NOBODY GOES TO THE TROPICS TO SEE GORGONIANS. The sea fans, plumes, rods and whips that make up Order Gorgonacea are just there, incidental bystanders on the “real” reefs of beautiful, stony corals. Sometimes, they’re in the way.
While none of the 500 or so species of fans, plumes, rods or whips can compare to the exquisite beauty that soft corals achieve (although some sea fans make a good effort), they’re part of the broad array of forms that makes reefs hot spots of life.
DIVERSITY, SANCTUARY, FOOD CHAIN As part of the reef landscape, they add to its diversity in and of themselves. Beyond this, they provide hiding places for small and juvenile fishes, snails and other creatures. And, their polyps represent a food source for some of the animals who hang around them.
While sea fans do have a presence in the Indo-Pacific, gorgonians overwhelmingly are creatures of the tropical Atlantic and Caribbean. Typically, gorgonians are found in relatively shallow depths but some live in waters as deep as 1,000 feet or more.
POLYPS, JUST LIKE “REAL CORALS” Although they’re eight-tentacled octocorals, in their individual selves they’re coral polyps just as much as the (six-tentacled) animals that build stony coral masses. They’ve just found alternative vehicles for earning their livings in the sea, foregoing calcium carbonate solidity for skyward reach. Like other coral polyps, gorgonian polyps hide below the surfaces of their branch-like structures and extend their tentacles outward to sieve plankton from the current.
MAXIMUM CURRENT ACCESS Except that their structures generally involve stems, branches and branchlets. By colonizing in flexible tree-like forms that can sway back and forth in the current, many species of gorgonians maximize plankton-filtering opportunities along reef crests, walls and channels, for example. Often, bulkiness creates efficiency. Sea fans face the currents broadside for the greatest exposure by the greatest numbers of polyps. Sea plumes grow as bushy structures that maximize exposure to the plankton.
SUBORDERS, BUT NOT TIDY ONES Gorgonians are placed in two suborders, depending on the composition of their stems and branches. The skeletons of Suborder Holaxonia are built with a complex protein called gorgonin – sometimes compared to horn (they’re sometimes referred to as horny corals). The skeletons of Suborder Scleraxonia get their strength from spicules, small, needlelike and tightly bound calcareous formations, similar to those found in soft corals and sponges.
SUBORDERS But while a tidy world would have all fans or whatever are in one suborder and all rods or whatever in the other, in fact some of each are found in each category.
The core stems and branches of sea fans and rods are surrounded by a gelatinous layer called the rind. Embedded in the rind layer, each polyp extends its tentacles through an opening called an aperture that is surrounded by a rim called a calyx.
On the other hand, lacking the extensive calcium carbonate production of stony corals, gorgonians don’t contribute significantly to the construction of the reef. Gorgonian structures are generally attached to the bottom by a single holdfast at the base of their stems.
ZOOXANTHELLAE, YES, BUT VIVID COLORS ARE “SPICTACULAR” Like stony corals, many gorgonians are abetted by the presence of algae – zooxanthellae – embedded in their tissues. In contrast to stony corals, which often get their exotic coloration from their zoox, gorgonians don’t. Zooxanthellae-hosting gorgonians are usually a dullish light brown. The vivid blues, reds, greens and other bright colors in sea fans and sea rods are due to the presence of the spicules within them.
AND HARD TO NAIL DOWN Beyond their basic structures as fans, rods or whips, etc., identifying a species of gorgonians relies on the arrangement of its polyps (neat rows, alternating rows, no discernable rows, for example) and the shape of its aperature and calyx. But accurate identification can be difficult and requires examination by an expert using a microscope.
Principal Sources: Coral Reef Idenification, Paul Humann, edited by Ned DeLoach; ReefEd (www.reefed.edu.au) Great Barrier Reef Marine Park Authority, Australia; Indo-Pacific Coral Reef Field Guide, Gerald R. Allen and Roger Steene; Coral Reef Animals of the Indo-Pacific, Terrence M. Gosliner, David W. Behrens, Gary C. Williams; National Oceanic and Atmospheric Administration