MBE 230-Chapter 10: Cnidarians (Coelenterates)

Cnidarians (10,000 species; 560-750 mya):

- They have radial symmetry and are gelatinous.

- They have an epidermis and gastrodermis, and the mesoglea is in between.

- They have a hydrostatic body.

- They have polyps and Medusae form (Medusae: a free-swimming sexual form of a coelenterate such as a jellyfish, typically having an umbrella-shaped body with stinging tentacles around the edge).

- They have a nerve net, statoliths, and primitive eyes.

- They have tentacles and nematocysts (or mucus)

- They are carnivorous

- They have a "dead end" gut

- Some have symbiotic algae or detritus

- Cosmopolitan (Can be found mostly everywhere)

Cnidarian Reproduction:

- Polyp --> asexual reproduction --> Medusae --> sexual reproduction --> polyp

Cnidarians: Anthozoans (Sea Anemones and Corals) (6,500 species):

- The polyp life cycle form dominates, and is mostly colonial.

- They are benthic carnivores, and use tentacles and nematocysts

- Hexacorals: Anemones, corals (CaCO3 skeleton)

- Octocorals: Gorgonians, sea fans, sea whips, sea pens (organic skeleton)

Anthozoans - Anemones:

- They are both solitary and colonial

- They use their gut for hydrostatic body

- They are benthic on hard surfaces, some are intertidal, they can also burrow or be in the pelagic (Cosmopolitan)

- They are carnivores. They use tentacles and nematocysts, as well as mucus.

- Some have symbiotic dinoflagellates

- Some are bioluminescent

- NO medusa stage. egg --> planula larvae --> adult

- They use budding in colonies. Sometimes binary fission is possible as well.

- Some can be symbiotic with other animals (clown fish and anemones)

Anthozoans - Ahermatypic and Scleractinian Corals

- Scleractinian corals (About 250 mya) --> CaCO3, reefs

- Ahermatypic corals don't have CaCO3

- Are more cosmopolitan

- They mostly brood eggs

Hard Corals:

- The polyp form dominates, and are mostly colonial with budding.

- They are benthic carnivores and use tentacles and nematocysts.

- Hermatypic corals with CaCO3 are only in shallow tropical waters. They build reefs over time.

- Hermatypic corals have mostly external fertilization

- Planula larvae can be from 2 days to 2 months

- Some can tolerate air exposure (more intertidal exposure in the pacific than in the Atlantic)

Hard Corals SLIDE 2:

Turbidity -- reduces light and interferes with feeding

- Corals must use mucus to eliminate sediments

- Corals are not near river mouths -- Turbidity and salinity (most corals are not in river mouths due to low salinity levels)

- Not along East coast of South America

- Corals that tolerate sediment tend to be solitary and erect (clonal on hard substrates, solitary on sediment)

- Tolerant species on back reef, lagoon, and sediments

Methods of Coral Nutrition:

Photosynthesis --> zooxanthellae (They help the coral survive by providing it with food resulting from photosynthesis)

Zooplankton --> Tentacles, mesenterial filaments (They are typically bright white and full of nematocysts—specialized stinging cells that corals use to capture and kill prey), and mucus

Detritus --> use of mucus and mesenteric filaments

DOC (marine dissolved organic carbon) --> microvilli on surface (They use the microvilli to collect falling dissolved organic carbon)

Hard Corals SLIDE 3:

- Branching: High S/V ratio, small polyps and they are more autotrophic

- Massive: Low S/V ratio, large polyps, and they are more heterotrophic

Perforate (have porous skeletons with connections between the polyps through the skeleton):

- Buds outside tentacular ring
- Discrete polyps

Imperforate (They are corals that have solid skeletons):

- Buds inside tentacular ring
- Long rows of tentacles and mouths

Horizontal sheets of tissue connect all of the polyps

- Branched Corals grow faster

- Some have distal tips: Fewer zooxanthellae and more calcification

Hard Corals SLIDE 4:

- Slow growers usually regenerate after damage is taken

- Fast growers don't regenerate or they only regenerate the young distal parts --> leaving their old parts to die

- Coral colony morphology -- adaptation for:

Light harvest
Zooplankton for harvest
Competition for space

Hard Corals SLIDE 5:

Shallow - bright light: High CaCO3 deposition/ tissue grow up

Deep - dim lighting: low CaCO3 deposition/ tissue grows horizontal

More calcification in brighter lights --> Overtopping, steep slopes

- More branches in turbulent water despite more toppling.

Less Disturbance:

Slow growing species
more aggressive (mesenteric filaments)
hang onto space

More Disturbance:

Fast growing species
Less aggressive (overgrow and shade)
Colonize and move

Coral Competition:

- Overgrowth

- Shading

- Mesenterial filaments (used by some coral species to attack and digest away their nearby coral competitors)

- Sweeper tentacles

Coral Reproduction:

- Colonial budding

- Fragmentation --> storm or boring induced, or purposely weakened.

- Polyp bail out --> drift away

- Sexual reproduction --> planula larvae

- They have both internal and external fertilization (Brooders and broadcast spawners)

- Sexual maturity in about 5-10 years

- Gamete release --> diel, lunar, and seasonal

- Most eggs --> larvae have zooxanthellae, but some don't

- Planula larvae settle after 2-40 days (prefer limestone over granite)

Most corals

Hard corals SLIDE 6:

- Zooxanthellae: 1-10% of coral tissue

- A few corals with zooxanthellae don't deposit CaCO3

- A few corals without zooxanthellae do deposit CaCO3

- Hermatypic corals die without light

- When starved of food or light, corals expel zooxanthellae (not digested)

Anthozoan - Octocorals:

- Gorgonians:

Sea whips
Sea fans
Sea pens
Fire coral

- Skeleton made of organic gorgonin

- They are benthic carnivores

- They use tentacles with nematocysts and mucus

- A few have symbiotic dinoflagellates

- Cosmopolitan

Cnidarians - Scyphozoans (300 species):

- The medusa form dominates

- True jellyfish
- Box jellyfish
- Stalked jellyfish (Benthic)

- Carnivores: they have drifting tentacles with nematocysts

- They have slow jet propulsion

- External fertilization --> planula larvae --> polyp --> bud off medusa

- Polyps can be benthic or pelagic, some can skip the polyp stage

- Cosmopolitan

- Box jellyfish have 24 eyes

Stauromedusae:

- They are benthic

Cnidarians - Hydrozoans:

- Both polyp and medusa forms in life cycle

- Hydroids
- Siphonophores

Hydroids:

- Colonial polyp with attached or separate medusae

Siphonophores (Portuguese Man O' War):

- Complex colony of polyps and medusae

- Differentiation among individuals

- Medusae: swimming, floats, and reproduction

- Polyps: for feeding

- They are ocean drifters (incapable of moving through the water on their own)

- Some are bioluminescent

- Cosmopolitan