Coral Field Guide

What is the distribution of Coral Communities in Hong Kong?

What are Corals?

Corals are animals belonging to Phylum Cnidaria, a group of animals that include hydroids, jellyfish and sea anemone. The word "coral" has been used to describe different organisms in this group, including the black coral, sea fan, soft coral and hard corals.

All these corals have a basic tube shaped, anemone-like body plan that is symmetrical about a central axis. (that is, they are radically symmetrical). The body has a sac-like cavity with a single opening that serves both as the mouth and the anus. This body opening is tinged by tentacles with stinging cells that help in defense and capture of food.

Soft coral commonly seen in Hong Kong waters These corals can be further divided into two main groups, depending on the number of tentacles they have. Those that bear tentacles in the multiple of eight are classified into a group called Octocorallia, and those that have tentacles in the multiple of six are grouped under Hexacorallia.

Gorgonian commonly seen in Hong Kong waters

Soft corals and sea fan belong to the former group. They are also characterized by the absence of external calcareous skeleton (exoskeleton).

For soft corals, their skeletons are in the form of needle-like structures called sclerites found inside their body. Sea fans also have this type of endoskeleton, but in addition, they also have axial skeleton mainly made up of proteinaceous horny materials that form the core of their branching colonies.

Hard corals belong to the group Hexacorallia, together with black corals. Hard corals have calcareous exoskeleton but do not have endoskeleton.

Black corals have axial skeleton that is also organic in nature. Hard corals grow progressively by depositing more and more calcareous materials underneath their living tissues.

So a living hard coral colony is made up of layers of dead skeleton materials (mainly calcium carbonate) with a thin veneer of soft tissue on top. It is these hard corals that build up the foundation of the coral reef with the calcium carbonate deposit over thousands and thousands of years.