Biology and Ecology of Coral Reefs

These flashcards cover key vocabulary related to the biology and ecology of coral reefs, including definitions of important terms, concepts, and processes that are integral to understanding coral reefs and their environments.

Coral Reefs

Underwater ecosystems characterized by reef-building corals.

Oligotrophic

Describes water that is nutrient-poor, promoting clear water that supports coral growth.

Biogeography

The study of the distribution of species and ecosystems in geographic space and through geological time.

Marine Protected Areas (MPAs)

Regions of the ocean designated to protect and conserve marine ecosystems and biodiversity.

Mutualism

A symbiotic relationship where both species benefit from the interaction.

Climate Change

Long-term alteration of temperature and typical weather patterns in a place, significantly affecting coral reefs.

Corallivores

Species that feed directly on coral tissues, impacting coral health and biodiversity.

Herbivory

The act of consuming plants, which in coral ecosystems is important for controlling algae that compete with corals.

Symbiodiniaceae

A family of algae that live in symbiosis with corals, providing them with nutrients through photosynthesis.

Acanthaster planci

Commonly known as the crown-of-thorns starfish, a major predator of corals that can cause significant damage to coral reefs.

Coral-algae phase shift

A shift from dominance by corals to dominance by macroalgae, often due to environmental stressors.

What are Coral Reefs?

Coral reefs are complex underwater ecosystems built by colonies of tiny marine invertebrates called coral polyps, which secrete calcium carbonate to form hard exoskeletons. These structures create a diverse habitat for a vast array of marine life.

Types of Coral Reefs: Fringing Reefs

Fringing reefs are coral reefs that grow close to the coastline throughout the tropics, directly attached to the shore or separated by a narrow, shallow lagoon. They are the most common type of reef.

Types of Coral Reefs: Barrier Reefs

Barrier reefs are extensive linear reefs that run parallel to the coast, but are separated from it by a wider and deeper lagoon than fringing reefs. The Great Barrier Reef is a famous example.

Types of Coral Reefs: Atolls

Atolls are ring-shaped coral reefs that enclose a central lagoon, often formed from the subsidence of a volcanic island. They are typically found in the open ocean far from continental landmasses.

Ecological Importance of Coral Reefs

Coral reefs are often called 'rainforests of the sea' due to their extraordinarily high biodiversity. They provide habitat, breeding, and feeding grounds for thousands of marine species, protect coastlines from erosion, and support fisheries and tourism.

What does Oligotrophic mean in the context of coral reefs?

Oligotrophic describes environments, such as tropical ocean waters where coral reefs thrive, that are characterized by low levels of dissolved nutrients (e.g., nitrates, phosphates). This condition leads to clear water, allowing sufficient light penetration for the symbiotic algae within corals.

Why are Oligotrophic Waters essential for Coral Reef Health?

Oligotrophic waters are crucial because they limit the growth of fast-growing macroalgae, which would otherwise outcompete slow-growing corals for space and light. The clarity of these waters also maximizes light availability for the photosynthetic Symbiodiniaceae within coral tissues.

What is Eutrophication?

Eutrophication is the process by which a body of water becomes overly enriched with nutrients, often from agricultural runoff or sewage, leading to excessive growth of algae and depletion of oxygen (hypoxia) as the algae decompose.

Impact of Eutrophication on Coral Reefs

Eutrophication can severely degrade coral reefs by promoting algal blooms that reduce light penetration for corals, increase competition for space with macroalgae, and can lead to oxygen depletion, ultimately stressing or killing corals.

What is Biogeography?

Biogeography is the scientific study of the distribution of species and ecosystems in geographic space and through geological time. It seeks to understand where organisms live, at what abundance, and why they are (or are not) found in certain geographical areas.

Coral Reef Biogeography

Coral reef biogeography examines the spatial patterns of coral species and reef ecosystems globally, considering factors like ocean currents, sea surface temperature, and historical geological events that influence their dispersal, colonization, and evolution.

Factors Influencing Coral Distribution: Temperature

Coral reefs are predominantly found in tropical and subtropical waters where sea temperatures range between 18-30^{\circ}C (64-86^{\circ}F), with optimal temperatures around 23-29^{\circ}C (73-84^{\circ}F). Prolonged exposure to temperatures outside this range causes stress, such as coral bleaching.

Factors Influencing Coral Distribution: Light Availability

Scleractinian (reef-building) corals rely on symbiotic algae (Symbiodiniaceae) for photosynthesis. Therefore, they thrive in clear, shallow waters where sunlight can penetrate, typically at depths less than 50 meters, although some species can exist deeper.

Factors Influencing Coral Distribution: Ocean Currents

Ocean currents play a vital role in coral reef biogeography by facilitating the dispersal of coral larvae (planulae) over vast distances, connecting disparate reef systems and maintaining genetic flow between populations.

What are Marine Protected Areas (MPAs)?

Marine Protected Areas (MPAs) are discrete geographical regions of the ocean, including the overlying water and seafloor, that are designated and managed to protect and conserve marine ecosystems, biodiversity, and cultural resources through specific regulations.

Purpose of Marine Protected Areas (MPAs)

The primary purpose of MPAs is to conserve marine biodiversity, protect habitats from human impacts, restore degraded ecosystems, and support sustainable fisheries by providing safe havens for marine species to reproduce and grow.

Benefits of MPAs for Coral Reefs

MPAs protect coral reefs by reducing destructive fishing practices, limiting pollution, managing tourism, and providing refugia where corals can recover from disturbances and act as sources for larval dispersal to surrounding areas.

Challenges in MPA Implementation and Management

Challenges include obtaining sufficient funding, ensuring effective enforcement against illegal activities, engaging local communities and stakeholders, and managing internal and external threats like climate change impacts and pollution originating outside MPA boundaries.

Types of MPAs: No-Take Zones

No-Take Zones are the most restrictive type of MPA, where all extractive activities, such as fishing, mining, and drilling, are strictly prohibited. These zones are crucial for allowing marine populations to recover and flourish.

What is Mutualism in ecology?

Mutualism is a type of symbiotic relationship between two or more species where all involved species derive a net benefit from the interaction. Both partners gain increased fitness, survival, or reproductive success.

Example of Mutualism: Coral-Symbiodiniaceae Symbiosis

A classic example of mutualism in coral reefs is the relationship between stony corals and photosynthetic dinoflagellates from the family Symbiodiniaceae (zooxanthellae). The algae live within the coral's tissues.

How Symbiodiniaceae Benefit Corals

Symbiodiniaceae provide corals with up to 90\% of their energy needs through photosynthesis, converting sunlight into sugars, amino acids, and other nutrients. This high energy input enables corals to grow rapidly and build large calcium carbonate skeletons.

How Corals Benefit Symbiodiniaceae

Corals provide Symbiodiniaceae with a protected environment within their tissues, access to sunlight, and a steady supply of essential nutrients like carbon dioxide and metabolic waste products (e.g., nitrogen and phosphorus) that would otherwise be scarce in oligotrophic waters.

What is Climate Change impacting coral reefs?

Climate Change refers to long-term alterations in global or regional climate patterns, driven significantly by anthropogenic emissions of greenhouse gases. For coral reefs, this leads to complex issues including ocean warming, ocean acidification, and changes in storm intensity.

Impact of Climate Change: Ocean Warming

The absorption of excess heat by oceans due to climate change leads to rising sea surface temperatures. Prolonged periods of elevated temperatures cause thermal stress on corals, leading to coral bleaching events.

What is Coral Bleaching?

Coral bleaching is the process where corals expel their symbiotic Symbiodiniaceae algae due to stress (primarily from elevated water temperatures, but also pollution or disease). The loss of these pigmented algae causes the coral to turn white, making it vulnerable to disease and starvation.

Consequences of Coral Bleaching

If bleaching events are prolonged or severe, corals will starve and die, leading to significant declines in coral cover, loss of reef structure, decreased biodiversity, and reduced ecosystem services such as coastal protection and fishery support.

What is Ocean Acidification?

Ocean acidification is the ongoing decrease in the pH of the Earth's oceans, caused by the uptake of atmospheric carbon dioxide (CO2) by seawater. As CO2 dissolves, it forms carbonic acid, reducing the availability of carbonate ions (CO_3^{2-}).

Impact of Ocean Acidification on Coral Reefs

Ocean acidification inhibits the ability of corals and other calcifying organisms (e.g., mollusks, coralline algae) to build and maintain their calcium carbonate (CaCO_3) skeletons and shells, leading to slower growth, weakened structures, and increased erosion rates.

Impact of Climate Change: Sea Level Rise

Sea level rise, largely caused by thermal expansion of seawater and melting glaciers and ice sheets, threatens shallow-water coral reefs by reducing light penetration, increasing sedimentation, and potentially drowning corals if they cannot grow upwards fast enough.

Impact of Climate Change: Extreme Weather Events

Climate change is increasing the frequency and intensity of extreme weather events such as hurricanes and cyclones. These powerful storms can cause severe physical damage to coral reef structures, breaking off corals, overturning entire colonies, and scattering debris.

What are Corallivores?

Corallivores are species of marine organisms that feed directly on coral tissues, polyps, or mucus, impacting coral health, growth rates, and community structure. They are a natural component of reef ecosystems, but outbreaks can cause significant damage.

Examples of Corallivorous Species

Corallivores include a range of organisms such as certain fish species (e.g., butterflyfish, puffers), snails (e.g., Drupella, Coralliophila), and starfish (e.g., Crown-of-Thorns starfish, Acanthaster planci).

Ecological Role of Corallivores

In balanced ecosystems, corallivores can play an important role in reef dynamics by pruning corals, controlling their growth, and promoting biodiversity by opening up space for other species to colonize. However, overpopulation can be destructive.

Impacts of Corallivore Outbreaks

Outbreaks of highly destructive corallivores, like the crown-of-thorns starfish, can decimate large areas of coral reef by consuming coral tissue faster than it can regenerate, leading to widespread coral mortality and ecosystem degradation.

What is Herbivory in coral reef ecosystems?

Herbivory in coral reef ecosystems refers to the consumption of plant and algal matter by marine animals. This process is critically important for maintaining the balance between corals and algae on the reef, especially macroalgae.

Importance of Herbivory for Coral Reef Health

Herbivory is vital for suppressing the growth of competitive macroalgae that can overgrow and smother corals, especially when reefs are stressed or damaged. By consuming algae, herbivores keep reef surfaces clear for coral recruitment and growth.

Key Herbivorous Species on Coral Reefs: Fish

Prominent herbivorous fish include parrotfish (scrape algae and consume dead coral, creating sediment), surgeonfish (graze on algal turfs), and rabbitfish (feed on macroalgae). These fish play a critical role in controlling algal abundance.

Key Herbivorous Species on Coral Reefs: Sea Urchins

Sea urchins, particularly diadema species, are voracious grazers of algae, often consuming turf algae and macroalgae. Their populations are essential for maintaining healthy reef ecosystems, and their decline can lead to algal overgrowth.

Consequences of Reduced Herbivory on Reefs

A reduction in herbivore populations, often due to overfishing or disease, can lead to an increase in macroalgae. This algal overgrowth can outcompete and smother corals, hindering coral recovery and contributing to coral-algae phase shifts.

What are Symbiodiniaceae?

Symbiodiniaceae is a diverse family of dinoflagellate algae (microalgae), formerly known as zooxanthellae, that live in a crucial endosymbiotic relationship primarily within the tissues of reef-building corals, but also in other marine invertebrates like anemones and clams.

Role of Symbiodiniaceae in Coral Nutrition

Through photosynthesis, Symbiodiniaceae convert sunlight into organic compounds (sugars, amino acids, glycerol) which they then translocate (transfer) to their coral host, providing the coral with the majority (up\ to\ 90\%) of its metabolic energy requirements.

Symbiodiniaceae and Coral Coloration

The diverse pigments within the various species and strains of Symbiodiniaceae are responsible for the vibrant colors observed in healthy corals. When corals expel these algae (bleaching), they lose their color and appear white.

Diversity of Symbiodiniaceae Types (Clades)

The Symbiodiniaceae family contains many different genera and species (clades), each with varying physiological tolerances and efficiencies. Different corals can host different clades, which can influence their adaptability to environmental changes like temperature shifts.

Factors Affecting Symbiodiniaceae-Coral Symbiosis Stability

The stability of the Symbiodiniaceae-coral symbiosis is highly sensitive to environmental stressors such as elevated sea temperatures, ocean acidification, pollution, and increased irradiance, all of which can lead to the breakdown of the symbiosis and coral bleaching.

What is Acanthaster planci?

Acanthaster planci, commonly known as the Crown-of-Thorns Starfish (CoTS), is a large, venomous starfish native to the Indo-Pacific region. It is a major corallivore, feeding almost exclusively on coral polyps.

Feeding Mechanism and Impact of CoTS on Corals

CoTS feed by everting their stomachs over coral colonies and digesting the coral polyps and tissue externally, leaving behind bleached coral skeletons. A single starfish can consume several square meters of coral per year.

Causes of Crown-of-Thorns Starfish Outbreaks

Outbreaks of CoTS (population explosions) are often linked to nutrient enrichment (eutrophication) which can boost their larval survival, and overfishing of their natural predators like giant triton snails and certain fish species.

Ecological Consequences of CoTS Outbreaks

During severe outbreaks, CoTS can decimate vast areas of coral reef, leading to significant reductions in live coral cover, shifts in coral community structure, and broader impacts on reef biodiversity and ecosystem function.

What is a Coral-Algae Phase Shift?

A coral-algae phase shift describes a fundamental change in the dominant benthic (seafloor) organisms of a coral reef ecosystem, typically from a state dominated by healthy reef-building corals to one dominated instead by fleshy macroalgae.

Primary Causes of Coral-Algae Phase Shifts

Phase shifts are primarily driven by chronic environmental stressors, including overfishing of herbivorous fish and sea urchins, nutrient enrichment (eutrophication) from land-based runoff, and disturbances such as coral bleaching events or disease outbreaks that reduce coral cover.

Characteristics of an Algae-Dominated Reef

An algae-dominated reef typically has extensive growth of macroalgae covering coral skeletons and other hard surfaces. This often leads to reduced structural complexity, decreased biodiversity of coral-associated fauna, and a decline in overall reef health and resilience.

Consequences of Phase Shifts for Reef Biodiversity and Function

Coral-algae phase shifts result in a severe loss of biodiversity as coral-dependent species lose their habitat and food sources. The change in reef structure also diminishes ecosystem services like coastal protection and fisheries productivity, impacting human livelihoods.

Strategies to Prevent or Reverse Phase Shifts

Preventing and reversing phase shifts involves reducing anthropogenic stressors such as controlling marine pollution, establishing and enforcing Marine Protected Areas to protect herbivore populations, and managing fishing pressure effectively to maintain grazing levels.