MB3190/MB5190 Coral Reef Ecology - Lecture 3: Importance of Corals (Especially Hard Corals)

Lecture Outline

Introduction to Corals
Origin and Diversification of Hard Corals
Functional Importance of Hard Corals
- Foundation species
- Reef building
- Habitat forming
Further Reading
- Connell JH (1978) "Diversity in tropical rain forests and coral reefs." Science 199: 1302-13010.
- Schuhmacher H, Zibrowius H (1985) "What is hermatypic? A redefinition of ecological groups in corals and other organisms." Coral Reefs 4:1-9.
- Wood R (1998) "The ecological evolution of reefs." Annual Review of Ecology and Systematics 29(1):179-206.
- McWilliam M, Pratchett MS, Hoogenboom MO, Hughes TP (2020) "Deficits in functional trait diversity following recovery on coral reefs." Proceedings of the Royal Society B. 287(1918):20192628.

Introduction to Corals

Definition of Corals: Primarily refers to hard (or stony) corals of the order Scleractinia.
Other Related Organisms: Includes various organisms that are similar but not closely related.
- Scleractinian corals may be either solitary or colonial and possess aragonite skeletons.

Had Corals – Closest Relatives

Phylum Cnidaria
- Class Hexacorallia
- Order Scleractinia: True or hard corals
- Order Antipatharia: Black corals
- Order Actiniaria: Sea anemones
- Order Corallimorphia: Corallimorphs
- Class Octocorallia
- Order Alcyonacea: Soft corals
- Order Pennatulacea: Sea pens
- Order Helioporacea: Blue coral
- Class Hydrozoa
- Order Anthoathecata: Fire corals, Lace corals
- Class Scyphozoa: Jellyfish
- Class Cubozoa: Box jellyfish

Origins of Reef-Building Corals

First Major Radiation: Hexacorals (order Tabulata and Rugosa) appeared approximately 450-480 million years ago (mya) during the Ordovician Period.
- Rugose corals featured calcite skeletons, a more stable form compared to aragonite.
- Adaptation to high levels of atmospheric CO2 and low pH in Paleozoic oceans.
- Extinction: These coral lineages went extinct over 20 mya before the origin of Order Scleractinia (Oliver 1980, Paleobiology).
Ancient Reef Structures: Established up to 570 million years ago, but not formed by carbonate accumulating organisms.
- Representation of Permian Reef Systems: Primarily by sponges and bryozoan communities (Wood 1998).
- Permian Period (250-300 mya): Home to Rugose and Tabulate corals but primarily formed by non-coral organisms.

Origin and Diversification of Corals

Modern Coral Reefs: Established approximately 34 million years ago, ongoing diversification since 5.3 million years ago (Bellwood et al. 2016 Biological Reviews).
- True corals (order Scleractinia) and reefs have existed for 250 million years.
- Modern coral reef ecosystems (dominated by extant species of hermatypic corals) emerged within the last 34 million years, peaking in diversity only in the last 5.3 million years.
Environmental Conditions: Established during periods of low (<400 ppm) atmospheric CO2.
- Ancestral State: Hard corals are not derived from calcite skeletons, suggesting ancestral forms were without skeletons (similar to anemones).

Coral Diversity

Diversity Hotspot: Highest in the Indo-Pacific Archipelago (also known as the Coral Triangle).
- Number of hard coral species significantly underreported.
- Coral diversity in the western Pacific (e.g., Australia's Great Barrier Reef) is at least five times greater than that in the Caribbean (Veron et al. 2015).
Molecular Phylogenetics and Evolution (Cowman et al. 2020): Limited resolution in distinguishing hard corals with traditional genetic tools; recent methods reveal six distinct clades of Acropora spp., transforming the understanding of phylogenetic relationships and nomenclature.
- Restructuring of some coral genera and families due to new insights.
World List of Scleractinia: Contains 1682 valid extant species names, with estimates of over 4500 distinct species names, reflecting extensive phylogenetic revisions.

Intermediate Disturbance Hypothesis (Connell 1978)

Key Insights:
- Rapid colonizers are often the only persistent species following disturbances.
- Best competitors monopolize space.
- High biodiversity in coral reefs may be influenced by moderate disturbance levels preventing dominance by a few species.

Coral Richness and Evidence of Intermediate Disturbance Hypothesis

Evidence indicates declines in coral diversity at very high coral cover levels.
Coral Cover and Disturbance: Highest coral cover often corresponds to a few dominant coral species, suggesting a competitive exclusion dynamic.

Coral Diversity Traits and Functions

Key Traits and Functions of Corals:
- Growth Rate (GR): Contribution to carbonate framework accretion and reef regeneration.
- Skeletal Density (SD): Influences carbonate framework accretion.
- Corallite Width (CW): Related to filter feeding and nutrient capture.
- Interstitial Branch Spacing (IB): Provides habitat support.
- Colony Height (CH): Affects carbonate framework accretion and habitat provision.
- Surface Area to Volume Ratio (SV): Impacts primary productivity and nutrient cycling.
- Colony Size (CS): Essential for carbonate framework accretion and habitat formation.
Functional diversity representation provides insights into redundancy and the multitude of roles corals play in their ecosystems, emphasizing that despite some traits being easy to measure, data remains limited for many species (Madin et al. 2016 TREE; McWilliam et al. 2020 Proc. B).

Importance of Hard Corals

Functional Importance of Hard Corals:
- Increasing recognition due to studies on coral reef ecosystems lacking hard corals (Cornwall et al. 2021).
- Contributions include:
- Primary production
- Reef accretion
- Habitat provision
- Trophic interactions
- Hydrodynamic regulation
- Sediment production
- Larval export
- Suspension feeding
- Carbonate storage
- Nutrient cycling.
Foundation Species: Play a key role in structuring communities and ecosystems by contributing to primary production, forming the basis of the trophic pyramid.
Primary Production and Productivity:
- Hard corals contribute to primary production via symbiosis with photosynthetic dinoflagellates (zooxanthellae).
- AUTOTROPHY: Involves the utilization of sunlight and nutrients from their symbiotic partners.
- HETEROTROPHY: Sourcing nutrients by filter feeding (shown in azooxanthellate corals).
- Productivity Rates (measured in mg C.g.day):
- Corals: 8 to 40
- Turf Algae: 17 to 280
- Crustose Coralline Algae (CCA): 0.1 to 12
- Macroalgae: 3 to 118
- Seagrass: 4 to 9
- Despite moderate contributions to reef productivity, hard corals profoundly impact coral reef structure and ecosystems.
Reef Building (Accretion): Coral reefs are created by calcium carbonate deposition and accumulation, with hard corals as essential building components.
- While hard corals are primary contributors, some reefs achieve net carbonate accretion even with minimal coral presence (as questioned by Cornwall et al. 2021).
Many Other Reef Organisms: Such as crustose coralline algae, also significantly contribute to carbonate production and reef accretion (Cornwall et al. 2023).
Hermatypic Corals: Define those that substantially contribute to reef-building, often noted for their symbiotic relationship with zooxanthellae.
- Not all corals living on reefs contribute to reef-building.
Habitat Formation: Hard corals, often recognized as ecosystem engineers, modify local environments, creating habitats vital for other species.
- Some fish species are explicitly reliant on live corals, while others depend on the structural complexity provided by diverse coral communities.
Research Findings (Muruga et al. 2024): Global meta-analyses indicate a positive yet weak correlation between coral cover and both fish abundance and species richness, raising questions about fish community resilience following extensive coral loss.
Ghost Skeletons of Corals: The remnants of dead corals can remain in the environment for years to decades, still contributing to habitat complexity. Increased algal cover may increase primary productivity, leading to discussions on ecological balances without coral presence.