ME&B LEC 2 - CORAL REEFS

Why are coral reefs geologically important?

Massive physical structures, islands and archipelagos

Why are coral reefs biologically important?

High diversity, many phyla

Why are coral reefs economically important?

Shoreline protection, harbours, fishing in developing world, tourism

What are coral reefs?

• Compacted and cemented assemblages of sediment and skeletons of sedentary organisms

• Constructional, wave-resistant features

• Built up principally by corals, coralline algae, sponges and other organisms

What are the factors coral reefs need to survive?

• High light - symbiosis with algae

• Warm sea temperature

• Open marine salinities

• Low turbidity

What are the factors that negatively effect coral reef survival?

• Strong sea water currents, wave action

• Reef growth a balance between accretion and bioerosion

• Reef growth must respond to rises and falls of sea leve

Biogeography of coral reefs

• Current division between Pacific and Atlantic provinces

• Strong Pacific diversity gradient:

  • Diversity drops with increasing longitude, away from centre of diversity near Phillipines and Indonesia

  • Diversity drops with increasing latitude, north and south from near equator

• Very diverse in numbers of species

What are coastal reefs?

Wide variety of reefs that grown on the shallow continental shelf, sometimes massive structures, e.g. GBR, down to small patches, e.g. Eilat, Israel

What are Atolls?

Reefs in form of ring or horseshoe-shaped chain of coral cays built up on open oceanic volcanic island - balance of sinking of island and upwrd growth of corals

What are hermatypic corals?

• Reef framework building, have many zooxanthellae, high calcification rates

• Belong to the phylum Cnidaria, Class Anthozoa, Order Scleractinia

• Secrete skeletons of calcium carbonate

• Colonies of many similar polyps

• Branching and massive forms

• Abundant endosymbiotic zooxanthellae

What are ahermatypic corals?

Not framework builders, low calcification rates

What is the branching growth form?

Grow in linear dimension fairly rapidly

What is the massive growth form?

Produce lots of calcium carbonate but grow more slowly in linear dimensions

What is the encrusting growth form?

Horizontal, expand outwards

Talk about how corals have phenotypic plasticity

• Many corals show plasticity in growth form depending upon conditions - ecomorphs

• E.g. chnaged by bidirectional or multidirectional flow

Discuss zooxanthellae in corals

• Considered as one species: Symbiodinium microadriaticum

• Dinoflagellate - without typical dinoflagellate pair of flagella

• Found in endodermal tissues, concentrated in tentacles

What are the benefits of zooxanthellae in corals?

• Nutrition - radio-labelled C taken up by zooxanthellae and transported to coral tissues

• Oxygen for coral respiration

• Facilitate release of excretion products

• Facilitate calcification - uptake of carbon dioxide by zooxanthellae enhances calcium carbonate deposition, inhibit photosynthesis and calcification rate decreases

Discuss depth zonation on reefs

• Reefs dominated by different coral species at different depths

• May be controlled by factors similar to rocky shores

• Also chnaging light conditions

What is the reef flat?

Shallow, exposed at low tide experiencing salinity and temperature variation, reduced water circulation and sediment accumulation - few corals but sea grasses and benthic algae

What is the reef crest?

• Breaking waves and exposed at low tide

• Few, robust coral species with encrusting red coralline algae or brown algae.

• Sometimes referred to as the algal ridge

What is the reef face/front?

• Increasing depth = reduced light intensity

• At surface there may be large corals

• 15-25m - maximum number of coral species

• > 30m hermatypic corals replaced by ahermatypic corals and encrusting organisms such as sponges

What are the primary producers in the reef food web?

Zooxanthellae, calcareous algae, algal mats, phytoplankton, epiphytic species

What are the primary consumers in the reef food web?

Corals, clams, sea urchins, crustaceans, gastropods, grazing fish

What are the secondary consumers in the reef food web?

Anemones, starfish, crustaceans, butterfly fish

What are the tertiary consumers in the reef food web?

Octopus, moray eels, barracuda

What is the range of gross primary production for tropical marine waters?

1500 - 5000 g C m² y⁻¹

What do many organisms in coral reefs do to cement the reef?

They deposit calcium carbonate in shells, skeletons, etc

What other process is carried out by organisms on coral reefs besides cementing?

Bioerosion of the reef

What are the dominant vertebrates in coral reefs?

Reef fish, which are mostly carnivorous

What is the Goreau Paradox?

It is the observation that measured calcification rates of many species on Jamaican reefs do not explain their relative abundance on the reef

What does the Goreau Paradox suggest about growth rates and species abundance on reefs?

It suggests that species' relative abundance is not necessarily linked to their growth

What is an example of a slower-growing species that dominates a particular depth zone on coral reefs?

The massive coral Montastrea annularis, which dominates a specific depth zone despite slower growth rates.

What did Judith Lang observe about Scolymia lacera and its ecological variants?

She observed that supposed ecological variants were placed next to each other, and a bare zone was established after mesenterial filaments were extruded through the polyp wall

What happens during aggressive interactions between polyps of different coral species?

Aggressive species extrude mesenterial filaments, and the tissues of less aggressive species are digested

In terms of aggression, how do massive/encrusting corals compare to branching/foliose corals?

Massive and encrusting forms (slow-growing) are more aggressive than branching and foliose forms (fast-growing)

How do fast-growing corals compete with other species?

They compete by rapid growth and shading

How do slower-growing corals compete with other species?

They use interspecific digestion and sweeper tentacle defenses

What is suggested about the duration of interspecific competition between corals?

Some evidence suggests that interspecific competition may be a short-term effect, which can be overcome in the longer term

Are all fast-growing coral species non-aggressive?

No, some fast-growing species are also aggressive

What does the urchin Diadema antillarum feed on in the Caribbean?

It feeds on sea grasses surrounding patch reefs and reef algae

What happens when Diadema antillarum urchins are experimentally removed from the ecosystem?

Strong seaweed growth occurs as a result

What happened in the 1980s that affected Diadema antillarum populations in the Caribbean?

A disease eliminated most of the urchins, which resulted in strong growth of seaweeds

What is a grazing halo, and what causes it?

A grazing halo is a cleared area around Diadema antillarum urchins caused by their grazing on algae

What was the result of the die-off of Diadema antillarum on Jamaican reefs?

The die-off of Diadema seems to have flipped Jamaican reefs into an alternative stable state, which was also influenced by storm damage

How did the die-off of Diadema antillarum affect the coral cover on Jamaican reefs?

Instead of rich coral cover, reefs now have poor coral cover and lots of algae

What are the effects of competition and herbivory on Porites cylindrica coral and Lobophora variegata alga on the Great Barrier Reef?

Competition (removal of tissue at interaction boundary) and herbivory (exclusion and partial cages) affect both the coral and alga, with the alga able to kill coral tissue, especially at the base where contact is made

How does Lobophora variegata alga affect Porites cylindrica coral?

The alga can kill coral tissue, particularly at the base where contact occurs, but has less effect on the coral's growth at the tips of its branches

How does Porites cylindrica coral affect Lobophora variegata alga?

The coral inhibits algal growth but is less competitive than the alga

What is the main conclusion about herbivores and their effect on corals and algae?

Reduction in grazing pressure leads to increased algal growth and increased coral mortality, meaning herbivores have an indirect effect on corals through their impact on algal competitors

Why is grazing important in maintaining pristine reefs?

Grazing helps control algal growth and is crucial for maintaining the balance of reef ecosystems

Besides invertebrate grazers like Diadema, which other organisms are important for grazing on reefs?

Herbivorous fish are also very important for grazing on reefs

How do herbivorous fish on reefs compare to other marine environments in terms of abundance?

Herbivorous fish are more abundant on coral reefs than in any other marine environment

How much algal production can herbivorous fish remove from coral reefs?

Herbivorous fish can remove 50-100% of the total algal production on reefs

Why do herbivorous fish need to consume large amounts of algae?

Algae have low nutritional value, so herbivorous fish need to consume large quantities to meet their dietary needs

What is the role of territorial damselfishes in reef ecosystems?

Some damselfishes defend "algal gardens," creating a complex relationship between algae and different life stages of corals

How might the interaction between algae and corals lead to a shift in coral types?

The complex relationship between algae and corals, particularly the influence of herbivory, may contribute to a shift toward faster-growing branching corals.

What was the main finding of the Cheal et al., 2010 11-year study on the Great Barrier Reef?

The study recorded a persistent coral-macroalgal phase shift on the Great Barrier Reef (GBR)

What event followed the coral-macroalgal phase shift on the GBR? (Cheal et al., 2010)

The phase shift followed extensive coral mortality due to bleaching and storm damage

How did the coral-macroalgal phase shift on the GBR compare to two other reefs in the Cheal et al., 2010 study?

The GBR experienced a persistent phase shift, while the two other reefs suffered similar damage but recovered quickly

What factors were associated with the persistent coral-macroalgal phase shift on the GBR? (Cheal et al., 2010)

The phase shift was associated with low herbivore fish diversity and low abundance of algal browsers (e.g., rabbitfishes) and grazer detritivores (e.g., surgeonfishes)

Despite high densities of parrotfish, why did the coral-macroalgal phase shift persist on the GBR? (Cheal et al., 2010)

The phase shift persisted despite high parrotfish densities, likely due to the low diversity and abundance of other herbivorous fish, like algal browsers and detritivores

What implication does the study have regarding the harvesting of herbivorous fish on the GBR? (Cheal et al., 2010)

The study suggests that if herbivorous fish are harvested, reefs may become vulnerable, even though these fish are not currently harvested on the GBR

What was the focus of the two experiments conducted over 2 years in the Burkpile and Hay, 2008 study?

The experiments focused on assessing the effects of different herbivore species and densities on seaweeds and corals

What were the experimental treatments used in the studies? (Burkpile and Hay, 2008)

The treatments included large cages enclosing single herbivore species (redband parrotfish, ocean surgeonfish, or two parrotfish species), equal densities of mixed herbivore species, or excluding herbivores

How did mixed-herbivore treatments compare to single-herbivore treatments in terms of macroalgal abundance? (Burkpile and Hay, 2008)

Mixed-herbivore treatments lowered macroalgal abundance by 54-76%

What effect did mixed-herbivore treatments have on crustose coralline algae? (Burkpile and Hay, 2008)

Mixed-herbivore treatments enhanced the cover of crustose coralline algae (preferred recruitment sites for corals) by 52-64%

How did mixed-herbivore treatments impact coral cover and coral mortality? (Burkpile and Hay, 2008)

Mixed-herbivore treatments increased coral cover by 22% and prevented coral mortality

What is the main conclusion about herbivore diversity in coral reef preservation? (Burkpile and Hay, 2008)

Maintaining herbivore diversity and complementarity of feeding is critical to the preservation of coral reefs

What is the general conclusion about the effect of parrotfish on coral reefs?(Mumby, 2009)

The balance of evidence suggests that parrotfish have a positive effect by reducing algal levels and encouraging coral recruitment and growth

Is there any evidence for negative effects of corallivory (coral-eating) by parrotfish on coral growth and spread? (Mumby, 2009)

No, there is no evidence that corallivory constrains coral growth and spread

What is a potential negative role of parrotfish on coral reefs, according to some suggestions? (Mumby, 2009)

Some suggest that parrotfish may be involved in the spread of coral disease

What species of starfish feeds on corals in the Pacific Ocean?

The Crown-of-thorns starfish (Acanthaster planci) feeds on corals

When did outbreaks of Acanthaster planci start occurring across the Indo-Pacific?

Outbreaks started in the 1960s

How did the behavior of Acanthaster planci change over time?

They changed from being rare and dispersed to herding together, and from being nocturnal to diurnal in their feeding habits

What is the COTSbot

COTSbot is designed to seek out and inject Crown-of-thorns starfish with a lethal dose of bile salts

How do corals reproduce asexually?

Budding from parent and main mechanism for expanding colony size - new colonies may also form by fragmentation from existing colony

How do corals reproduce sexually?

Main mechanism for establishment of new colonies - most corals are hermaphroditic but some are dioecious

How do most corals reproduce?

Most corals are broadcast spawners, while some are brooders that release offspring at the planula larval stage

When does coral spawning typically occur, and how is it synchronized?

Coral spawning is usually seasonal, and many corals release gametes synchronously, often linked to the lunar cycle (e.g., on the Great Barrier Reef)

How do diseases spread in marine environments, and what is one example of mass mortality caused by disease?

Diseases spread through currents and can cause mass mortality of species, such as the common black sea urchin (Diadema antillarum) in the 1980s

What is an example of a disease affecting corals, and what causes it?

An example is black band disease, caused by the cyanobacterium Phormidium corallyticum

How does black band disease affect corals?

A dense mat of filaments develops between the coral’s living tissue and its skeleton, leading to secondary bacterial infections

What is the trend regarding coral diseases?

Coral diseases, like black band disease, are becoming more prevalent

What do Lesser et al. (2007) suggest about the nature of most coral diseases?

Lesser et al. (2007) suggest that the majority of coral diseases are opportunistic secondary infections following earlier physiological stress, such as temperature changes

What did Maynard et al. (2015) predict in their modelling study regarding pathogens?

Maynard et al. (2015) predicted that the abundance and virulence of pathogens would increase due to the predicted rise in global temperature

How do increased temperatures affect coral pathogens?

It appears that pathogens involved in coral diseases are favored by increased temperatures

How does tidal exposure affect reef flat species?

In regions like the Red Sea, very low tides kill most reef flat species, with fast-growing branching species being more susceptible

How does wave action impact coral reefs?

Wave energy decreases with depth, with most energy concentrated at the reef crest, which often lacks corals and is dominated by an algal ridge

Which species are most affected by storm damage on coral reefs?

Most storm damage occurs to shallow species on coral reefs

How do regular, intermediate disturbances affect coral reef diversity?

Evidence suggests that regular, intermediate disturbances help maintain coral reef diversity

What effect do big storms have on coral reefs?

Big storms have the opposite effect of regular disturbances, potentially reducing coral reef diversity

How does sedimentation affect coral reefs?

• Sedimentation affects coral reefs by:
  a) Smothering corals and increasing turbidity, which inhibits feeding and/or photosynthesis, and
  b) Covering hard substrata, making it unsuitable for colonization

How do corals differ in their ability to cope with sedimentation?

Corals have varying abilities to remove sediment, which affects the distribution of coral species

What causes corals to become more susceptible to sedimentation?

Corals are more susceptible to increased sediment loads due to local dredging activities or increased runoff from land management practices

How do algae typically appear in healthy reefs?

Algae are usually inconspicuous in healthy reefs but play an important role in the ecosystem

What role do encrusting red coralline algae (e.g., Lithothamnion) play on reefs?

Encrusting red coralline algae help in "cementing" reef structures.

What is the role of the "algal ridge" on reefs?

The algal ridge reduces the velocity of waves at the windward edge of reefs

How do calcareous green algae (e.g., Halimeda) contribute to the reef ecosystem?

Calcareous green algae contribute to lagoon sands

How do algae contribute to overall reef productivity?

Algae contribute to overall reef productivity by playing key roles in cementing, sand formation, and wave energy reduction

What is notable about coral reef fish communities?

Coral reef fish communities are the most diverse fish communities on Earth