Welcome to Lecture 3: Reef Coral Symbiosis.

Reef corals are first and foremost symbioses. These symbioses involve two very different kinds of organisms that have been separated for most (if not all) of their evolutionary history.

The reef coral "host" is an invertebrate animal in the phylum Cnidaria. The reef coral "symbionts" are photosynthetic dinoflagellate algae that reside within the cells of their animal host.


The symbiosis is thus a very intimate one (an intracellular "endosymbiosis"). Scleractinian (stony) corals are the dominant reef-builders of shallow tropical seas, and have been for much of the Cenozoic (past 65 million years). Scleractinians themselves have a fossil record dating back to the mid Triassic (220 Mya). Diverse (molecular, stable isotopic, ecological) evidence suggests scleractinian corals formed symbioses with algae soon after their appearance in the fossil record. Scleractinian reef coral symbioses are ancient associations whose importance in the ecology of coral reefs has probably reached its apex in the present day.

Algal symbiosis may have been important in even more ancient reefal ecosystems.

In this lecture we will concentrate on the scleractinian (stony) corals and their symbiosis with dinoflagellates in the genus Symbiodinium.

These organisms are the dominant reef-builders in the world's shallow tropical seas, and the most widely studied symbiosis on the coral reef.

The objective of the lecture is to give an appreciation of the importance of this symbiosis for our understanding of coral reef ecology and conservation in the face of contemporary environmental threats.

The material provided in this website is designed as background information to foster questions and discussion in this (or subsequent) lectures. This lecture will focus on scleractinian reef-building corals and their symbiosis with photosynthetic dinoflagellates in the genus Symbiodinium.