Sea cucumbers may help protect coral reefs

Meaning:To stifle something is to smother it — to hold it down so it cannot grow, spread, or come out. You can stifle a yawn in the middle of class, and a rule can stifle an argument before it starts. In the article, algae spread in thick mats over a reef and block the light the coral needs, so the coral's growth is stifled.

Meaning:A delicacy is a food people treat as special and expensive, usually because it is rare or hard to get. What counts as one depends entirely on where you live. In the article, dried sea cucumber — sold as bêche-de-mer — is a prized delicacy in parts of east Asia, which is why so many are hauled off the sea floor.

Meaning:Sediment is the loose material — sand, mud, grit, and bits of dead plants and animals — that settles to the bottom of water and sits there. Rivers, lakes, and oceans all collect it. A sea cucumber crawls along the sea floor swallowing sediment, takes in the scraps of food mixed through it, and pushes the leftover sand back out.

Coral looks like a colorful rock, but a reef is built by animals. Each one is a tiny, soft creature closely related to a sea anemone — scientists call them anthozoans — and each builds a hard stony case around itself. Stack billions of those cases up over hundreds of years and you have a reef.

Here is the strange part: those animals cannot catch nearly enough food to live on. Packed inside their bodies are huge numbers of tiny algae, and the algae use sunlight to make sugars. Most of that food passes into the animal. In exchange the algae get a protected spot in the sunlight and the raw materials they need — ammonia, for instance, which algae use to build protein.

Biologists call two different kinds of living thing living together like this symbiosis, from Greek pieces meaning 'living together'. When both sides gain, as here, it is called mutualism. You carry an example around all day: the bacteria in your gut get a warm place and leftovers from your lunch, and in return they break down food your own body cannot digest.

So a reef is not one living thing but two, locked together. That is also why reefs are so fragile — damage either partner and the whole arrangement stops working.

The sea is slowly turning more sour, and that is bad news for anything that builds a shell.

Chemists sort liquids by how acidic they are. Lemon juice and vinegar are strongly acidic, plain water sits in the middle, and dish soap is at the far other end. Seawater normally sits a little past the middle on the non-acidic side. Acid dissolves chalky material: leave a chicken eggshell in a cup of vinegar overnight and the shell fizzes and goes soft, because the acid eats away the chalk it is made of.

Now the connection. Coral skeletons and clam shells are made of almost exactly that chalky stuff, calcium carbonate. The air today holds more carbon dioxide than it used to, and some of that gas dissolves into the sea, where it makes the water slightly more acidic. That is ocean acidification. The shift is small, but it makes the building material harder for coral to lay down and easier for seawater to dissolve — rather like building a sandcastle while the tide keeps washing at the base.

That is why the carbonate a sea cucumber releases matters: it nudges the water right around the coral back toward the non-acidic side, so that patch of reef has easier conditions to build in. Whenever you read about the ocean and carbon dioxide, remember there are two separate problems — the water is getting warmer, and it is getting more acidic.