Food in Exchange for Shelter

What am I looking at?

This is a darkfield, focal stacked image of coral polyps (1) in the genus Acropora. It was taken under a light source designed to highlight naturally fluorescent compounds within the polyps, as well as in the zooxanthella algae that live inside them. The polyps appear maroon and the algae appear pink.

Biology in the background

Coral reefs are made of communities of coral animals living on the calcified skeletons of their ancestors. An individual coral animal is called a polyp. These polyps live in communities that form the structures which most people think of when they hear the word “coral.” Each polyp adds its skeleton to the reef structure during its lifetime, making the reef larger with each generation. Coral polyps are filter feeders – extending retractable protrusions into the surrounding water to pull small plants and animals into their stomachs.

However, this is not their only mechanism for gathering food. These polyps also have a symbiotic relationship with a collection of single-celled algae called zooxanthellae. These algae, which live inside the coral polyps, generate food in the form of energy-rich molecules through photosynthesis. The algae pass some of these molecules along to the cells of the coral polyps, which in turn provide protection for the algae.

Unfortunately, rising ocean temperatures and pollution are now causing coral polyps to expel symbiotic algae, a process called “coral bleaching.” This can lead to widespread polyp mortality and result in deterioration of reef ecosystems, some of the most biodiverse habitats on the planet.

The polyps you see in this image are about 2 millimeters long, or roughly 10 times smaller than a human thumbnail.

Technique

This image was created using darkfield microscopy, focal stacking and custom light sources.