Partners of the Ocean Reef
By Mary Jonilonis • Photos by the author
When I learned to scuba dive, I was fascinated by the many interactions among sea creatures.
Of course, these relationships are not always friendly. In some cases, one creature benefits while the other receives no harm or benefit. In other relationships, one creature benefits while harming the other creature.
But by far the most interesting to me are relationships in which both creatures benefit.
Divers can see coral reefs. But the most common partnership in the reef cannot be seen without using a microscope. It is the life-sustaining partnership of algae and stony corals.
A stony coral is a tiny soft-bodied creature that lives by eating small creatures that float by. Each coral uses minerals from the water to make a hard, protective skeleton around itself. New corals build their skeletons on top of the old ones. Over time, the skeletons build up and form a reef such as this one.
What a diver cannot see is that inside each tiny coral are more living things called algae. Each of these two gets important nutrients from the other. The algae get nitrogen from the corals, which take it from the creatures they eat. The corals get sugars and starches from the algae, which make them by storing the energy of sunlight.
One of the most colorful relationships on the reef is between the clownfish and the anemone (uh-NEM-uh-nee). The beautifully colored clownfish hovers above the anemone. When a predator comes along, the clownfish seeks a safe hiding spot among the anemone’s poisonous tentacles. The fish’s special slippery covering lets it brush against the anemone without triggering the poisonous stingers. And the anemone seems to benefit from the clownfish. In some cases, the fish protects its anemone from butterfly fish (not shown), which can eat the stinging creature.
Two kinds of shrimp share the burrow of this conger (KONG-ger) eel. The tiny shrimp near the eel’s eye is a cleaner shrimp. It stays busy (and full) eating parasites off the eel. The red-and-white shrimp are called banded coral shrimp. They keep the burrow clean by eating tidbits of food dropped by the eel. Both kinds of shrimp are safe here. The eel won't eat them, and other fish stay away to avoid becoming the eel’s dinner.
If I watch closely, I can usually find a “cleaning station.” Cleaner wrasses (the two blue-and-yellow fish) eat tiny parasites off other fish. Soon after they start cleaning a fish, other kinds of fish begin to line up to take a turn at having their parasites removed. Here, two gold-ring surgeonfish have moved into the station. The surgeonfish get a cleaning, and the wrasses get a meal.
Turtles also get parasites on their skin and shells. One day I was lucky to find a turtle in a cleaning station. Turtles usually swim away, but that turtle stayed still during its cleaning, even while I slowly approached to take this picture.
We hear a lot about wild animals competing with and eating one another. That’s a good way to understand how nature works. But there are many ways that different species depend on one another to make a living. I like to look for surprising cases of living things that depend on one another to survive.