Phylum Cnidaria

Question 1

Cnidarians

Answer 1

11,200 spp in 6 classes
Radially symmetrical
Diploblastic
Polyp and Medusa forms
Gelatinous mesoglea
2-way digestive tract
Apomorphies:
- Cnidae
- Planula larvae
(Have tissues, muscles, NS, DS, morphs)

Question 2

Cnidae

Answer 2

Organelle in a cnidocyte (cnidoblast, hematoblast)
Explosive discharge
30 kinds
Capsule - one open end, usually with lid (operculum)
• coiled thread - inside-out sock
• cluster of cilia at edge of opening
Cnidocil - trigger

Question 3

Osmotic hypothesis

Answer 3

Water rushes into the organelle

Pressure increases and capsule opens

Question 4

Tension hypothesis

Answer 4

Capsule pressure always high

Pressure released when lid is triggered

Question 5

Contractile hypothesis

Answer 5

Contractile units squeeze capsule

Question 6

Types of cnidae

Answer 6

Sticky
Wrap
Penetrate
Toxic

Question 7

Functions of cnidae

Answer 7

Food collection (paralyze/hold prey)
Defense
Locomotory? Attachment?

Question 8

Location of cnidae

Answer 8

Feeding tentacles
Epidermis
Gastrodermis (not in Hydra)

Question 9

Problems with a 2-way digestive tract

Answer 9

Can’t ingest when egesting
GVC distorts with movement
Gametes and gonads take up space (in some)

Question 10

Nervous system

Answer 10

Diffuse nerve net (no brain, spinal cord, longitudinal major nerve)

Question 11

Slow nerve network

Answer 11

Multiple synapses; multiple direction for transmission

Nerve signal spreads from site of stimulation

Question 12

Fast nerve network

Answer 12

Fewer branches in neurons
Less “net-like”
Transmission more directional 
Larger cell process diameter
Just under epithelium

Question 13

Food processing

Answer 13

• cnidae on tentacles
• “mouth” to gastrovascular cavity
• digestive enzymes in gastrovascular cavity
• food into cells of gastrodermis
• waste out through mouth

Question 14

Muscles

Answer 14

Not mesodermally derived (no mesoderm)
Epitheliomuscular (from ectoderm)
Nutritive muscular (from endoderm)
Layers:
longitudinal - shortening
circular - squeezing for extension (with hydrostatic skeleton)

Question 15

Gas exchange

Answer 15

Lack gills or other specialized gas exchange structures

Gases diffuse across exposed epidermal and gastrodermal surfaces

Question 16

Sexual Reproduction

Answer 16

Medusae usually gonochoristic (separate sexes)

• reproduce sexually (external and internal fertilization)
• planktonic, ciliated planula larva produced

Question 17

Asexual reproduction

Answer 17

produce genetically identical medusae and polyps

Question 18

Class Anthozoa

Answer 18

(anemones, corals, sea whips, sea pens, sea fans, and sea pansies)
• ~ 6,000 species, ~ 70 % of Cnidaria
• polyp only, no medusa stage
• absence (not loss!) of operculum on cnidae
• ciliated grooves (siphonoglyphs) in pharyngeal wall leading from mouth
• mitochondrial DNA is circular (as most eukaryotes)
not linear like other cnidarians

Question 19

Siphonoglyph

Answer 19

A ciliated groove at one or both ends of the mouth of sea anemones and some corals. The siphonoglyph extends into a pharynx and is used to create currents of water into the pharynx. These water currents are important for respiration and maintenance of internal pressure.

Question 20

Mesentries

Answer 20

Infoldings (septa) of gastroderm and mesoglea which greatly increase the SA available for secreting digestive enzymes and absorbing nutrients

Question 21

Acontia

Answer 21

Thin filaments loaded with nematocysts and secretory cells
Used offensively and defensively and may function in digestion
Extended outside body wall (openings)

Question 22

Cnidarian Evolution

Answer 22

Most of the recent evidence appears to reject Hydrozoa as basal lineage
• Supports Anthozoa as basal lineage
– suggests medusa was not present in common
ancestor of Cnidaria
– Also a good lesson that basal (primitive) does not mean “simple, less complex”

Question 23

Corals

Answer 23

Some reef-building
• skeleton external to animal - sits on top
• can retract within
• polyps connected by thin layer of tissue
• zooxanthellae (symbiotic dinoflagellates) provide energy via photosynthesis

Question 24

Infoldings (septa) of gastroderm and mesoglea which greatly increase the SA available for secreting digestive enzymes and absorbing nutrients

Answer 24

Mesenteries

Question 25

A ciliated groove at one or both ends of the mouth of sea anemones and some corals. The siphonoglyph extends into a pharynx and is used to create currents of water into the pharynx. These water currents are important for respiration and maintenance of internal pressure.

Answer 25

Siphonoglyph

Question 26

Thin filaments loaded with nematocysts and secretory cells Used offensively and defensively and may function in digestion Extended outside body wall (openings)

Answer 26

Acontia

Question 27

Pharynx

Answer 27

Tubular | Leads to GVC

Question 28

Class Scyphozoa

Answer 28

• ~ 200 species (~ 2.3 % of Cnidaria) • medusoid stage dominates, often large (polyps small or absent) • asexual replication by strobilation - apomorphy • 100% marine Gut of medusa 4 -chambered • Mesoglea acts as skeleton, stretching muscles • When muscles relax, bell “pops” back into shape

Question 29

Scyphozoa Sensory Organs

Answer 29

• balance organs: statocysts – Hold weighted statoliths (see fig. 6.8b) • light receptors: ocelli (not image forming) • touch receptors (some spp): sensory lappets • held in rhopalia (singular = rhopalium)

Question 30

Scyphozoan reproduction

Answer 30

Planula larva becomes a scyphistoma This grows & buds off new scyphistomae & strobilates, producing genetically identical ephyras, which are juvenile medusae

Question 31

Class Cubozoa

Answer 31

``` (Box Jellyfish, Cubomedusae, Sea Wasps) 20 species • medusa box-like, 4 corners each with tentacles • complex, lensed eyes (but no brain!?) • medusae small, usually only a few cm • highly virulent cnidal toxin • each polyp => 1 medusa • rhopalia hold statocysts & ocelli • rhopalia in Scyphozoa & Cubozoa • Cubozoa lensed eyes are among the most complex in the invertebrates ```

Question 32

Class Hydrozoa

Answer 32

(Hydras, men-o’-war, hydrocorals) ~ 2,700 species • gastrodermal tissue without cnidae, epidermis only • mesoglea without cells • freshwater but mostly marine • hydrocorals - convergent • velum - shelf of tissue around inner edge of bell of the medusa Velum - shelf of tissue around inner edge of bell of the medusa (increases velocity of ejected water) • polyp predominates • hydromedusae smaller than scyphomedusae • most colonial (e.g. Obelia) • - each polyp a zooid • connected - share gastrovascular cavity • often with protective perisarc -

Question 33

Colonial polymorphism

Answer 33

* gastrozooids (hydranths): feeding * gonozooids: reproduction * dactylozooid - with cnidae for prey capture & defense

Question 34

Genus Hydra

Answer 34

Atypical Hydrozoan 1. Not colonial 2. Not marine 3. No medusa

Question 35

Siphonophores

Answer 35

Genus Physalia Genus Praya - colony of both polyp-zooids & medusa-zooids - all from 1 planula

Question 36

Myxozoa

Answer 36

``` Spore-forming protozoa • parasitize fish and aquatic annelids • degenerate multicellular animals • molecular data - Cnidaria • strange polar capsules ```

Question 37

Coral reefs

Answer 37

* reef-building corals (hermatypic) restricted to clear, warm waters * usually shallow tropical - <90 m - need sunlight * diverse ecosystem: * algae, protozoans (foraminifera), shelled molluscs, tube-dwelling polychaete annelids, bryozoans give structure * most from corals * calcification - additional 10 kg calcium carbonate per m2 annually * carnivorous * live in areas with low productivity of plankton

Question 38

Zooxanthellae

Answer 38

• endosymbiotic dinoflagellates • photosynthesize - provide corals with energy- rich organic compounds • 20-95% of products of photosynthesis released to host • zooxanthellae use metabolic wastes of coral - carbon dioxide and nitrogenous wastes • protected from predators NEGATIVES • toxic forms of oxygen build up (hydrogen peroxide, oxyradicals) • coral tissue counteract with high levels of enzymes to detoxify oxygen toxins

Question 39

Coral bleaching

Answer 39

* Loss of zooxanthellae * high water temperatures * adaptation to allow colonization by more appropriate zooxanthellae?

Question 40

Genus Chironex

Answer 40

Cubozoa • 3 - 60 m long tentacles • ~ 1 human death per year in N. Australia • death in 3 minutes