A Guide to Soft Corals (Octocorals)

Soft corals refer to the organisms in the class Octocorallia, which includes gorgonians, sea fans, sea pens, sea feathers, and blue corals. These corals have a flexible, sometimes leathery, appearance. Although many resemble plants, they are actually animals.

Soft corals are colonial organisms, which means they are formed of colonies of polyps. The polyps of soft corals have eight feathery tentacles, which is why they are also known as octocorals. One way to tell the difference between soft corals and hard (stony) corals is that the polyps of hard corals have six tentacles, which are not feathery.

Here are some stony coral characteristics, with some of the key differences with soft corals identified:

• They have polyps that secrete a cup (calyx or calice) in which they live. The polyps of soft corals usually have feathery tentacles.
• They may harbor zooxanthellae, algae that live within coral polyps and can produce brilliant colors. Others may be colored by bright pink, blue or purple pigmentation.
• They may contain spikes called sclerites, which are made of calcium carbonate and protein, and are located within a jelly-like tissue called coenenchyme. This tissue lies between the polyps and contains canals called solenia, which transport fluids between the polyps. In addition to providing structure to the coral and protection from predators, the shape and orientation of the sclerites can be used to identify coral species.
• They have an inner core made out of a protein called gorgonin.
• They may have a variety of shapes, including fan-like, whip-like or feather-like, or even leathery or encrusting.

Classification

• Kingdom: Animalia
• Phylum: Cnidaria
• Class: Anthozoa
• Subclass: Octocorallia
• Orders:
  • Alcyonacea (the horny corals, also known as the gorgonians, sea fans and sea feathers)
  • Helioporacea (blue corals)
  • Pennatulacea (sea pens)

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Habitat and Distribution

Soft corals are found worldwide, primarily in tropical or subtropical waters. Soft corals do not produce reefs but may live on them. They may also be found in the deep sea.

Feeding and Diet

Soft corals may feed during the night or day. They use their nematocysts (stinging cells) to sting passing plankton or other small organisms, which they pass to their mouth.

Reproduction

Soft corals can reproduce both sexually and asexually.

Asexual reproduction occurs by budding when a new polyp grows out of an existing polyp. Sexual reproduction occurs either when sperm and eggs are released in a mass spawning event, or by brooding, when only sperm are released, and these are captured by female polyps with eggs. Once the egg is fertilized, a larva is produced and eventually settles to the bottom.

Conservation and Human Uses

Soft corals may be harvested for use in aquariums. Wild soft corals may also attract tourism in the form of dive and snorkeling operations. Compounds within the tissues of soft corals may be used for medicines. Threats include human disturbance (through humans stepping on corals or dropping anchors on them), overharvesting, pollution, and habitat destruction.

Examples of Soft Corals

Soft coral species include:

• Dead Man's Fingers (Alcyonium digitatum)
• Sea Fans
• Sea Pens

Sources and Further Reading

• GBR Explorer. Soft Corals. ReefED.
• NOAA. Coral Anatomy and Structure. NOAA Coral Reef Conservation Program.
• Simpson, A. 2009. Reproduction in Octocorals (Subclass Octocorallia): A Review of Published Literature. Version 16 July 2009. In Deep-Sea Corals Portal.
• South Carolina Department of Natural Resources. Octocoral Morphology.
• Tan, Ria. 2008. Soft Corals. Wild Fact Sheets.
• Wet Web Media. The Soft Corals, Order Alcyonacea; Use In Marine Aquariums.