Coral Reefs: Biology and Ecology

• Coral reefs are explored in this material
• Six learning objectives are presented.
• It looks at the ecological roles and reproductive cycles of reef building corals
• Describes coral biology, including reproduction, growth, development, and symbiosis with zooxanthellae
• Coral reefs are highly productive ecosystems, despite developing in nutrient-poor waters
• It reviews coral reef ecosystems, including organisms, depth zonation, coral reef types, and ecological interactions
• A general food web of a coral reef system, with organisms is shown
• geographic diversity patterns seen in coral reef organisms are discussed

Marine Habitats

• There is a diagram displaying the major marine habitats
• It includes the intertidal, neritic, pelagic (oceanic) zones
• The benthic zone is shown, including subtidal, shelf break, bathyal, abyssal and hadal regions.
• The pelagic zone is further divided into epipelagic, mesopelagic, bathypelagic, abyssopelagic and hadopelagic zones, with depths indicated

Topic outline

• The course will cover the following topics
• What is a coral
• Factors that limit the distribution of corals
• The role of Zooxanthellae
• Coral Reef Structure and Associates
• Productivity in a Nutrient Poor World
• Food Webs and Interactions

Phylum Cnidaria

• Corals are classified within the Phylum Cnidaria
• This phylum has three major classes
• Hydrozoa: diverse cnidarians
• Schyphozoa: true jellyfish
• Anthozoa: include anemones and corals

Scleractinia

• Scleractinia, the stony corals, are further classified
• Cnidaria includes Hydrozoa, Anthozoa, Schyphozoa
• Anthozoa include Octocorallia, Hexacorallia
• Hexacorallia include Scleractinia and Actinaria

Coral Polyps

• Corals are composed of individual polyps
• Polyps are connected by tissue
• They have tentacles, a mouth, and a gut cavity
• The mesenterial filaments are within the gut cavity of the polyp
• Polyps secrete a bare skeleton

Reef-Building corals

• Reef building corals are hermatypic
• Coral skeletons show individual polyp skeletons
• Montastrea cavernosa is a Caribbean coral

Hard and Soft Corals

• Phylum Cnidaria, Class Anthozoa includes subclasses of corals
• Octocorallia (soft corals), and Hexacorallia (anemones and stony corals are described

Hard Corals Morphology

• Hard Corals - Brain Corals exist, such as Grooved Brain Coral (Diploria labyrinthiformis)
• Hard Corals - Pillar and Branching Corals are Dendrogyra cylindricus (pillar) and Acropora palmata (elkhorn)
• Acropora cervicornis (staghorn) are other examples.
• Encrusting and Leaf Corals also exist
• Mustard Hill Coral (Porites asteroides)
• Lettuce Coral (Undaria agaricites)

More about Coral Anatomy

• Coral anatomy and growth involves corallites
• Most corals are colonial
• Polyps sit in corallite cups
• Individuals in a colony are interconnected by cenosarc tissue that grows over the corallites

Octocorallia

• Octocorallia include soft corals
• Common Sea Fans (Gorgonia ventalina), and Sea Whips (Order Alcyonacea ) are octocorallia
• Giant gorgonian, sea fans are collected in Palau
• Gorgonians exist underwater too

Factors affecting coral distribution

• Reef-forming corals are limited by factors
• They are found in tropics and subtropics
• They are more abundant along eastern margins of continents
• Salinity must be average, about 35 ppt

Coral Reef Biogeography

• Corals are restricted to tropical and subtropical waters
• The distribution can be seen on a Pacific ocean map
• It features warm and cold ocean currents around Central and South America, and Asia and Australia

Limiting Factors

• Limiting factors of coral reefs include warm sea temperature
• Corals usually cannot grow in habitats with winters cooler than 18 C
• It cannot be too warm, a current problem of global sea surface temperature rise
• High light for symbiosis with algae
• Open marine salinities is key
• Corals usually cannot grow in lower salinities
• Low turbidity to prevent continent areas
• Low nutrients to avoid competing with algae
• Strong waves and sea water currents slows growth
• Erosion causes reef growth between growth and bioerosion
• Sea level limits when corals cannot handle emersion stress
• pH? - Increasing ocean acidity

Global Distribution

• There is a map displaying global distribution of coral reef
• It highlights warm water regions
• © 2006 Thomson-Brooks Cole

Coral Reef Biogeography

• Indo-Pacific region is more diverse than the Caribbean sea region
• Strong Pacific diversity gradient has several factors
• Diversity decreases with increasing longitude
• The diversity extends away from the center of diversity near Phillipines and Indonesia
• Diversity decreases with increasing latitude, north and south from near equator.

Reproduction

• Corals can reproduce sexually or asexually
• Asexual reproduction forms individual polyps
• Sexual reproduction spreads sperm and eggs

Coral Growth

• Table coral likely originated from single coral planula
• Maturation can take 7-10 years

Mass Spawning

• Most corals have planktonic gametes
• Many species of corals spawn at the same time on the Great Barrier Reef and Texas
• It raises questions on what advantage does mass spawning have?
• Cues may include the time of year, water temperature and tidal and lunar cycles.

Larval development

• Fertilized corals become planktonic larvae, called a planula
• Planula usually has their own collection of zooxanthellae
• The planktonic period is often only a few days
• The planula can survive for longer and morph into adult form

Reproduction Diagram

• Diagram of coral reproduction is displayed
• Floating eggs form slicks on the surface
• They drift with currents for 4-5 days
• Cell division begins 1-2 hours after spawning
• The development of zygote happens with progressive cleavage at 2-18 hours
• Planulae larvae develops and drifts with currents for 4 days
• Planulae begin searching for a suitable substrate
• Planulae settle 4 or more days after spawning
• Polyp begins to develop and lay down a skeleton
• Colony begins to grow through polyp budding
• This starts with synchronous release of sperm and egg bundles

Zooxanthellae

• Zooxanthellae live within reef-building coral tissues.
• This limits light that corals can absorb

Zooxanthellae Definition

• Zooxanthellae are a dinoflagellate with several benefits
• Found in species of anemones, hermatypic corals, octocorals, bivalve Tridacna.
• Symbiodinium microadriaticum was once one species, but is now at least 10 distinct taxa.
• Found in corals within tissues and concentrated in tentacles
• Picked up by larvae, juveniles by infection from the environment

Function of Zooxanthellae

• In corals, photosynthesis involves zooxanthellae
• Carbon dioxide and water come into the coral
• With sunlight, photosynthesis happens
• Organic matter and oxygen are produced

Benefits of Zooxanthellae

• Radiocarbon-labeled carbon taken up by zooxanthellae and transported to coral tissues
• Corals feed on microzooplankton in this process
• Facilitate calcification
• The uptake of carbon dioxide by zooxanthellae enhances calcium carbonate deposition
• Photosynthesis inhibition decreases the rate

Other Benefits

• Sources of oxygen for coral respiration, but corals are often in oxygenated water too
• Facilitate release of excretion products, though corals are generally well circulated in water

Coral Bleaching

• Coral Bleaching = loss of algae and stress
• Causes - expulsion of zooxanthellae
• Mechanisms - not fully understood, but zooxanthellae cells appear to die and are expelled

Reef Formation Stages

• Reef building corals settle on hard surface, often pre-existing reef
• Interstitial spaces are filled in by CaCO3 sediments
• Sediments are cemented together by encrusting organisms, building new reef 'rock'.

Bioeroders

• These erode corals and cause loss of coral
• They include Burrowing worms, Boring Sponges, Parrot fish, etc.

Corals that build reefs

• Reefs are not built by coral alone
• Coraline Algae, Bivalves help as well

Biological Competition

• Corals can compete with one another by damaging the other directly
• They can be overgrown

Competition

• Competition involves shading, overgrowth, interspecific digestion, sweeper tentacles, allelopathy, et al. Acropora palmata,Montastrea annularis are shown

Reef breakdown

• Sponges, worms, and other boring organisms weaken base
• Storm surge or colony's weight causes coral to topple
• Exposed surfaces colonized by coralline algae and bryozoans
• Boring organisms continue to erode coral, causing it to break into smaller pieces
• Fine sediment from boring settles to the bottom and covers exposed surfaces

Reef composition

• Calcareous shells of dead animals pile up; they fill cracks and spaces between coral pieces
• Planula larvae start new colonies in these cracks.

Coral Reef Types

• Over time, coral communities form reefs
• Coastal reefs include massive structures and small patches
• such as Great Barrier Reef, Eilat, Israel
• Atolls - horseshoe or ring-shaped island chain of islands atop a sea mount

Coral Diversity

• Usually found in the Pacific, atolls display excellent coral growth
• A diagram illustrates terminology such as a fore-reef and a back-creek

Characteristics

• Vertical zonation indicates that reefs are dominated by different coral species
• This happens due to several factors dependent on depth, light, and other rocky-shore qualities.
• Caribbean depth zonation happens

Environment

• Coral reefs involve a reef crest by the sea
• There are heavy branching corals
• Seagrass beds and sand are typical terrain

Animal Dominance

• Animal biomass, rather than algae biomass dominates coral reefs
• Darwin's Paradox occurs in tropical reefs due to productivity
• the environments have few nutrients

Darwin's Paradox Solution

• The paradox is solved via these explanations
• Symbiosis: symbiosis with dinoflagellates makes primary producers
• High nutrient recycling: the shallow reefs aid recycling, and sponges play a key role in this activity
• Nitrogen fixation/capture: fixation of atmospheric nitrogen
• High photosynthetic efficiency: Corals symbionts reach high photosynthetic

Food Web

• The coral reef food-web displays the types of species interaction
• Consumers are: Grazers, Detritus feeders, Coral and coral mucus feeders, Plankton feeders
• Producers are: Seaweeds, coralline algae, photosynthetic bacteria and plankton

Communities

• Coral reefs have tremendous biodiversity
• Variety of marine life is possible

Competition

• Species in coral reefs compete with the same resources.
• It defies competitive exclusion principle which suggests the species can occupy one role
• 60-70% of reef fishes are general carnivores
• ~15% are coral algae grazes or omnivorous

Competition Model

• There are several Competition models
• Competition is based on the time of day, the prey, or just what part of the water column
• predation disturbance assumes that predation can affect populations or prevent competitive exclusion
• lottery says that chance determines which species settlings in plankton can colonize the reef
• resource limitations suggest larval populations do not have enough fish to support populations in the habitat

Interactions

• Interactions involve many animals
• Competition and Mutualism can be mutually beneficial
• Grazing can introduce predators, and Larvae are recruited to the surface
• Disease can spread too

Mutualism Example

• Anemone and clownfish protect each other
• Also, anenome zooxanthellae can gain fish waste
• It is proposed that Clownfish can reduce the parasitic load

Cleaning Station

• Cleaner fish remove parasites from coral, and earn food
• There are cleaning animals throughout the reef.

Grazhing

• Urchin species grazing is important
• Urchins remove photosynthetic competitors, like seagrassed
• Usually fish and parrotish eat seaweed
• Fish remove grazing seaweed

Predation Model

• Urchin, Diadema, is a sea urchin that grazes on algae
• Parrotfish will eat algae too
• Crown-of-thorns Star are predators in the pacific
• A single thorn star is capable of eroding 2-6 meters of coral

Conservation

• Reefs are already mostly destroyed
• Tourism directly affects reefs
• Boats anchor themselves and harm slow growing structures Swimming can harm reef structures

Storm erosion

• Extreme weather can have several effects
• Storms often do not completely destroy them
• They break coral and clear predators

Climate Change

• Addition of gases such as carbon cause ocean reduction
• Coral secrete aragonite, which get dissolved in acid
• Algae gain advantage over reef in coral

Black Band

• Black Band - these bands corrode coral
• They lead to sulfide production
• Toxicity occurs
• White band is another coral disease, known to affect Acropolid corals.
• White plaque erodes as well.