Coral = animal + plant!

Most of us fortunate enough to have had exposure to coral reefs, have come to recognise that a special relationship is involved.

Corals are essentially anemone-like animals that secrete a skeleton whilst living in a symbiotic relationship with algae. A single unit is called a polyp; collectively they form a coral colony. Most corals that make up coral reefs, such as the Ningaloo Reef in W.A., are colonial.

A schematic representation of general structure of a polyp and its underlying skeleton.

Painting: Geoff Kelly – Australian Institute of Marine Science, 2013.

The coral provides the algae with a protected environment and compounds they need for photosynthesis. In return, zooxanthellae supply the coral with the products of photosynthesis and help it to remove wastes. The mutualistic relationship between the algae and coral polyp facilitates a tight recycling of nutrients in the fairly nutrient-poor waters of the tropics.

The success of the symbiotic relationship between these ‘hard’ corals (an animal) with zooxanthellae algae (a plant) to harness the energy of the sun, powers the process of reef-building.

Why do these organisms invest so much energy into building something that is dead? That is, the calcium carbonate skeleton of the polyp which makes up the reef foundation. - One aspect is that these skeletons provide the necessary structure that is needed to form colonies, which are required to build large wave-resistant reefs.

Colony formation and algal symbiosis are two evolutionary innovations that clearly go together and have very likely evolved in conjunction. The importance of this is verified by the fact that most living Cnidaria (the Phylum which includes corals) that reap the advantages of reef-building, are both colonial and symbiotic.

It is not necessarily what species of plant and animal, but the relationship between them is what is critical to what are the most prominent modern reef-builders.

This specific symbiotic relationship allows the limitless resources of seawater carbonates and sunlight to be harnessed and harvested to construct an ecosystem.

In effect, this special relationship allows reefs to be built by animals because it gives them the energy-generating capacity of plants.

Undoubtedly, the evolutionary advantage of this symbiosis is prominent, but the cost is also great. These zooxanthellate corals are limited to the most hostile of marine environments, the ocean’s surface, both physically and biologically. This symbiosis also constrains corals to live in habitats in high competition with macroalgae. Coral-algae symbiosis is therefore ultimately responsible for the geographic constraints of coral reefs, as well as their construction.

This short video gives a brief introduction to coral anatomy and the importance of coral reefs in simple terms. With its cute cartoon animations, it is a cry out for coral reef peril and sends a great message advocating for coral reef conservation.

 BIBLIOGRAPHY:

·         Baird, A., Cumbo, V., Leggat, W., & Rodriguez-Lanetty, M. (2007) Fidelity and flexibility in coral symbioses. Marine Ecology Progress Series, 347, 307-309.

·         Marine Pollution Bulletin (2004) Coral symbiosis under the spotlight. Marine Pollution Bulletin, 49(7-8), 529-529.

·         Rowan, R., & Knowlton, N. (1995) Intraspecific diversity and ecological zonation in coral-algal symbiosis. Proceedings of the National Academy of Sciences of the United States of America, 92(7), 2850-2853.

·         Veron, J.E.N. (2013) coral info sheets, Australian Institute of Marine Science, viewed April 2013, <http://coral.aims.gov.au/info/structure.jsp >.

'Reticulate Evolution' of CORALS

Beautiful hard corals of the Ningaloo Reef - Ningaloo Kayak Adventures, Coral Bay 2013

A small animal that, collectively, is the foremost integral member of the ‘Coral Coast Crew’, is of course, CORAL!

The purpose of this blog is to briefly introduce you first to the evolutionary history of the ‘modern corals’. Corals are a major constituent of the Coral Coast and are essentially what structurally and fundamentally makes up the Ningaloo Reef ecosystem.

The term ‘coral’ is generally used for both ‘soft’ and ‘hard’ corals, sometimes encompassing other colonial Cnidaria (also commonly called Coelenterata). All, except the Stylasterina, of the four extant orders are partly or wholly zooxanthellate (corals which have symbiotic blue-green algae called zooxanthellate in their tissues) and are consequently constrained to sun-lit shallows and warm water.

In brief, the evolutionary history of ‘modern corals’ may be divided into three geological intervals:

(1) Paleogene – the survivors of end-Cretaceous and Late Palaeocene extinctions proliferated into a diverse cosmopolitan fauna;

(2) Miocene – subdivision of this fauna into the broad biogeographic provinces we have today and when the immediate ancestors of most extant species (primarily Indo-Pacific) evolved.

(3) Plio-Pleistocene to present – global glaciation mode and when modern distribution patterns emerged.

Compared with most other major taxa (groups) of animals, coral genera are long-lived in geological time and have low extinction rates: nearly half of all extant genera extend as far back as the Oligocene and nearly a quarter extend back to the Eocene.

Species are the fundamental units (or ‘building blocks’) of Nature defined in a limited geographic space, however, this concept breaks down when applied to corals over large geographic ranges.

The fundamental reason for this is that coral species exist as interlinked patterns in geographic space that change continuously so that variation within a single species becomes indistinguishable from the variation between comparable species. The majority of coral species do not exist as geographically or taxonomically definable units. Therefore, it may be necessary to consider evolutionary change in a different manner from that which has become generally accepted in both popular and scientific literature. ‘Reticulate evolution’, the development of a network of closely related taxa within and at the species level, is a fundamentally distinct concept (paradigm) which involves a different way of looking at what species are, the geographic patterns they make, and their change over evolutionary time.

Stunning hard coral garden inside the lagoon of the Ningaloo Reef - Migration Media, Ningaloo Reef Dive, Coral Bay 2013

As with most other forms of life, coral evolution can only be inferred from other studies - palaeontology, taxonomy, biogeography and genetics. Palaeontology shows something about how life was in the distant past and how it has changed, whilst genetics has the potential to reveal a great deal about mechanisms of evolutionary change. Biologists depend to a large extent on taxonomy and biogeography to provide an insight into how species evolve. Geographic space and evolutionary time interact.

Evolutionary changes will include distribution and genetic changes occurring in response to fluctuations in ocean currents. As a result, a coral species are genetically as well as geographically changed irregularly over the species’ geographic range and evolutionary history. The species may break apart and then re-form into a slightly different unit, creating a ‘reticulate’ pattern in both geographic space and evolutionary time.

A hypothetical view of reticulate evolutionary change within a group of species. 

Australian Institute of Marine Science 2013

The reticulate ‘re-packaging’ occurs constantly at all scales of space and time and is not confined to a single phylogeny or evolutionary clade but involves many simultaneously. Reticulate evolution is hence a mechanism of slow arbitrary change acting on genetic composition is under physical environmental control changes patterns of genetic connections. This is again in sharp contrast with a major aspect of the traditional view where evolution is largely controlled by competition between species, resulting in morphological changes.

The concepts of reticulate evolution and the traditional view of evolution are not compatible – they are two paradigms which become increasingly mutually exclusive with increasing amounts of space and time.

Image: Australian Institute of Marine Science, 2013

These are hypothetical representations of the same gene pool under different regimes of ocean currents. (A) The gene pool forms a single cohesive species with strong currents, (B) currents are decreasing and the gene pool forms a single species but some parts of it are partly reproductively isolated (represented by overpasses), (C) currents are weak and the species is broken up into isolated pockets.

Reticulate evolution is primarily driven by changes in surface circulation patterns causing changes to the dispersal patterns of larvae. If dispersion by all the ocean currents stopped, every reef, island and headland would be genetically isolated. The corals of each location over time would gradually become distinct from those of every other through the processes of Darwinian natural selection, amplified by genetic drift and mutations. In time, every location would develop a unique reef fauna and every coral species would have a distribution range of just that situation. Thus, if currents remained constant throughout evolutionary time there would be general uniformity in species and their distribution. Nevertheless, the variations in ocean currents due to geo-climatic cycles result in constant changes in dispersion and genetic connectivity: they generate reticulate patterns. It is argued that the concept of reticulate evolution is vastly explanatory about coral taxonomy observations and biogeography and is starting to be supported by genetic studies.The reticulate evolution concept is also strongly supported by what is known of coral reproduction.

More on these members and their coral reefs of the ‘Coral Coast Crew’ are to feature in the following blogs!

Bibliography:

Arnold, M.L. & Fogarty, N.D. (2009) Reticulate evolution and marine organisms: the final frontier?, International journal of molecular sciences, 10 (9), 3836-3860.

Frank, U. & Mokady, O. (2002) Coral biodiversity and evolution: recent molecular contributions, Canadian Journal of Zoology, 80 (10), 1723-1723.

Stanley, J.G.D. & Fautin, D.G. (2001) Paleontology and evolution: The origins of modern corals, Science (New York, N.Y.), 291 (5510), 1913-1914.

Veron, J.E.N. (2013) coral info sheets, Australian Institute of Marine Science, viewed April 2013, <http://coral.aims.gov.au/info/coral-reefs.jsp>. 

The oldest members of the Coral Coast Crew – living fossils!

Hamelin Pool stromatolites, Shark Bay World Heritage Area, Western Australia (2009).

They may not be as energetic and entertaining as the other members of the Coral Coast Crew, but they are an interesting and important component for evolutionary biology. 

Shark Bay is one of only two places in the world where extant marine stromatolites exist.

Stromatolites are photosynthetic biosedimentary structures of ‘microbial mats’. They are constructed by the entrapment and binding of sediments by cyanobacteria and other microorganisms, which have been formed throughout the earth's evolutionary history.

The world-famous stromatolites at Hamelin Pool, on WA’s Coral Coast, are the only known occurrence of extant stromatolites forming in hypersaline coastal environments.

watch: Stromatolites of Hamelin Pool - Petbur

[This is an older video clip, but has some nice footage and explanation.]

 

Living stromatolites, Shark Bay, Western Australia - Paul Copper (2013) 

These halophilic ("salt-loving") Archaea have adapted to living in extremely hostile environments. Halophilic Archaea are chemoorganotrophs and belong to the class Euryarchaeota. Halophilic Archaea are of particular interest as they are astonishingly robust organisms, able to survive being desiccated into a crust of solid salt, resulting in extremely high longevity (millions of years and possibly indefinite!) entrapped in salt crystals. Stromatolites grow successfully and undisturbed at Hamelin Pool because the sea water is twice as saline (as usual sea water) due to partial isolation by a sandbar across the entrance of the bay and the rapid evaporation from its shallow water. Halophilic Archaea thrive in concentration of salt even five times greater than that of the ocean (higher than those used in any food pickling processes!). They actually require high salt for growth and they are adapted to environments which have little or no oxygen available for respiration.

·         Fun Fact! The Shark Bay World Heritage area has the Westernmost Point of Autstralia – Steep Point, Western Australia (26° 09' 5" S113° 09' 18" E).

How microbialites form, Government of Western Australia (2013).

These ancient structures are monuments of what life on Earth was like 3.5 billion years ago as they contain living microbes (that build the stromatolites) similar to those found in 3,500 million year old rocks - the earliest record of life on earth! The stromatolite structures which are found to be up to a metre high are believed to be hundreds to thousands of years old as they grow at a maximum of 0.3mm per year.

Hamelin Pool is home to the most abundant and diverse examples of living stromatolites in the world. Several halophilic archaea, belonging to the genus Halococcus, have been identified at the Coral Coast location, preserved in the geological record for over 3 billion years. As such, these stromatolites are considered ‘living fossils’ which provide a record of local environmental changes. The Coral Coast Hamelin Pool stromatolites are considered the oldest and largest ‘living fossils’ part of the Earth's evolutionary history.

Watch: Stromatolites, proof of Life's Origin HD- Western Australia

[There’s quite an interesting introduction into the Earth’s history leading up to the stromatolites 

(just under 3 minutes into the video if you wanted to skip to it).]

Watch: Why do we care about stromatolites - Virtual Field Trips

BIBLIOGRAPHY:

Goh, F., Leuko, S., Allen, M.A., Bowman, J.P., Kamekura, M., Neilan, B.A. & Burns, B.P. (2006) Halococcus hamelinensis sp. nov., a novel halophilic archaeon isolated from stromatolites in Shark Bay, Australia, International journal of systematic and evolutionary microbiology 56 (6), 1323-1329.

Papineau, D., Walker, J.J., Mojzsis, S.J. & Pace, N.R. (2005) Composition and Structure of Microbial Communities from Stromatolites of Hamelin Pool in Shark Bay, Western Australia, Applied and Environmental Microbiology 71 (8), 4822-4832.

Renee, C. & Wandersee, J. (2013) STROMATOLITES, The Science Teacher 80 (2), 60.

Paul Copper (2013) image: Living stromatolites, Shark Bay, Western Australia. Australian Institute of Marine Science, <http://coral.aims.gov.au/info/reefs-palaezoic.jsp>.

Government of Western Australia (20013) image: How microbialites form, Government of Western Australia <http://www.dmp.wa.gov.au/5257.aspx>.

Shark Bay World Heritage Area (2009) image: Hamelin Pool stromatolites, Shark Bay World Heritage Area, Western Australia <http://www.sharkbay.org/stromatolites.aspx>.