (word that means “true, higher-level animal”)

What are Keystone species?

an organism that has a massive impact on its ecosystem compared to its small population size

Which type of Cnidaria is very important ecologically as a keystone species (like beavers)?

Corals

What type of symmetry are Cnidarian?

Radial

Oral end

Where the mouth is located

Aboral

opposite end of the oral end (where the mouth is)

Cnidocyte

Cell with a stinging Organelle

Nematocyst

Stinging Organelle

Nematocysts

Some “sting” prey by penetrating and injecting them with venom

Other types capture prey by entangling them

Others can stick to the substrate, holding the animal in place

How are Nematocysts Triggered?

automatically by physical contact with prey

Nervous system

Cnidaria have cell-tissue level of organization

True tissues include the decentralized nerve net

Bidirectional nerve cells (unlike other animals)

-electrical signals to flow in both directions

Radial symmetry means decentralized nervous system

neuromuscular system

Nerve cells connect to muscular cells

Basic structure

digestive sac with one opening (mouth/anus), thus, a two-way digestive system

They are diploblastic, which means they have

Two embryonic germ layers: 

-ectoderm (“outer skin”)

-endoderm (“inner skin”)

Body Plan

Epidermis (derived from ectoderm), Gastrodermis(endoderm), mesoglea is between these two layers, innermost cavity is gastrovascular.

Gastrovascular Cavity

for digestion and circulation of nutrients/wastes/gases

Polyp

Sessile “hydroid” form

Medusa

Mobile “jellyfish” form

polyp form

Medusa

Polyp: asexual reproduction

Podocyst formation, Budding, Fission

Podocyst formation

The base of the polyp creates hard, resistant cysts (podocysts) that stay dormant on a surface until conditions improve and grow into a new polyp.

Budding

A new, small clone grows directly out of the side or base of the parent polyp

Fission

The polyp body divides horizontally

Medusa

Sexual Reproduction

Hydrozoa

Hydra, Obelia, Physalia

Scyphozoa

Aurelia

Cubozoa

Chironex

Anthozoa

Metridium and corals

Hydrozoa

Mostly marine, colonial animals

freshwater, polyp form only – no medusa

both sexual and asexual (budding) reproduction

dioecious (male and female are separate animals) in most species-sometimes having an egg on the outside

Epidermis: cell types

epitheliomuscular cells, cnidocytes, sensory cells, gland cells, interstitial cells, nerve cells

Epitheliomuscular cells

longitudinal muscle layer for movement of body-move

Cnidocytes

protection/prey capture-stinging

Sensory Cells

sensing chemical and tactile stimuli-sensing

Gland cells

secrete adhesive, gas bubbles, mucus-secretions

interstitial cells

stem cells to replace others

Gastrodermis: cell types

gland cells secrete enzymes for extracellular digestion

-nutritive-muscular cells for:

1) phagocytosis and intracellular digestion

2) contraction of gastrovascular cavity –   weak circular muscle layer

Hydrozoa: Physalia

marine, floating, colonial (and somewhat dangerous) hydrozoan

polyp and medusae are present in the same colony

Hydrozoa: Obelia

a marine, sessile, colonial hydrozoan

Different types of polp in Obelia

Hydranths-feeding

Gonangia-produce medusa buds via asexual reproduction

Obelia-Hydrocauli

The branching stalks where Polyps occur with vascular system for whole colony

Obelia: sexual reproduction

Gonangia(reproductive polyps) produce small male and female medusa buds

Medusae grow and develop, then form a zygote (sperm and egg)

Zygote develops into a blastula and then a free-swimming planula larva (Fig 13.7)

Planula settles and starts a new colony of polyps, completing the life cycle

Scyphozoa

Often called the “true jellyfish”

Common example: Aurelia, the "moon jellyfish"

Polyp stage of Scyphozoa is not colonial, but solitary

Jellyfish reproduction: sexual phase

Male and female medusae produce gametes

Male releases sperm, female takes sperm into gastrovascular cavity to the ovaries for fertilization

After fertilization, zygote develops into planula

Planula settles on substrate to become scyphistoma polyp

Jellyfish reproduction: asexual phase

Scyphistoma goes through strobilation(division) to produce the strobila stage of the life cycle

Strobila buds to release multiple clones:

-ephyra larvae

Ephyra develop into male and female medusae

Scyphozoa medusa anatomy

relatively large compared to the polyp stage, since it is a predator on other animals

many radial canals radiate from central gastrovascular cavity, which contains four horseshoe-shaped gonad. They grab prey with these. (word for “sex organs”).

Cubozoa

Square-shaped jellyfish; formerly classified as Scyphozoa

dangerous – can be deadly to humans

Common names: "sea wasp”, box jellyfish

They are good swimmers

Anthozoa

Sea Anemones:

Unlike other cnidarians, the body is a large, densely muscular polyp

gastrovascular cavity divided by numerous septa

no medusa stage – polyp only

Anthozoa-acontia

(stinging “threads” inside gastrovascular cavity) to quiet prey. They grab then quiet them.

“true” or “stony” corals

Colonies of small, interconnected polyps (no medusa)

secretes calcareous cup that over the years develops into a massive “colonial skeleton" called coral rock

gastrovascular cavities of different individuals connected by a sheet of tissue covering entire surface. There have caps on them too.

mutualistic dinoflagellates (zooxanthellae) provide food via photosynthesis and also enhance the ability of coral to secrete calcium carbonate