Every fact in the three coral reef lessons is listed here with where it comes from. The numbers in [brackets] on the lesson pages link to this list.
How we rate sources
High: space agencies (NASA), peer-reviewed open textbooks (OpenStax, Rice University) and professional scientific societies (American Physical Society).
Medium-high: reputable science news reporting directly on a primary document.
Medium: single-author essays and popular history sites. We only use these for history details, and never on their own: each one is paired with a second source.
Wikipedia was not used as a source. Numbers are rounded (for example 9.8 m/s² for Earth's gravity). The text was drafted with help from AI (Claude) and then checked against these sources. Corrections: hello@echoesof.science.
Where the facts are simplified or less certain: growth rates of corals vary a lot between species and places; NOAA gives 0.5–2 cm per year for massive corals, up to about 4.5 cm in good conditions, and up to about 10 cm per year for branching corals [5][6]. The share of ocean species living on reefs ('nearly one-quarter') is an estimate [4]. The temperatures and depths where reefs grow are typical values, not strict limits: many corals tolerate colder water [5]. Our shell experiment uses vinegar, which is far more acidic than sea water will ever become, so it shows the chemistry quickly but greatly exaggerates the speed of real ocean acidification; NOAA's ocean pH values (about 8.1 now, perhaps about 7.8 by 2100) are averages and projections [7]. In the 15–16 lesson we work out the acidity change from the pH scale ourselves; NOAA rounds the change from 8.2 to 8.1 to 'about 30%' [7].
All sources
[1] Is coral an animal or a plant?
NOAA National Ocean Service · Reliability: High
Type: National ocean agency (fact page)
Corals are animals, because they do not make their own food as plants do. Corals have tiny, tentacle-like arms that they use to capture food from the water. Each soft-bodied polyp, most no thicker than a nickel, secretes a hard outer skeleton of limestone (calcium carbonate). Most corals contain algae called zooxanthellae, which use the coral's waste for photosynthesis and supply oxygen and organic products. Hard (stony) corals build the limestone base of reefs; soft corals include sea fans and sea pansies. Coral reefs are the largest structures of biological origin on Earth.
NOAA National Ocean Service (education tutorial) · Reliability: High
Type: National ocean agency (education tutorial)
Zooxanthellae supply the coral with the building blocks of sugars and proteins from photosynthesis; as much as 90 percent of the organic material they produce is transferred to the coral's tissue. The coral provides a protected environment and the carbon dioxide and water they need, and they help remove the coral's waste. They give many stony corals their colours. Because they need sunlight, reef-building corals need clear water with little suspended material. Stressed corals can expel their zooxanthellae (bleaching); prolonged bleaching can lead to starvation and death.
When water is too warm, corals expel the algae (zooxanthellae) living in their tissues, which provide most of the coral's food and its colour, and the coral turns completely white. A bleached coral is not dead; corals can survive bleaching if the stress is mild, but they are under more stress and more likely to die. Cold water can also cause bleaching (Florida Keys, January 2010). Other causes include runoff and pollution, very strong sunlight and extreme low tides. In 2005 the U.S. lost half of its Caribbean coral reefs in one year in a massive bleaching event.
Coral reefs occur in less than 1 percent of the ocean but are home to nearly one-quarter of all ocean species. Reefs are natural breakwaters that buffer shorelines from waves and storms. Many fish and shellfish such as grouper, snapper and lobster depend on reefs. Threats include pollution and runoff, sediment, marine debris, overfishing, disease, invasive species, rising water temperatures (bleaching), ocean acidification, ship groundings and careless diving and boating.
NOAA Coral Reef Information System (CoRIS) · Reliability: High
Type: National ocean agency (information system)
More than 6,000 coral species are known; stony corals are the main reef builders. Colonies are made of hundreds to hundreds of thousands of polyps, each usually 1–3 mm across, and can weigh several tons. Reefs begin when free-swimming coral larvae attach to submerged edges of islands or continents. Fringing reefs (most common) grow out from the shore; barrier reefs are separated from land by a lagoon; atolls are rings of reef around a lagoon that form when a volcanic island sinks while the coral keeps growing up. Massive corals grow 0.5–2 cm per year (up to about 4.5 cm); branching corals up to about 10 cm per year. Reefs need water that stays at or above 18 °C; most grow best at 23–29 °C, in clear water, mostly in the top 70 m, between about 30° N and 30° S.
A coral reef is made of thin layers of calcium carbonate. Stony coral polyps each produce a calcium carbonate skeleton, and colonies of millions of polyps grow on top of the limestone remains of older colonies. Polyps average one to three millimetres across. Massive corals grow 0.5 to 2 cm per year, and in favourable conditions (lots of light, consistent temperature, moderate wave action) some species grow up to 4.5 cm per year.
NOAA Office of National Marine Sanctuaries · Reliability: High
Type: National ocean agency (classroom lesson)
Classroom experiment: clean clam, mussel or oyster shells are left for a week in vinegar, half vinegar and half water, carbonated water and salt water; pH is measured and shells are compared and optionally weighed before and after. The ocean absorbs about a quarter of the CO2 we release every year. Its average pH is now around 8.1; a drop from 8.2 to 8.1 is an increase in acidity of about 30%, and by 2100 surface pH could be around 7.8, about a doubling of acidity. Acidified water corrodes calcium carbonate and reduces the carbonate dissolved in sea water, making it harder for animals such as corals and shellfish to build shells and skeletons.