Episode 36 – Reefs

Listen to Episode 36 on PodBean, Spotify, YouTube, or your favorite podcast source!

In this episode we dive into the concept of reefs. Reefs today are one of the most crucial environments in the oceans. They promote extremely high biodiversity and protect coast lines. Today we are accustomed to coral reefs, but throughout history many organisms have built reefs that proved just as important to ancient ocean life.

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What is a reef?

A reef is technically just a shallow ridge that reaches near or above the ocean’s surface. This means that there are actually a number of things that can count as reefs, typically falling into three categories: inorganic, artificial, and organic.

Nusa Lembongan Reef is a classic image of a coral reef habitat. But this is only one form that organic reefs have taken over time. Image from Wikimedia Commons by Ilse Reijs and Jan-Noud Hutten.

Inorganic reefs include rock structures and sand bars. Artificial reefs are built by humans for various purposes including agriculture, wave protection, conservation, and even to make more ideal surfing waves. Organic reefs are those built gradually by organisms, including coral, algae, molluscs, and more.

A sunken ship, one of the most common kinds of artificial reefs today. Image from Wikimedia Commons by Gareth Richards.
Oyster reefs are a modern organic reef that is not built by corals. Image from Wikimedia Commons by Jstuby.

Reefs Through Time

During the vast span of time that life has existed in the oceans, reefs have been built by a diversity of organisms going all the way back to the Cambrian (530 million years ago). Reefs have been built by ancient sponges, clams, and algae, as well as extinct and modern corals.

Archaeocyatha were likely ancient sponge ancestors with cone-like construction that formed some of the first reefs in Earth’s history, first appearing in the early Cambrian. Image from Wikimedia Commons.
Vaccinites. These two fossil rudist clams were reef builders during the Jurassic to Cretaceous, growing on top of one another. Image from Wikimedia Commons by Wilson44691.
Grewingkia canadensis. Rugose coral, otherwise known as horn coral, was one of the major groups of now-extinct reef-building hard coral during the Ordovician to Permian Periods. Image from Wikimedia Commons by Wilson44691.
Syringoporidae. The tabulate corals were around at the same time as the rugose corals. These had a unique partitioned internal structure. Image from Wikimedia Commons by Wilson44691.

Coral Reefs

Modern reef-building corals are the stony Scleractinia. Coral is a member of the group Cnidaria, which also includes jellyfish, anemones, and sea pens. Corals are typically colonial, formed by thousands of coral polyps. The polyps build calcium carbonate skeletons to support and protect their soft bodies.

An internal look at a coral polyp with its calcium carbonate skeleton beneath it. Image from Wikimedia Commons by NOAA.
Mushroom Coral (Fungia sp.). The remains of a coral skeleton after all the polyp tissue has died away. This is ultimately what the core of a coral reef is built from. Image from Wikimedia Commons by Jon Zander.

Coral can form a variety of reef types. Fringing reefs form along shores and extend like shelves. Barrier reefs stand apart from the shore, closing off a shallow lagoon. Finally atolls are barrier reefs formed around an island that has eroded away, leaving the reef as a circle enclosing a lagoon in the ocean. One reef type can actually progress into the next over time.

Top: In this gif watch a fringing reef form, gradually becoming a barrier reef and ultimately an atoll as the island erodes away.  Bottom: a real atoll in the Maldives Islands. Images from Wikimedia Commons by NOAA and B166-er respectively.


Coral reefs are extremely important to modern environments and economies, but are highly threatened. Major threats include climate change, chemical and physical pollution, sediment run off, and physical damage to reef from anchors and tourism.

But there are efforts to preserve and regrow coral. A few organizations that are making efforts to save the reefs are the Coral Restoration Foundation and The Florida Aquarium.

Recent discoveries also bring some hope that coral reefs may be more resilient than previously thought. The Amazon Reef is an unexpected and beautiful example of corals surviving in harsh conditions.

Check out The Encyclopedia Earth: Coral Reef for more of an overview of coral reefs.

If you enjoyed this topic and want more like it, check out these related episodes:

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One thought on “Episode 36 – Reefs

  1. Cheryl Resnick (@watch4rx) July 7, 2018 / 2:20 pm

    This was a fun episode to listen to as it brought back fond memories from my favorite undergrad strat paleo class. I appreciate this episode’s format starting with definitions of reefs & corals, to descriptions of ancient and modern coral reefs. We live in Pennsylvanian age rocks so my students are always excited to find rugose corals on field trips. The Amazonian reef was a big surprise to hear about given the harsh sediment-laden water conditions. I’ve always been curious about deep-sea cold water corals. A quick internet search says they range from solitary, to colonial, to reef-building structures. The corals can’t depend on zooxanthellae at that depth so I’m curious how they build their structures. I’ll ask my bio colleagues when the semester starts in August.

    I also find it fascinating how the growth rings of ancient corals show that the length of Earth’s day has gotten shorter over time, thereby showing Earth’s rotation is slowing down.

    As always, thanks for answering some questions and encouraging new ones.

    Liked by 1 person

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