Topic 14: Coral Reefs

Biotic community

Assemblage of organisms that live together within some definable area or habitat

Ecosystem

Biotic plus abiotic community through which organisms transfer and/or exchange energy and nutrients

Reefs

The largest biologically constructed formations; underwater ecosystem characterized by reef-building marine invertebrates (corals)

• Found throughout the ocean (cold-water corals), focus on those in shallow, warm water areas

Structure of coral reefs

Consist of a living framework with internal cavities filled with sediment and a surrounding area of reef-derived skeletal grains

3 Characteristics of coral reefs

1. Densely populated and diverse community

2. Actively growing coral colonies, also contain fragments of dead coral and other material

3. Higher energy environment (attempt to be 1st to obtain nutrients)

3 main types of coral reefs (based on stages of formation)

1. Fringing reefs

2. Barrier reefs

3. Atolls

Fringing reef

A reef that may connect directly to land with a non-existent or shallow back reef; located nearshore

• The most common reef type

• Living organisms are concentrated on the seaward edge of reef

Barrier reef

Reefs separated from land by a deep channel or lagoon with a raised outer (seaward) edge (for greater food supply and therefore faster growth)

Atoll

A ring-shaped island of coral reefs and coral debris that encloses or partially encloses a shallow lagoon

• Potential to be highly affected by sea level rise

Darwin’s ideas on reef development + flaws

Theory that various types of coral reefs and atolls could be explained by uplift and subsidence of Earth’s crust under the ocean; based on observations made during voyage on the Beagle

Flaw in Darwin’s ideas?

• Failed to consider sea level rise and fall due to glaciation (not commonly known in his time)

Who builds the reef?

Phylum Cnidaria (includes corals, sea anemones and jellies)

2 basic body forms of phylum Cnidaria

1. Free-swimming / floating (nekton) medusae

2. Sessile/benthic polyps anchored to substrate

Distinguishing feature of phylum Cnidaria

Specialized cells (cnidocytes) used for protection and capturing prey that contain nematocysts (stinging organelles)

Stony/reef-building corals

Any corals that contain zooxanthellae (tiny photosynthetic algae living in their tissue; dinoflagellates)

Purpose of zooxanthellae

1. Create a hard, calcareous skeleton

2. Mutualism with coral– corals supply nutrients, zooxanthellae supply food!

3. Gives colour to corals

Body of coral polyps

• Coral polyps are all stomach and tentacles

• With stinging cells on tentacles

• They secrete CaCO3 to build a hard outer skeleton

Mixotrophs (i.e reef corals)

Mix of autotrophs (feed themselves) and heterotrophs (feed on others)

How reef corals obtain food

10% from food capture (used for growth and respiration

90% from zooxanthellae

• Most of the carbon forms photosynthate (carbs and lipids) which forms a protective mucus

• Also used for respiration

Reef-building corals thrive in _______ environments

Oligotrophic (low-nutrient)

How increased nitrate affects reef-building corals

Zooxanthellae retain that nitrate for their own growth which harms the coral’s metabolism, making the coral supsceptible to disease and bleaching

6 necessary conditions for coral reef growth

1. Clear (nutrient poor) water with low turbidity (little sediment)

2. Brightly lit (so zooxanthellae can photosynthesize)

3. Warm, shallow water

4. Normal to slightly elevated salinity (25 to 35%)

5. Strong wave or current activity (to bring nutrients and O2 in)

6. Temperature is 25 to 29 degrees C (temp. that the zooxanthellae like)

Cause of coral bleaching

Death of zooxanthellae due to drastic changes in conditions

Coral reef zonation

They exhibit horizontal and vertical zonation; different coral assemblages and animals live in different places

• A result of changes in sunlight as well as other factors (salinity, nutrition, temp)

Growth form and environment of different coral

Environment of encrusting corals

Found in very high energy conditions, grow/live directly on the reef substrate

Reef slope

The fore-reef or reef front

Inhabitants of the reef slope

Delicate growth forms and larger corals; species diversity decreases when moving deeper

Reef crest

Highest part of the reef, exposed at low tide with the greatest wave action

Inhabitants of the reef crest

Dominated by robust branching corals and coralline algae

Reef flat

Area of protected calm, shallow water

Inhabitants of the reef flat

Corals and other marine organisms that can tolerate a wide range of temperatures, light intensity, and salinity and adapt to low levels of dissolved oxygen in seawater

• Gorgonian corals (soft corals) that do not produce hard CaCO3 structure (also known as sea fans or sea whips); still have polyp

• Sea cucumbers (scavengers and deposit feeders; echinoderms)

Deposit feeding

Feeding on food items that occur as deposits; some organisms ingest sediment and extract organic matter from it

6 Organisms found in reefs

1. Calcareous algae

2. Fish

3. Sharks and rays

4. Phytoplankton and zooplankton

5. Molluscs

6. Sponges

4 Categories of organisms found in reefs

1. Builders (corals, calcareous red algae)

2. Dwellers (bivalves, calcareous green algae, fish, sponges, anemones, crabs…)

3. Grazers (echinoderms, gastropods, some fish)

4. Destroyers (sponges, bivalves, annelids)

What are sponges

• Sessile, benthic filter feeders

• Also mixotrophs

• Symbiotic relationship with algae where they derive part of nutrition from symbiote

• Primitive animals

3 classes of molluscs

1. Cephalopoda (octopus, squid…)

2. Bivalvia (clams, oysters, mussels…)

3. Gastropoda (snails, slugs, periwinkles…)

Molluscs

Several classes of organisms (from mussels to octopus) that differ in size, shape, habitat, feeding strategies but have similar body plans and share a common ancestor

4 Parts of a mollusc + their basic function

1. Foot: movement

2. Mantle: creates the shell; covers the visceral mass

3. Shell: protects the visceral mass (‘guts’); 3 layers

4. Radula: tiny teeth; a scraper

3 life strategies of molluscs

1. Carnivorous feeding

2. Siphoning

3. Filter (suspension) feeding

Siphon (molluscs)

Siphon promotes water flow and used for locomotion, feeding, respiration, and reproduction

• Symbiotic algae living in its mantle

Filter (suspension) feeding

Organism cannot move and attached to a substrate (e.g. clams, oysters)

• Have special structures where water passes through – filter food particles out

Ecological role of parrot fish

They browse on and ingest corals and algae; excrete carbonate sand

4 reasons why coral reefs are so important

1. Home to >> 1 million diverse aquatic species

2. Have a global estimated value of $3 trillion per year, including tourism and food

3. Provide coastal protection (reduce shoreline erosion by absorbing energy from the waves) which protects coastal housing, agricultural land and beaches

4. Home to species that contain pharmaceutical compounds— treatments for illnesses and diseases

4 factors that can lead to coral bleaching

1. Climate change causing changes in water temp

2. Extremely low tides causing exposure

3. Overexposure to sunlight

4. Pollution