Caribbean Gorgonians: Beauty in Motion

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Anyone who has been diving extensively in the Caribbean has surely encountered the special reef habitats unique to the region: hardbottoms and reefs populated by conspicuous, highly diverse species of zooxanthellate gorgonians. These habitats are analogous to the fields of diverse species of Alcyonacean soft corals found in the western Pacific.

The back and forth sway imparted by surge on these gorgonians on a sunny day is mesmerizing. In an aquarium their moving branches contrasts with the stationary sps corals. I have been fortunate to live just a few minutes from where I can observe these lovely creatures in their natural setting, and they have always been among my favorite types of corals for a reef aquarium display.

Classification

Sea Fans, Sea whips, or gorgonians are octocorals that often have a flexible proteinaceous horny skeletal axis. They belong to the class Anthozoa, subclass Octocorallia (Alcyonaria), order Alcyonacea. The Caribbean photosynthetic gorgonians belong to two groups (sometimes treated as suborders), the Holaxonia and the Scleraxonia.

Here is a classification diagram for the common Caribbean photosynthetic genera:

Phylum Cnidaria Class Anthozoa Subclass Octocorallia Order Alcyonacea, Suborder Holaxonia Family Plexauridae Plexaura spp. Pseudoplexaura spp. Plexaurella spp. Eunicea spp. Muricea spp. Family Gorgoniidae Pseudopterogorgia spp. Pterogorgia spp. Gorgonia spp. Suborder Scleraxonia Family Briareidae Briareum asbestinum Family Anthothelidae Erythropodium caribbaeorum

 

Sexual reproduction in these gorgonians follows the pattern characteristic of all anthozoans, where both internal and external larval development having been recorded. Most gorgonians have separate male and female colonies. In some the eggs and sperm are released simultaneously for an external fertilization, but in most gorgonians the female catches the sperm, and broods the eggs. A swimming planula larva develops from the fertilized egg. The larva metamorphoses and settles on the substrate to form a founder polyp, from which secondary polyps bud off asexually to form a branched "colony."

Asexual propagation is therefore the mode of growing, but it is also the principle means of forming new colonies in many photosynthetic gorgonians. Sea fans, Sea Whips and other gorgonians reproduce asexually by colony fragmentation. Portions of the colony frequently break off and drift away to settle down elsewhere and expand into a new colony. Such colony fragmentation can be the most important means of colonizing reefs in the case of some gorgonians. Thus large populations may develop from a single, sexually produced founding colony. Lasker (1984) found that 94% of the colonies of photosynthetic gorgonians at a site in the Caribbean were the products of colony fragmentation. This fragmentation is sometimes assisted by the formation of tissue constrictions on the branches that increase the likelihood of breaking, a feature also seen in soft corals in the Indo-Pacific (for example Capnella spp.).

The yellow form of the seafan Gorgonia flabellum is extremely abundant in the Bahamas and some Caribbean localities. It makes a beautiful addition to an aquarium, but is seldom available due to restrictions on its harvest.

Feeding

While the species I cover in this article possess symbiotic zooxanthellae and benefit from their photosynthetic products, the most important mode of feeding for gorgonians in general is the filtering of plankton from the surrounding water. When illuminated with sufficient light the Caribbean photosynthetic gorgonians do not require much feeding to grow in captivity, but they do feed voraciously when fine particulate food is offered, and such feeding increases their growth. The feeding capacity is enhanced by water movement. Gorgonians occur in benthic habitats where there are persistent currents. To catch enough food in a given time, filter-feeding gorgonians need a large volume of water to pass over the polyps. Gorgonian colonies therefore thrive in positions where they are swept by vigorous currents.

Structure, Growth, and orientation

The skeleton may be made of sclerites, like other octocorals, but is frequently a combination of sclerite pieces and a horny but flexible core of protein called gorgonin that is similar to the collagen in our tendons but with at least twice the tensile strength (Jeyasuria and Lewis, 1987). Gorgonians must avoid being damaged by currents and at the same time remain erect in order to feed effectively. These two requirements necessitate a compromise between rigidity and flexibility, so that colonies typically having the elasticity of stiff rubber. In sea whip gorgonians the branches are like rods, whereas sea fans are highly branched forming an interconnected mesh, often growing in a single plane to form a fan shape. Colonies of either fans or sea whips may vary their growth form and orientation depending upon the type and direction of prevailing currents. In turbulent currents that come from variable directions, the growth form is typically bushy. The flat meshed fan shape, characteristic of the gorgonians known as "sea fans", is found where the currents are uni- or bi-directional, or
where the fan is exposed to a prevailing surge that makes it sway to and fro. The different morphologies have developed to maximize the amount of plankton entering the sieves of the polyps, and the amount of light they receive, which benefits the symbiotic zooxanthellae. This is achieved by reducing the amount in which one part of the colony 'shades' another part of the colony from the current or light. In fan shaped colonies the back and forth flapping caused by surge shades the lower side while the upper is illuminated. When there is no surge, the fan sits upright and both sides are illuminated equally.

Clonal propagation

Castanaro and Lasker (2003) clipped branches off colonies of Pseudopterogorgia elisabethae at 2 sites in the Bahamas to study how colony growth responds to disturbances such as harvesting, grazing, and storm damage. Colonies were clipped so that either 4 branches or 10 branches remained, and the growth rates of branches were monitored over 1 year and compared with controls (nearby unclipped colonies). Branch extension rates were equal among the 3 treatments, but newly formed branches had significantly greater extension rates in all treatments compared to branches present at the start of the experiment. The per colony branching rates were greatest on the more severely clipped colonies and were lowest on control colonies (which were not clipped).

Interestingly, the absolute number of branches that became mother branches did not differ among treatments. It did not matter whether the colonies were clipped so that 4 or 10 branches remained. At the end of the experiment they had the same average number of mother branches per colony. There was no significant difference between treatments in the average number of new branches formed on the colonies.

Per capita branching rates were significantly different among treatments only because the relative proportion of branches that became mother branches was higher in colonies with four branches than in treatments with more initial branches. Total growth (cumulative growth on all branches) was not significantly different between the 2 clipped treatments.

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Predators

There are a few predators of Caribbean photosynthetic gorgonians that should mentioned. First, the common flamingo tongue snails (Cyphoma spp.) that are sometimes offered for sale to aquarists feed exclusively on photosynthetic gorgonians. Their preferred prey is Plexaurella spp., but they also like the Sea Fans Gorgonia ventalina and G. flabellum. In the absence of these choices they can still survive on species of any of the other genera mentioned in this article. Harvest of these snails for aquariums does not make sense because they can only survive if given photosynthetic gorgonians to feed on. They quickly strip a gorgonian of its live tissue in an aquarium, even though they rarely cause such complete loss of tissue and death of colonies in the natural habitat.

Small white frilly seaslugs in the genera Tritonia and Tritoniopsis feed on Pseudopterogorgia spp. and Gorgonia spp., but generally do little damage unless they develop large populations. Their presence does tend to prevent the polyps from expanding, however.

Butterflyfishes and some angelfishes will feed on gorgonian polyps or tissue. The latter, however, are often safe to house with gorgonians, while the former will constantly nip the polyps.

Disease

Photosynthetic gorgonians are resilient and resist most diseases that afflict other corals. They can succumb to black band and red band infections, caused by the cyanobacteria. The genus Gorgonia suffers from fungal infections in the natural habitat, an extremely unusual occurrence for the marine environment, and not reported in aquariums.

A survey of the common photosynthetic gorgonians from the Caribbean.

 

	Pterogorgia sp. shedding Eunicea spp. Muricea sp. Muriceopsis sulphurea Erythropodium growing on the underside of a ledge in the Bahamas.

Pseudopterogorgia

Members of the genus Pseudopterogorgia have narrow smooth branches and the colonies are commonly pinnate (feather shaped). The most common colors are pale lavender and brown, but they may be gray, deep purple or yellow. In Florida there are five common species found on the reefs and numerous other species that occur only in backreef areas and in bays. The most popular species that is commercially harvested for aquariums is Pseudopterogorgia elisabethae, known in the aquarium trade as the "Purple Frilly." It occurs on outer oceanic reefs, attached to vertical faces and the undersides of ledges. This contrasts with most other species in this genus, that normally are attached upright on hard flat bottoms, or on the tops of reef structures. Pseudopterogorgia bipinnata occurs with P. elisabethae, attached in the same orientation, and ranges slightly deeper. It looks quite similar to P. elisabethae, but has finer branches. It may be deep purple, but more commonly is pale gray.

Pseudopterogorgia americana is a giant species with a rapid growth rate. It may reach heights in excess of six feet and colony diameters of more than eight feet. It is distinctly slimy. The elongate branches, which look like hair blowing in the currents, slip through the fingers if you try to hold them.

Potent anti-inflammatory agents have been identified from the Purple Frilly gorgonian, Pseudopterogorgia elisabethae collected in the Florida Keys (Sprung and Delbeek, 1987). New compounds are being discovered from extracts of this species and other members of the same genus. The compounds include novel pseudopterosins, seco-pseudopterosins and elisabethadione.

Pterogorgia

The three species of Pterogorgia are common on backreefs and nearshore habitats, growing upright in extremely shallow water. They tolerate very bright illumination, and thrive where other gorgonians would quickly become smothered by algae. Their success is largely due to their capacity to shed a waxy surface coating that prevents algae from gaining a foothold on them. They are not successful at preventing another type of gorgonian from growing on them. The encrusting gorgonian, Briareum asbestinum commonly grows onto and over Pterogorgia, kills it, and uses the former's axis for structural support.

Of the various genera covered by this article, Pterogorgia has the poorest long-term survival record in aquariums. This stands in contrast with its position as among the most tolerant of wide fluctuations in temperature and water quality in the habitat where it naturally occurs. It has a very good survival rate in shipping, with mortality being basically non-existent.

The poor survival in captivity relates to this genus seeming to require very strong illumination. It also seems to need some supplemental feeding. When strong light is given the members of this genus grow well in captivity, and supplemental feeding keeps them opening the polyps daily. The members of this genus frequently close the polyps for days or weeks at a time, and then shed a waxy skin, which leads a novice aquarist to believe the gorgonian is dying. As mentioned previously, the skin shedding prevents algae from growing on them. This genus grows upright on hardbottoms and should therefore not be positioned as if it was growing out of a wall of rock. It should instead be growing out of a rock on the bottom of the aquarium, with its branches facing upwards.

Gorgonia

There are three species of seafans in this genus, G. flabellum, G. ventalina, and G. mariae. This genus is not widely available to aquarists in the USA, although it is very common in nature, fast growing, and easy to keep in the high light high flow aquarium systems employed for keeping sps corals. The reason for its scarcity in the aquarium trade is a ban on its harvest from Florida and most Caribbean locations. The bans were put into effect because large seafans were commonly harvested and dried for the curio trade that was especially popular in the 1950's and 1960's. An aquarium trade in small three inch sized fans would hardly make an impact on the two common species in this genus, but such details cannot easily be accommodated by bureaucracy. Furthermore the fungus Aspergillus sydowii has been responsible for the mass destruction of Gorgonia ventalina especially over the last 15 years (Nagelkerken et al., 1997). This fact is sure to eliminate much hope of someone proposing a commercial harvest of this genus, although I hear that Caribbean-collected specimens are occasionally available to European aquarists.

Almost ten years ago I submitted a grant proposal to demonstrate that one inch by one inch squares of Gorgonia ventalina could be harvested from large adult colonies (which can be more than 36 inches across and multi-lobed) using a scissors. The small fragments could be attached to limestone rocks and cultured for sale to the aquarium industry or used in reef restoration as a means of rapidly generating new colonies without having any removal of established colonies. I included the proposal that the idea could be extended to the harvest of small branches from other photosynthetic gorgonians. The grant was submitted as part of a request for applications demonstrating novel harvest approaches. It was rejected on the basis that there was no prior literature that showed my proposed method would work! The recent study by Castanaro and Lasker (2003) mentioned earlier partially fills that literary void. Hopefully the scientist who rejected my proposal has found the time to examine things outside the lint collecting in his navel.

Plexaura

The genus Plexaura has two common Caribbean species, P. flexuosa and P. homomalla, and at least a couple additional uncommon species. The former is commonly purple while the latter is a distinctive chocolate brown. This genus, P. homomalla in particular, gained attention in the scientific literature because it produces protaglandins, steroids known usually from mammals. Prostaglandins have also been found in Plexaura nina and the arctic soft coral Gersemia fruticosa.

These compounds are believed to be used for defense, to prevent predators from eating the gorgonian. Gerhart (1991) found that although the gorgonians are initially palatable to fish, the prostaglandins may function as defensive toxins by inducing vomiting, and subsequent learned aversions in its potential predators.

Members of this genus do not ship well and are therefore rare in captivity. Small colonies (up to four inches tall) can be shipped more successfully than larger colonies. In aquariums purple or chocolate brown ones must be placed where they will receive strong water circulation and bright light. Pale gray P. flexuosa can be kept under lower light intensities.

Eunicea

Members of the genus Eunicea typically have knobby protrusions marking the calyces from which polyps emerge. This lends an Acropora-like appearance to their branches. They often have purple sclerites, though the surface color is usually brown or gray. Dark purple Eunicea occur on brightly illuminated shallow back reefs in some localities. Some Eunicea species have very large feathery polyps.

Eunicea species prefer surge zones or at least strong currents. They suffer from low oxygen levels in shipping bags, but small colonies can be shipped and they quickly grow into beautiful large colonies in an aquarium.

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Plexaurella

The slit-pore gorgonians belong to the genus Plexaurella. They are all pale brown or yellowish, and form colonies with club-tipped branches. Large specimens may have many branches, but average sized colonies consist of a main axis and just a few branches, giving a shape like a big sahuaro cactus. These are very hardy and can be recommended as a "beginner's" coral. They must not be allowed to rest against the substrate, however, as the portions of branches that lay on the sand or against a rock will suffocate and rot, turning black and falling off the axis. Tips of branches under bright illumination (close to a 250 watt or 400 watt metal halide) may fail to open. This is a response to photosynthetically produced active oxygen. Shading will stimulate expansion of the polyps in this case.

Pseudoplexaura

One of my all-time favorite gorgonians is Pseudoplexaura. There are three common species in Florida and an unknown number of others in Florida and the Caribbean. Pseudoplexaura species are similar to Plexaurella, but are distinguished by having purple sclerites, and round pores. All Pseudoplexaura spp. have a slimy consistency, while only one species of Plexaurella (that appears to be a mimic of the former genus) does.

Pseudoplexaura spp. grow very rapidly, and new branches seem to appear literally overnight sometimes. A swelling of tissue precedes their development. This rapid growth seems to be promoted mostly by the symbiotic zooxanthellae rather than food additions, though this species readily accepts fine particulate foods. It is a simple matter to cut off branches and propagate this species, though the slime production makes them poor shippers.

Muricea

The various members of the genus Muricea, which includes photosynthetic and non-photosynthetic species, are a bit delicate in captivity, so I don't recommend them to the novice. The deepwater fine branched "silver bush" gorgonian from Florida is an exception. It ships well and is hardy in aquariums.

Other species require very strong water currents or surge to survive, and they ship poorly, often decomposing a day or so after being transported.

Muriceopsis

This genus has two common species in the Caribbean, M. flavida and M. sulfurea. The former is sold under the common name "purple bush." The latter is not harvested commercially for the US and European aquarium trade. It occurs in the southern Atlantic and Caribbean and is common in Brazil, where it attains larger dimensions than elsewhere.

Members of this genus are capable of developing sweeper tentacles (Sprung and Delbeek, 1997).

Muriceopsis flavida is hardy and easy to propagate in aquariums. Branch fragments of this genus were washed on the shore of Miami Beach shortly after Hurricane Andrew in 1992. I collected some branches from the beach for Peter Wilkens when he visited me in Miami and gave a lecture at a MACNA conference held at that time. Peter doubted the colonies would survive, since they were collected out of water among seaweed and seagrass washed up on shore. Offspring of these branches now grow in numerous aquariums in Europe.

Briareum

This genus is well known to aquarists as "green star polyps" from Indonesia. In the Caribbean colonies of Briareum lack the fluorescent green pigment of their Indonesian cousins, but they are nevertheless beautiful and hardy. Reef aquarists should be careful with this genus, because Briareum has a habit of encrusting live rock and then overgrowing other corals attached to the rock. Encrusting forms of Briareum should be kept on the sand and not allowed to grow on the rocks. They are easy to keep when given bright light and strong water currents.

Although only one species, Briareum asbestinum, is recorded in the Caribbean, it seems to be a highly variable species, or there seem to be at least two or three undescribed species. One form, which is most common on offshore reefs, is upright with long smooth straight branches attached to a flat base. Other forms are encrusting, and may be smooth or with projecting calyces. The polyps are brown and may or may not have white centers and a white line down the midrib of each pinnate tentacle. The different polyp forms are so different that they seem like different species, in much the same way that different forms of Tubipora musica in the Indo-Pacific appear to be different species. Briareum spp. brood their larvae on the surface of the colony.

Erythropodium caribbaeorum

This encrusting soft coral is easily and often confused with encrusting Briareum. In fact they are not closely related, Erythropodium being a closer relative of the knobby gorgonian Diodogorgia nodulifera, which is azooxanthellate. Erythropodium has zooxanthellae, so it is brown. The underside of a colony has a purple color due to purple sclerites. The polyps of Erythropodium caribbaeorum are brown, without stripes, and form elongate hair-like tentacles in strong water motion. When the polyps are withdrawn, the calyces are lighter than the rest of the surrounding tissue, so they appear as light spots. In Briareum asbestinum the closed polyps are darker than the surrounding tissue.

Erythropodium caribbaeorum is very easy to keep and propagate. It grows well on the walls of the aquarium, and it is best kept there or on the sand or gravel bottom in a region with strong water flow. If it is allowed to grow on the rocks it will encrust over stony corals.

The various photosynthetic gorgonians from the Caribbean lend a refreshing display of motion to a reef aquarium exhibit and include species that are easily maintained and propagated. Their habitat in the wild is a real treat to observe and is an important feature of Caribbean reefs.

 

(Click images for a larger view)
Compare the closed appearance of Briareum asbestinum, left, to Erythropodium caribbaeorum, right.	Cyanobacteria in the genus Phormidium, which cause "black band disease" in corals, also affect gorgonians, such as this Pseudopterogorgia sp. They can be siphoned off as a means of control. Otherwise they slowly smother the coral and cause the live tissue to decompose.	Thick branched varieties of Eunicea are especially beautiful. This scene was photographed on a reef in the Florida Keys.
Branches cut off with a scissors can be attached to rock using underwater epoxy. By using several species one may create a unique and beautiful piece.	The bristle worm Hermodice carunculata feeds on gorgonians (as well as other cnidaria). This one has just consumed the tip of a branch of Pseudoplexaura.	Muriceopsis flavida is sold under the common name "purple frilly.' It has a pinnate shape, like Pseudopterogorgia, but its branches are round in cross section, not flattened.
Plexaura flexuosa from the Caribbean growing with Indo Pacfic Xenia in an aquarium.	Two color forms of Pseudopterogorgia sp. washed up along a shallow rocky shore in the Florida Keys. The yellow form is rare. Storms break colonies loose and many perish. Aquarists can rescue them for aquariums.	Pterogorgia guadalupensis expanded in an aquarium.
Plexaurella sp. expanding its lovely polyps in an aquarium.	Note the slit shaped pores on this Plexaurella sp.	(no larger photo) The circular pores (calyces) of Pseudoplexaura spp. are distinct from the similar looking Plexaurella spp, which have slit-shaped pores.

 

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References and suggested reading