The Accretionary Wedge #37: Crazy Coral

I have decided to join the ranks of other geology bloggers who participate in the monthly edition of the Accretionary Wedge. The Accretionary Wedge is a blog carnival, a term I was not familiar when I first read it. A blog carnival is simply a monthly collection of posts that are all related to a particular topic and collected in a central location. Since Accretionary Wedge is the geology themed blog carnival it fits beautifully into the mission of this blog.

The topic of the month for August is "Sexy Geology". So what aspect of geology makes my palms sweaty and my heart race? Well, when I was doing my undergraduate I was lucky enough to participate in a sedimentology field trip to Bermuda, where we spent a week snorkelling, swimming and drinking. It was great! One thing that struck me as soon as I got in the water was the diversity and beauty of the coral reefs we were snorkelling on. In short, I thought they were pretty sexy.

Millepora alcicornis (Fire Coral) colonizing a Gorgonian. (Photo: Bill Martindale)

Why do I think coral are so sexy? Well, I could tell you tales about their gently reaching arms and ethereal beauty, but that just wouldn't describe it, although those are fair points for why I like looking at coral.

No, the real reason is more geologically oriented. Corals are some of the most diverse and interesting organisms that inhabit Earth. They have existed for a very long period of time in one form or another and their reef building abilities have been crucial to the evolution of marine life. The rest of this post will examine some of the most interesting features of coral.

Sexy Bermudan coral (Photos: Bill Martindale)

Coral Evolution and Characteristics

Coral as we know it today first evolved in the middle Triassic. However prior to the appearance of today's scleractinian corals there were other reef building organisms that evolved and went extinct throughout geologic time. The earliest reef builders were the stromatolites, which were primitive mounds of photosynthetic cyanobacteria. They were the first large reef builders and were just large colonies of single celled bacteria and sediment. They are mostly found in the fossil record but they do still exist today in one location: Shark Bay, Australia. The early Paleozoic reef builders were mostly dominated by stromatoporoid sponges, which were very similar to modern coral in many respects such as size and shape. Other corals unrelated to those of today also existed during the paleozoic. The late Paleozoic saw the extinction of the stromatoporoids at the end of the Devonian and the rise of reefs made mostly of algae and other encrusting organisms. However, the Mesozoic was a time of massive reef development sparked by the evolution of the scleractinian corals the same order as coral today. There was also a time in the Cretaceous when reefs were built by giant clams called rudists.

Geologic timescale showing episodes of major reef development and the major type of reef building organism (accessscience.com)

Modern corals belong to the phylum Cnidaria, which notably includes jellyfish. Their order is called Scleractinia and is defined by their ability to build skeletal structures of calcium carbonate. Coral are basically composed of two parts: the calcium carbonate skeleton and the living coral polyps, which are similar in appearance and function to the tentacles of a sea anemone or a jellyfish. One of the most unique features of modern coral is their symbiotic relationship with the single celled algae called zooxanthellae. The zooxanthellae live within the tissue of the coral and photosynthesize feeding the coral and helping to produce more calcite. This relationship allows coral to thrive in ocean environments that have very low nutrients, which often prevent other ecosystems from succeeding.

 A cutaway view of a modern scleractinian, zooxanthellate reef coral. Figure shows the massive and complicated underlying skeleton (white), all of which was secreted by the soft polyps and tissue of the living surface (colored). In the cutaway view of one polyp, are seen the tentacles, mouth and enteron, where captured prey are digested. Box in upper left (enlarged), shows a view of a tentacle and the encysted zooxanthellae (round bodies) which reside in great abundance in the endodermal tissues. From Veron (2000).

Coral is a somewhat picky organism with respect to the environment it can thrive in. I suppose part of the sex appeal of coral is the way it plays hard-to-get in most of the ocean. A former professor of mine, Noel James, called this the Goldilocks Principle (I am not sure if he coined this term, but it is still great). The Goldilocks Principle, like it namesake, states that in order for coral to thrive everything must be just right. This means that the temperature, salinity, turbidity, light conditions, wave energy and nutrient levels must all be within a specific range in order for coral to grow. If these conditions change suddenly the coral will die. Talk about needy!!

The Goldilocks Principle

The Importance of Coral

Coral is undoubtedly one of the most important organisms on Earth for a host of reasons. This biggest reason why coral is important to the ocean is the incredible ecosystems that grow and thrive around reefs. Without the coral these diverse, and beautiful reef environments would not exist as the coral provides shelter and a source of food in places where the lack of nutrients would generally inhibit such abundant ecosystems.

Fossilized coral in the sedimentary record is also useful to geologists. When I find coral in the fossil record there are many things it can tell me. The coolest is what the paleoenvironment was. When we look at rocks today it can be very difficult to ascertain what the climate and ocean conditions were like millions of years ago when the rock formed. However, by using the fossils in that sedimentary rock, particularly coral, it is possible to look back in time and see what the environmental conditions were like when the sediment forming that rock was deposited. As I mentioned above coral only grows in a very specific range of ocean conditions. Therefore, if I find coral in a sedimentary rock I know exactly what the ocean conditions were like millions of years ago. Another really useful feature of coral is its shape. All ancient and modern corals have very specific shapes that are dictated by the depth of water they grow in and the wave energy around them. For example, coral that is in shallow water, but has lots of waves will be very robust both now and in the past. However, coral that grows in much deeper water, where there is less light, but no waves, will have large. delicate plate like surfaces to capture as much sunlight as possible. Therefore, when I see coral of a certain shape and growth morphology in the fossil record I can look back in time to discover how deep underwater it was growing and what the wave energy was like.

Zonation of a near shore reef

Current Reef Health


It is no secret that the coral reefs of the world today are threatened and many formerly vibrant reef ecosystems are now bleached skeletons of their former selves. One of the greatest threats to coral are changing ocean conditions. For example, the El Nino event of 1998 led to change in ocean temperature that caused widespread coral bleaching (Souter, 2000) Furthermore, increasing coastal populations are affecting coral by increasing the nutrient load and turbidity near reefs resulting in their death.

Brain coral (Diploria) that is partially bleached

This post turned into more a factual exploration of why I think coral are amazing. All I can say is they rev my engine for a host of reasons. I am off to the Yukon/NWT tomorrow bright and early so stay tuned for a field update with lots of photos of the Arctic in autumn...yeah, it is autumn there.

References:

David W. Souter, Olof Linden, The health and future of coral reef systems, Ocean & Coastal
Management, Volume 43, Issues 8-9, 2000, Pages 657-688, ISSN 0964-5691, DOI:
10.1016/S0964-5691(00)00053-3. 
(http://www.sciencedirect.com/science/article/pii/S0964569100000533) 

Rachel Wood, The Ecological Evolution of Reefs Annual
Review of Ecology and Systematics 
Vol. 29, (1998), pp. 179-206 

Veron, J. E. N., Odorico, D. M., Chen, C. A., & Miller, D. J. (1996). Reassessing
evolutionary relationships of scleractinian corals. Coral Reefs, 15(1), 1-9.