The Soft Side of Corals

Another Lens on Life blog post from Chuck Kopczak, PhD

09/28/2020
The white, soft structure of a coral blossoms with bright pink polyps.
Chuck Kopczak, PhD

A soft coral (Dendronephthya sp.) begins to inflate as the current begins to pick up.

Today's photo was taken on a reef known as Coral Garden on the northwest coast of Taveuni, the third largest island in the archipelago. It was made with a Canon 100 mm f/2.8 macro lens mounted on a Canon 5D Mk. III in an Ikelite underwater housing. It was illuminated by two Ikelite DS-161 strobes set on eTTL. The exposure was set to 1/200 sec at f/11 and ISO 1600.

For most of us, the word "coral" conjures images of hard branching structures, or massive forms that look like the wrinkled convolutions of a brain. In any case, the most common feature we think of is probably the hard nature of corals. But not all corals build hard skeletons of limestone in which the individual polyps reside. Species of corals are divided into two primary groups based on the dimensional symmetry of the individual polyps. In the case of hard, or reef-building corals, the polyps have six-fold symmetry, while in the soft corals, the polyps have eight-fold symmetry. This means that polyps of hard corals typically have 6 tentacles each, while the polyps of soft corals have eight tentacles each.

Like their hard cousins, soft corals are filter feeders, catching food as it drifts by on ocean currents. But unlike their hard cousins, especially the reef-building corals, the soft corals are not limited to tropical waters, but rather are found over a wider range of latitude and depth.

While polyps of hard corals can retract into their limestone skeleton, the basic structure of the colony remains visible, but soft corals, because they lack a hard skeleton, can retract or deflate almost to the point of disappearing. When currents are slow, soft corals will retract, waiting for sufficient current speeds to bring plankton to them. When this occurs, a patch of reef that may otherwise looks somewhat unremarkable, becomes a brilliant patch of color as soft corals expand to feed.

The pinnacle of soft coral species diversity seems to be centered in Fiji in the south Pacific, located at approximately 18° S latitude and 179° E longitude. Known as the soft coral capitol of the world, Fiji is an island nation composed of 332 islands about 2/3 of the way between Hawaii and New Zealand.

Sources

https://coral.org/coral-reefs-101/coral-reef-ecology/soft-corals/

https://www.livingoceansfoundation.org/corals-without-skeletons/

https://coralreef.noaa.gov/education/coralfacts.html