Introduction to the Octocorallia

Although commonly called "soft corals," the Octocorallia are not close relatives of the Scleractinia, or "true corals" living today. Unlike true corals, which have hexaradial symmetry, octocorals have eightfold radial symmetry -- count the tentacles on the polyps of this Muricea, from the Bahamas. Notice also that the tentacles of Muricea, like those of all octocorals, are pennate -- small branches come off of the main tentacle to give a more or less feather-like appearance. All octocorals are colonial polyps, and in some, such as the Pennatulacea or "sea pens," the polyps are specialized for various functions.

Octocorals are traditionally divided into six orders:

The Telestacea and Alcyonacea are two types of "soft coral." Muricea, pictured above, is an alcyonacean soft coral.
The Stolonifera includes the so-called "organ-pipe corals."
The Gorgonacea includes the sea fans and sea whips.
The Pennatulacea includes the sea pens and sea pansies.
The Helioporacea includes only one genus, Heliopora, the so-called "blue coral" of the Indian and Pacific Oceans.
Except for the "blue coral" and "organ-pipe corals," few octocorals produce substantial calcium carbonate skeletons; hence the name "soft coral" for many of them. However, most octocorals form spicules within their tissues, and some produce calcified holdfast structures or long, rodlike internal supports; these parts can be preserved as fossils. Such fossils have not received much study by paleontologists, but have been found at scattered localities throughout the Phanerozoic. Rare fossils that preserve impressions of the soft tissues of octocorals have been found in the Cambrian Burgess Shale and elsewhere.

A searchable bibliography of the Octocorallia is now available from the National Museum of Natural History.