Anthozoans and the evolution of reefs

General Cnidarian features

• Tentacles
• Nematocysts
• Polyp and Medusa
• Planula larva

Problems of sessility

• Must wait for food
• Must encounter food while resisting predators, wave action
• Need to obtain and defend space
• Sexual reproduction can be difficult

"Hydrozoan" sessility

• Prominent polyp stage
• Limited by small size of polyps
• Typically lack armor
• Perisarc - thin protection

"Hydrozoan" sessility

• Prominent polyp stage
• Limited by small size of polyps
• Typically lack armor
• Perisarc
• Hydrocorallines (fire coral)
  • CaCO3
  • Big nasty nematocysts!

"Hydrozoan" solutions

• Solitary species
• usually cryptic
• Colonial species
• specialized defensive polyps
• Dactylozooids
• Coloniality very important
• E.g., Physalia

Anthozoan solutions to sessility

• More complex polyps
• Tentacles more numerous
• Nematocysts more ubiquitous
  • Gastrodermal and Epidermal

Anthozoan solutions to sessility

• More complex polyps
• Tentacles more numerous
• Nematocysts more ubiquitous
  • Gastrodermal and Epidermal
• Tubular Pharynx
• Siphonoglyphs
  • (provide biradial symmetry)

Radial vs.
Biradial symmetry

(slide--know)

Anthozoan solutions to sessility

• More complex polyps
• Tentacles more numerous
• Nematocysts more ubiquitous
  • Gastrodermal and Epidermal
• Tubular Pharynx
• Siphonoglyphs
• Very fibrous mesoglea
• Septate Gastrovascular cavity

Anthozoan Reproduction

• Asexual
• Pedal Laceration/Fragmentation
• Lateral fission common
• Colonies of clones
• Sexual
• Produce eggs, sperm, planula
• Hermaphrodites common
  • simultaneous
  • sequential
• Separate sexes
• Some brood, some broadcast

Anthozoans - Three subclasses

• Alcyonaria
• Soft corals
• Ceriantipatheria
• Tube, thorny corals
• Zoantheria
• Anemones, stony corals

Subclass Alcyonaria = Octocorallia

• 8 part symmetry
• All colonial with small or large polyps

Subclass Alcyonaria

• 8 part symmetry
• All colonial, polyps may

• Feed on plankton
• Gastrovascular cavities

• Fig. 6-25

Subclass Alcyonaria

• 8 part symmetry
• All colonial with small or large polyps
• Feed on plankton sea fans, sea whips
• Solenia
• Fibrous protein skeleton

Soft sediment alcyonarians - sea pens

• Primary axial polyp
• Supports whole colony
• Skeletal spicules deter predators
• Can take advantage of underused habitat
• Bioluminescent -
  spectacular!

Subclass Zoantheria = Hexacorallia

• Order Scleractinia Stony Corals
• Order Actinaria Anemones

Subclass Zoantheria = Hexacorallia

• Stony corals - Order Scleractinia
• Anemones - Order Actinaria
• Six part symmetry

Order Scleractinia

• Most abundant sessile cnidarians
• Both solitary (primitively) and colonial
• Solitary forms have very large polyps
• Colonial forms have small to moderate polyps
• Solitary corals once very successful, relatively few species remain

Scleractinian advances

• Increased number of simple tentacles
• Can capture larger prey
• Increased number of septa
• Improved digestion and support
• Fight for space
• Edge polyps divide & grow laterally, septal filaments emerge & secrete
  enzymes, "sweeper" polyps develop with long
  tentacles and big mean nematocysts!
• Can become very large (>10 m)
• Sclerosepta improve attachment
• Less susceptible to predators

Sclerosepta

(slide)

Colonial zoantharians

• Stony corals are the most abundant zoantharians
• Polyps range from small to moderately large (1-10 mm)
• Polyps secrete calcareous skeleton
• Each polyp in a calyx
• Neighboring polyps attached. Colony can share food!

Colonial zoantharians
Four Sources of Nutrition

• **"Normal" - feed on zooplankton using nematocysts on tentacles
• Mucous entrapment of detritus
• Direct absorption of organic molecules
• **Symbiotic zooxanthellae
• Only in reef-building ["hermatypic"] corals___
• Dinoflagellates live inside coral cells, may either invade or be passed in egg
• **Accelerate CaCO₃ production
• Provide organic molecules
• Dinoflagellate ecology--on being leaky

Environment and Colonial Coral Morphologies - Within species

(slide)

Environment and Colonial Coral Morphologies - Among species

• Branched corals
• Large surface area
• Small attachment area
• Open structure dissipates wave energy
• Very rapid growth; not aggressive
• Tabular corals
• Massive corals
• Platey or encrusting corals

Environment and Colonial Coral Morphologies - Among species

• Branched Corals
• Tabular corals
• Large surface area
• Small attachment area
• Easily damaged by waves; live in sheltered lagoons
• Rapid growth; not aggressive
• Massive corals
• Platey or encrusting corals

Environment and Colonial Coral Morphologies - Among species

• Branched Corals
• Tabular Corals
• Massive Corals
• Moderate surface area
• Broad stable base
• Usually resistant to wave shock; but become bowling balls in a hurricane!
• Slow growth - may live 1,000 years; very aggressive
• Platey or encrusting corals

Environment and Coral Morphologies

• Branched Corals
• Tabular Corals
• Massive Corals
• Platey or encrusting corals
• Some massive species become plate-like in deep water to gather more light (within species)
• Some platey species are all small, disc-shaped; most species live in deeper water (fragile - frequent partial mortality)
• slow growing; most aggressive of all

Reef Structure

Three types of coral reefs

• Fringing reef
• Small lagoon (if any)
• Barrier reef
• Linear lagoon
• Atoll reef
• Circular lagoon

Three types of coral reefs

• Fringing reef
• Small lagoon (if any)
• Barrier reef
• Linear lagoon
• Atoll reef
• Circular lagoon

Three types of coral reefs

• Fringing reef
• Small lagoon (if any)
• Barrier reef
• Linear lagoon
• Atoll reef ("ring islands" -- Darwin)
• Circular lagoon

3 pinnacles of biodiversity on Earth

• Rainforests, the deep sea, and coral reefs
• Of these, coral reefs have the smallest area, only 0.2% of the surface of the Earth (rainforests are 8% and deep sea about 60% of Earth’s surface)
• About 100,000 known species
• 1-5 million total [known + unknown] species (1/10 to 1/3 of total [known + unknown] species on Earth)

Dangers to coral reefs

• Siltation/nutrient enrichment/pollution:
• Land-use practices
• Destructive harvest
• Cyanide
• Explosives
• Overfishing
• Temperature increase
• Storms
• Predator and disease outbreaks

Anemones: Naked zoantharians

• Similar to corals, but …
• Lack protective skeleton
• Individuals can be very large (to >1m, e.g., Great Barrier Reef)
• Huge number of septa
• Increased area for digestion
• Increased support for body wall
• Usually solitary, but a few species are clonal - fight other clones for space

Anemone Characteristics

• General anthozoan features, plus ...
• Complex musculature

Anemone Characteristics

• General anthozoan features, plus ...
• Complex musculature
• Acontia for defense
• Large, muscular pedal disk
• Can move via cilia/mucous
• Extensive nervous system
• Seeking behavior
• Shielding behavior

General Cnidarian Evolution

Phylum Ctenophora

• Previously allied with Cnidarians due to primitive similarities
• Radial/biradial symmetry
• Diploblastic
• Gelatinous mesoglea
• Diffuse nerve net
• Nematocysts, when present, are stolen from Cnidarians

Ctenophore relationships

(slide)

Ctenophores and Bilaterians

• Ctenophores are the sister-group of
• Bilateria (general view)
• All other Eumetazoa (your text)
• Deuterostomes (a few people)
• True muscle
• Like Bilateria

Ctenophore characteristics

• Ctenes in 8 comb rows
• Colloblasts for prey capture
• Complex gut with anal canals

Ctenophore characteristics

• Ctenes in 8 comb rows
• Colloblasts for prey capture
• Complex gut with anal canals

Ctenophore locomotion and feeding

• Ctenes beat in oral to aboral wave
• Mouth forward
• Sophisticated relative to Cnidarians