MBE 230-Chapter 10: Cnidarians (Coelenterates)

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

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 <10 years Many corals >100 years Some clones >1,000 years

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