1. Coral
Pretty

2. Coral Corals consist of anemone-like animals called polyps.
Just like plants with their chloroplasts, coral polyps contain photosynthetic algae called Symbiodinium (older books call them zooxanthellae).
The algae give corals their color.

3. Coral

4. Mandatory shout-out
Corals and sponges are animals, a theory originally proposed by the Persian polymath Rayhan al-Bīrunī ( ).
European natural philosophers considered corals to be plants, but ~1800 they accepted that they were actually animals with algae inside.
Al-Bīrunī wrote some 146 books, of which about 20% survive. He correctly measured the radius of the Earth over 500 years before Europeans managed it.

5. Coral reefs
Since they require sunlight, corals only grow in shallow water where there is still light to harvest.
Colonies form when one coral polyp attaches itself to a rock and divides, cloning itself.
Most coral species have a calcareous (limestone, CaCO3) skeleton.

6. Calicles (Coral skeletons)
Successive generations of corals build a reef, literally on the bones of their ancestors.

8. Reef locations (NASA image)

9. Rainforests of the Sea
Reefs cover 0.1% of the ocean surface & are home to 25% of known marine species.
But tropical waters tend to be nutrient- poor (they’re oligotrophic).
In 2002, two scientists found: “The key, it turns out, is that corals have extremely rough surfaces. Objects under water are usually surrounded by a small boundary layer of still water, which effectively acts as a barrier to the rest of the ocean. But when waves break, the ultra-rough edges of corals amplify the water turbulence at a microscopic level. This disrupts the boundary layer, allowing the corals to hoover up what sparse nutrients there are.” – New Scientist 2361, September 21, 2002
This mystery only took 160 years to solve! Yay science!

10. Rainforests of the Sea
Since primary producers are base of any food chain, productive corals allow a ridiculous amount of life to live in the same habitat.

11. Threats to reefs
Just like rainforests on land, we have a lot to lose if they are destroyed.
Eutrophication (excess nutrient supply) can allow algae to smother corals.
Ocean acidification (CO2 absorbed by oceans, increasing acidity) reduces the ability of marine organisms to form shells.
Warming of water (especially in El Niño years) can cause coral bleaching.
Tourism.
These factors individually may only weaken coral. Together, they kill it.

12. Coral bleaching
If the algae inside polyps die, the polyp will also die due to lack of nutrients (I know I’d love to be able to photosynthesize).
In 1998, 16% of the world’s reefs died due to unusually warm water temperatures. More on that on block day.
Any sessile (fixed, immobile) organism is particularly vulnerable to environmental change.

14. Coral bleaching

15. Artificial reefs
The idea of artificial reefs to improve fishing goes back a few hundred years.
The US has sunk old ships, subway cars, and even planes to “seed” new reefs.
If sea temperatures continue to rise, corals may need assistance to relocate to cooler waters.
There are artificial reefs worldwide.

16. Artificial reefs
Craziness!