Scleractinian Corals

Question 1

What phylum are scleractinian corals found within taxonomically? What is typical about organisms in this phyla?

Answer 1

Cnidaria

• Radial symmetry
• All have cnida/nematocysts -> a harpoon-like structure within a specialised organelle (vesicle with a membrane known as a nematocyst vesicle) that creates a stinging sensation.

Question 2

What sub-clade are scleractinian corals found within taxonomically?

Answer 2

Anthozoans

Question 3

What are the two main classes within Anthozoans?

Answer 3

Octocorallia (multiples of 8) and Hexacorallia (multiples of 6)

Question 4

What are Scleractinia known as?

Answer 4

“Hard/stony corals”

Question 5

What relationship do most scleractinia exhibit?

Answer 5

Symbiotic relationship with dinoflagellates/zooxanthellae (photosynthetic unicellular organisms).

Note: this is not algae as they belong to the alveolate [they look more like algae because they lose their flagella, but are still dinoflagellates]

Question 6

What does it mean to be hermatypic?

Answer 6

Reef-building

Question 7

How do Scleractinian corals help to build up the reef?

Answer 7

They precipitate a CaCO3 skeleton via accretion processes.
This is then consumed by organisms (soft and hard coral tissue), such as invertebrates and fishes, which produces fine CaCO3 as a byproduct. This is slowly compacted over time by abiotic and biotic processes, cementing the structure and forming the reef itself (this cement is the reef!)

Question 8

How is the rate of reef growth linked to Scleractinian corals?

Answer 8

Reef growth is directly linked to CaCO3 skeletal growth.

Question 9

Is the skeleton external to the soft tissue in Scleractinian corals or vice versa?

Answer 9

Solid calcareous skeleton external to the soft tissues (fleshy part = polyp)

Question 10

What is a polyp?

Answer 10

A polyp is the single organism within the coral colony.

Question 11

What does the polyp sit in?

Answer 11

Part of the coral skeleton known as the corallite, which forms the cup structure that the polyp sits in.

Question 12

What is the main structure of a polyp?

Answer 12

• Two cell layers (inner = endoderm, outer = ectoderm) separated by the mesoglea.
• Some specialised cells for carnivorous diet -> note there is no nervous system.

Question 13

What are the two cell layers associated with in a polyp?

Answer 13

Endoderm = symbiont
Ectoderm = calcification when in contact with the skeleton

Question 14

Where does calcification occur in Scleractinian corals? What are the cells involved known as?

Answer 14

Occurs at the interface between the ectoderm/ outer cell layer and the skeleton using specialised calicoblastic cells (specific part of the ectoderm between mesoglea and skeleton)

This is known as ECM = Extracellular Calcifying Medium

Question 15

What four morphological characteristics can be used to identify species?

Answer 15

1. Corallite structure
2. Corallite size
3. Colonial vs solitary
4. Colony morphology

Question 16

What are the four main types of colony morphology?

Answer 16

1. Branching
2. Foliose (plate-like)
3. Massive
4. Encrusting

Question 17

What type of reefs are coral reefs?

Answer 17

Natural, biogenic reefs

Question 18

What type of reefs are sub-tropical and tropical reefs normally (i.e., the main ones studied in this module)?

Answer 18

Warm-water shallow corals

Question 19

Where are deep warm water corals found?

Answer 19

The Red Sea - (which is deep and tropical with clearer water allowing mesophotic ecosystems to form)

Question 20

What are the main depths of corals?

Answer 20

Shallow water corals (up to ~20-30 metres deep - sometimes even up to 40 depending on the light conditions) = euphotic corals

40-80 metres = mesophotic corals

Deep-sea corals = aphotic corals, 150 metres + -> these are

Question 21

Are nutrient concentrations in seawaters associated with corals typically high or low? Why?

Answer 21

Corals typically live in nutrient poor (oligotrophic) waters associated with tropical/sub-tropical environments (remember the water is typically very blue as limited phytoplankton, allowing light to penetrate deep which is good for corals)

Question 22

What is the paradox identifying the conditions that corals thrive in?

Answer 22

Darwin Paradox -> coral ecosystems thrive despite limited nutrients

Question 23

What are the three important nutrients biologically and what are their uses?

Answer 23

Carbon (proteins (amino acids), lipids and DNA/RNA)

Nitrogen (amino acids, DNA/RNA)

Phosphorus (DNA, phospholipid membranes)

Question 24

How do corals thrive in oligotrophic regions?

Answer 24

Endosymbiotic intracellular symbiosis allows access to 2 independent sources of nutrients:

1. Coral can feed on what is available -> particulate organic N/P etc.
2. Dissolved inorganic N/P can be taken up by the algal partner and organic C provided by photosynthesis

Question 25

What is exchanged between the coral and the algal symbiont?

Answer 25

- Inorganic N/P (waste) is passed from the coral to the algae - Organic N/P, sugars, lipids (C) and amino acids (N) (excess) is passed from the algae to the corals

Question 26

Why do nutrient concentrations in the seawater around coral remain low?

Answer 26

As nutrients are recycled within the holobiont (host)

Question 27

What is another example of a reef organism that is crucial for nutrient cycling?

Answer 27

Sponges - increase concentrations of inorganic nutrients

Question 28

What happens if the Scleractinia are removed from the system?

Answer 28

The reef structure is no longer replenished as these are the main reef-builders, breaking down the 3D habitat. This means that the ecosystem starts to fall apart.

Question 29

What are three different types of reefs that exhibit slightly different reef zones?

Answer 29

1. Fringing reef 2. Barrier reef 3. Atoll reef

Question 30

In a fringing reef, what are the three main zones in order of closest to the land?

Answer 30

1. Reef flat - intertidal region along shoreline (not good for corals as varying conditions) 2. Reef crest - highest part of reef, receiving most sunlight, but must withstand intense wave action) 3. Reef front/fore reef (gradually deepens, with highest biodiversity at shallowest part, and coral becoming wider and flatter to maximise sunlight access with depth)

Question 31

What are the four main zones of a barrier reef?

Answer 31

1. Lagoon 2. Back reef 3. Reef crest 4. Reef front/fore reef

Question 32

What are the five main zones of an Atoll reef?

Answer 32

1. Lagoon 2. Back reef 3. Reef flat 4. Reef crest 5. Reef front/fore reef

Question 33

How can reef zones be distinguished?

Answer 33

Location in the reef Depth Wave action Light intensity Temperature Water chemistry