Marine Invertebrates of Bermuda Common Sea Fan (Gorgonia ventalina) Tabitha A. Baker and James B. Wood (Ed) Abstract Taxonomy  Habitat  Ecology  Recent Research  Commercial Importance  Bermuda Laws  Personal Interest  References  Links

Taxononmy

Phylum: Cnidaria
  Class: Anthazoa
    Subclass: Alcyonaria
      Order: Gorgonacea
        Suborder: Holaxonia
          Family: Gorgoniidae


Abstract

Gorgonia ventalina is a unique coral that grows in the shape of a fan and has a distinctive purple coloring, which is where it gained the common name “purple sea fan”. G. ventalina can be found in many environments and varying depths throughout the Caribbean and Bermuda reefs. With their surface sclerites and chemical compounds they are able to defend off predators and diseases. One of the diseases that do infect this sea fan is Aspergillosis, which has been well studied in the recent years. G. ventalina is seen as a nice souvenir from many places in the Caribbean, however personally I think it adds to the overall beauty of the reefs here in Bermuda and is better left in it’s natural habitat where it is protected so by the Coral Reef Preserve Act of 1966.


Habitat

Gorgonia ventalina can be found in many different habitats, it was once believed that their growth was restricted to outer and rim reefs (Sterrer 1986), however they have since recruited to shallower waters in Bermuda and elsewhere in the Caribbean. The reason for their growth in many areas is because they flourish in areas with consistent water flow for feedings and respiration reasons (Matsumoto 2004).


Ecology

Gorgonia ventalina can be distinguished by their purple tissue and fan shape hence the common name ‘purple sea fan’. They are most commonly purple, however they can be yellow or brown in color but these colors are much rarer (Sterrer 1986). Their branches are rounded and slightly compressed in the plane of a fan, with small calyces located in 2 rows along the edges of these branches (Sterrer 1986). They can grow to be about 180cm tall by 150cm wide (Sterrer 1986). The time needed to grow to this length is estimated to be about 2 to 5 years; however they do continue to grow beyond the average but at a much slower rate (Cary 1915). The life span of a gorgonian is unknown, however Cary (1915) states that there is no evidence from fossil or current records of a sea fan dying from old age. The most common death to a sea fan is destruction by wave energy and overgrowth of their tissues by organisms such as Millepora alcicornis and some encrusting bryozoans (Cary 1915). The rate at which gorgonian death occurs is about 1/5 of the population in an area annually (Cary 1915), however it has been seen that recruitment vs. mortality is nearly balanced (Grigg 1977).

G. ventalina has a unique growth style that is influenced by feeding style; because they are filter feeders their growth form allows for optimal contact between living colony tissue and water (Matsumoto 2004). A study by Matsumoto (2004) looked at the growth of gorgonians in Japan, through these studies the growth form of G. ventalina is more known about because of the similarities between sea fans in the two regions. It was once thought that this shape was symmetric however recent studies have shown that the growth of the branches is not continuous and homogenous but rather believed that each of the main branches in the sea fan grows independently. Gorgonian growth is influenced by the variation in seasonal growth which can include water temperature, food abundance, and seasonal reproduction. When reproduction and water temperature are at the greatest the growth rate is slowest, therefore the highest growth rates have been seen in the winter when the water temperatures and reproduction are at the lowest. The reason behind this is because the energy that was once used on reproduction can then be used on growth. Growth rates are also highest in injured gorgonians because of the need to regain the optimal size for feeding and respiration requirements. Size limitation for gorgonians is due to water flow and allowing for the largest contact to occur between the tissue and water. Because of the reasons that they grow to allow for largest contact then it is reasonable to say that their growth rate would decrease with age, the reason that this occurs is because there are many risks and few advantages for the colonies to grow above optimal size. These risks include aerial exposure and set the upper limit on the size of colonies because of the probability of whole-colony mortality (Birkeland 1974).

The shape and form of a gorgonian is one that faces directly into the water flow to increase feeding rates and respiration (Matsumoto 2004). A study by Grigg (1972) showed how the orientation of a sea fan in respect to water flow is important. He made field observations in shallow water to see the orientation of sea fan colonies with respect to other sea fans and water flow. He discovered that gorgonian sea fans grow in planes perpendicular to the current direction, which has also been observed in greater depths. When measuring this orientation against other sea fans it was seen that this orientation agreement increased with height of the colony. Younger smaller corals do not get the true current because of boundary layers the current is multidirectional, however as they grow into the true current they twist within their holdfast to obtain the must efficient direction.

G. ventalina similar to other corals within a coral reef is symbiotic with zooxanthallae (de Putron 2006). Zooxanthallae is the dinoflagellate Symbiodinium spp. which measures approximately 10 to 12 microns in diameter (de Putron 2006). Its coloring is brown from chlorophyll A and C that it contains (de Putron 2006). Zooxanthallae provides the coral with carbon compounds; increases its growth, reproduction, and calcification; conserves nutrients; and provides a higher surface area to volume that favors prey capture and light capture (de Putron 2006). It has been seen that corals change their zooxanthallae during season changes from one clade to another (Goulet 2006). However, a study by Goulet (2006) looked a corals that did not change their zooxanthallae during these seasonal changes, one coral was G. ventalina. It was seen that they host a single clade B sub-class year around (Goulet 2006).

G. ventalina have structural and chemical defenses to predators; these defenses increase in areas where predation is at its highest (Puglisi et al. 2000). Structural defense comes in the form of sclerites. Sclerites are similar to the spicules found in sponges. They are known to make up almost 80% of the dry weight and when they come in high ratios they act like reinforcing fibers (Harvell 1989). Sclerite concentrations typically increase from the distal part of the colony to its base (Van Alstyne & Paul 1992). The sclerites from the surface of G. ventalina are in the shape of a scaphoid, which is bent down at the tips and encircled with a series of tubercle belts (Lewis & Von Wallis 1991). They form the surface layer of the sea fan and are in a parallel orientation, which can vary depending on the tension of the animal (Lewis & Von Wallis 1991). Sclerites are not only defenses for predators but also for foreign pathogens, for instance Osstreobium and Aspergillosis (Alker et al. 2004). With the invasion of these foreign objects the sea fan will increase purple sclerites in the area to try to contain it from spreading (Alker et al. 2004). Chemical defenses in G. ventalina have not been studied to the full extent that structural defenses have been studied to. However, it is believed that they contain about a dozen different chemical compounds that remain consistent in concentrations throughout the entire colony (Van Alstyne & Paul 1992).


Recent Research

Recent research that has been completed is on the sea fan disease, Aspergillosis. Aspergillosis is caused by the fungus Aspergillus sydowii which originates in soil and is also known as a food contaminant and occasionally as an opportunistic pathogen in humans (Alker et al. 2001). The symptoms of this disease include lesions, galling, and purpling of the tissue, which can together lead to mortality of the colony (Alker et al. 2001). Aspergillosis like any disease is dependent on different factors for infection: host of health, colony size, and chemical compounds. The geographical region that this disease is prevalent in is also important because it is only found within the Caribbean which is a disease hot spot (Alker et al. 2001; de Putron 2006).

The intensity of the disease as well as being infected by this disease is dependent on many factors. The first factor was discussed by Alker et al. (2001) and that was health of the host. Host health is important in sea fans the same way as it is in humans; and similar to the human immune system this health can be compromised by stress. One major stressor is that of temperature. If the temperature reaches a point that is higher then the threshold adaptation of the sea fans then zooxanthallae photosynthesis will be decrease, the sea fan will become stressed from the lack of nutrients; therefore infection chances are increased. The second factor was discussed by Dube et al. (2002) and that is the size of the colony. It was seen in their study that the larger the size of the colony the higher the likelihood of being infected. This was backed up by three different hypotheses: 1) a positive relationship between surface area and probability of infection, 2) a greater age and therefore longer exposure time, 3) smaller sea fans have greater chemical, cellular, and/or structural resistance to disease. The third factor in being infected by the disease Aspergillosis was studied by Kim et al. (2000). This factor is disease resistance through chemical compounds. Gorgonians are known to have secondary compounds within their tissue to defend against predators and it is thought that these same chemicals also protect against disease. If these chemical compounds were decreased because of outside factors then their defense against the disease would lessen and infestation is a greater risk. It was also seen by a studied done by Morse et al. (1977) that gorgonians that were diseased were found in reefs exposed to environmental stressors such as intensive and highly localized storm damage, chronic exposure to the conditions of low salinity and high turbidity; for instance reefs that were found in areas near the mouth of a river.

Aspergillosis was first documented in 1995 near Saba, Bahamas; following this discovery locations throughout the Caribbean basin and the Florida Keys also reported the disease (Kim & Harvell 2004). It is thought that the first epizootic that occurred was in the 1980s along the Central American coast and within the Caribbean, however was not identified until recently; this was discovered because of the similar lesions that are on older sea fans that are similar to the lesions from the current disease outbreak (Kim & Harvell 2004). These break outs are thought to be around the times of rapid temperature increase where one has occurred in the early 1980s around the time that the first epizootic occurred and the second was in the 1990s during the time of the second epizootic (Kim & Harvell 2004). In recent time this epizootic is subsiding which is thought to be caused by sea fan prevalence to the disease and the ever changing environmental conditions (Kim & Harvell 2004).

Another recent study that is ongoing is the research on the nudibranch Tritonia hamnerorum which feeds on Gorgonia ventalina (Cronin et al. 1995). T. hamnerorum feeds on this sea fan for the reason of acquiring its chemical compounds which are then incorporated in its body to defend against predators (Cronin et al. 1995). It was seen in a study by Cronin et al. (1995) that these specialized nudibranchs were found in great abundance on G. ventalina in Key Largo, this was thought to be because of an outbreak that occurred. Although this relationship is not unhealthy for the sea fan, in great abundance these nudibranchs can kill whole gorgonian colonies, for instance it was reported to NOAA by Thad Murdoch (2005) that sea fan colonies in Bermuda were graved to death by these nudibranchs in 2005.


Commercial Importance

Gorgonia ventalina are commercially important in a couple of different aspects. They are used by aquarium owners who have saltwater reef aquariums to add color and beauty (Westrum 2001). They are also sold and taken as souvenirs throughout the Caribbean (Westrum 2001). G. ventalina is indirectly commercially important because they add to the beauty of a coral reef, and attract tourist to the area through snorkeling and scuba diving.


Bermuda Laws

G. ventalina is protected in Bermuda by the Coral Reef Preserve Act of 1966. This act prohibits the removal of any coral living or dead from any coral reef within the North and South coral reef preserve (Wood and Jackson 2005). They are also protected within the 29 different no take zones that have been established throughout the Bermuda platform (de Putron 2006).


Personal Interest

I have been interested in this coral since my first snorkeling trip at Whalebone bay. I was intrigued by its beauty and ranging in sizes as I got deeper. I decided on that day that I wanted to learn more about this “purple sea fan”. Therefore, I have done this web report on them and have continued to study them through my final research here in Bermuda. My final research is going to look at population dynamics and colony health in the sea fans within coastal reefs on the north shore compared to non coastal coral reefs.


References

Akler AP, Kim K, Dube DH, Harvell CD. 2004. Localized induction of a generalized response against multiple biotic agents in Caribbean sea fans. Coral Reefs. 23:397-405.

Akler AP, Smith GW, Kim K. 2001. Characterization of Aspergillus Sydowii (Thom et Church), a fungal pathogen of Caribbean sea fan corals. Hydrobiologia. 460:105-11.

Birkeland C. 1974. The effect of wave action on the population dynamics of Gorgonia ventalina Linnaeus. Studies in Tropical Oceanography, Miami. 12:115-26.

Cary LR. 1915. The Alcyonaria as a factor in reef limestone formation. In: Proceedings of the National Academy of Sciences of the United States of America; May 15. 1(5):285-9.

de Putron SJ. 2006. Coral reef ecology lecture notes.

Dube D, Kim K, Alker AP, Harvell CD. 2002. Size structure and geographic variation in chemical resistance of sea fan coral Gorgonia ventalina to fungal pathogen. Marine Ecology Progress Series. 231:139-50.

Goulet TL. 2006. Most corals may not change their symbionts. Marine Ecology Progress Series. 321: 1 – 7.

Grigg RW. 1977. Population dynamics of two gorgonian colonies. Ecology. 58:279-90.

Grigg RW. 1972. Orientation and growth form of sea fans. Limnology and Oceanography. 71: 185 – 92.

Harvell CD, Fenical W. 1989. Chemical and structural defenses of Caribbean gorgonians (Pseudopterogorgia spp.): intracolony localization of defense. Limnology Oceanography. 24(2): 382-9.

Kim K, Harvell CD. 2004. The rise and fall of a six year coral fungal epizootic. The American Naturalist. 164:S52-S63.

Kim K, Kim PD, Alker AP, Harvell CD. 2000. Chemical resistance of gorgonian corals against fungal infections. Marine Biology. 137:393-401.

Lewis JC, Van Wallis E. 1991. The function of surface sclerites in Gorgonians (Coelonterata, Octocorallia). Biology Bulletin. 181:275-88.

Matsumoto AK. 2004. Heterogeneous and compensatory growth in Melithaea flabellifera (Octocorallia: Melithaeidae) in Japan. Hydrobiologia. 530/531:389-97.

Morse DE, Morse AN, Duncan H. 1977. Algal ‘Tumors’ in the Caribbean sea-fan Gorgonia ventalina. In: Proceedings of the Third International Coral Reef Symposium; 1977 May; Miami, Florida. P623-30.

Murdoch TJT. 2005. Tritonia hamnerorum in Bermuda. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from http://www.seaslugforum.net/find.cfm?id=14424

Puglisi MP, Paul VJ, Slattery M. 2000. Biogeographical comparisons of chemical and structural defenses of the Pacific gorgonians Annella mollis and A. reticulate. Marine Ecology Progress Series. 207: 263-72.

Sterrer W. 1986. Marine fauna and flora of Bermuda. Wilsey-Interscience: New York.

Westrum TS. 2001. West Papua: Only the village people can save their reefs and rainforests. Biodiversity. 2(1):15-19.

Wood, J, Jackson. 2005. Bermuda. In: Caribbean Marine Biodiversity; the Known and the Unknown. Pennsylvania. DEStech Publications, Inc. P19-36.

Van Alstyne KL, Paul VJ. 1992. Chemical and structural defenses in the sea fan Gorgonia ventalina: effects against generalist and specialist predators. Coral Reef. 12:155-9.

Links

NOAA’s Coral Reef Conservation Program
Ocean Conservancy
Reef Base