Written by: Maren Toor

You may remember the building blocks of coral from the February Coral Chronicles that described the individual parts that comprise a stony coral: the polyp, the calcium carbonate skeleton, and the zooxanthellae. Now we’re going to look inside the coral and get a closer look at the tissue that comprises the coral polyps!The coral tissue wall is made of two layers or epithelia composed of specialized cells, an outer layer (the epidermis) and an inner layer (the gastrodermis), which are attached to a connective tissue layer called the mesoglea. All polyps in a colony are joined to each other by common tissue known as the coenosarc or coenenchyme through which pass numerous tubes called gastrovascular canals opening into the central gastrovascular cavity. The tubes and cavity are lined by the gastrodermis through which water and nutrients are transported, ensuring that all polyps have a similar growth rate and don’t compete with each other (Veron 2013).


Source: NOAA

There are a lot of technical terms when it comes to examining the inside of coral polyps but it’s important to understand how a polyp functions in order to study subjects such as coral disease. We see some diseases on our corals in the nurseries and on the reefs here in the Keys and it’s important for us to understand how diseases affect survivorship amongst the corals we plant. Research is progressing in the fields of histopathology (the study of disease through microscopic examination of biological tissue) and microbiology, but relatively little is known about pathogens that cause coral diseases. Some bacteria, fungi, and ciliates are known to cause a few and abiotic factors also impair coral health (such as temperature extremes and chemical contaminants), but with more than 40 different coral diseases observed worldwide, only a handful of pathogens have been identified, making further research of coral tissue very important.


Source: Histology image of staghorn polyp from Esther Peters. Note that the polyp is embedded in the skeleton, which has numerous gastrovascular canals connecting the polyps along the branch and through the middle of the skeleton. The actinopharynx is a short tube that leads from the mouth (not seen) to the gastrovascular cavity.

While the internal structure might be complicated to understand, it’s incredibly important to study the cells and tissues of these tiny polyps to advance scientific knowledge and techniques of how to better protect and restore coral reefs around the world.

I would like to give special thanks to Dr. Esther Peters for her assistance and providing me with the most up to date knowledge on coral histopathology.