Purple Sea Fans
This topic submitted by Sam McBride (
samacmcbride@gmail.com) at 5:37 PM on 6/7/08.
A green
snake close-up, Hospital Pt, Panama.
Gorgonian ventalina: The Purple Sea Fan
The purple sea fan, or Gorgonian ventalina is a unique coral that gets its name from its purple coloring and its fan-like shape. According to Tabitha A. Baker and James B. Wood from the Bermuda Institute of Ocean Studies, G. Ventalina can be found in many environments and varying depths. A typical sea fan described by Baker and James in their research for the Bermuda Institute of Ocean Studies is:
ÒÉBranched and 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 150 cm wide (Sterrer 1986). The time needed to grow 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 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).Ò
G. Ventalina is a beautiful coral specimen and grows in colonies that are stunning under water. Unfortunately, because of their beauty, they tempt divers and snorkelers to take them as souvenirs and for the same reason locals have created a market for the fans. Outsiders to the purple sea fanÕs environment are not the biggest threat. The biggest threat to the purple sea fan is organic, it is a disease caused by a fungal pathogen, Aspergillus sydowii. While its environment has negative side effects such as Aspergillus sydowii, the purple sea fan interacts with its environment in very positive ways as well. Purple sea fans grow in colonies that are structured based on the natural current in the water. Their growth style is influenced by their feeding style; because they are filter feeders their growth form allows for optimal contact between living colony tissue and water (Baker and WoodÑMatsumoto 2004). The growth of the coral is influenced by a variety of factors. Some include: water temperature, food abundance and seasonal reproduction (Baker and Wood). 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 the lowest (Baker and Wood). The reasoning behind this thinking is that energy that would be used for growth is instead used for reproduction and visa versa (Baker and Wood). In addition to interacting with their environment to grow and reproduce, these seemingly simple creatures provide a home and a source of food to crustaceans such as the flamingo tongue. While the flamingo tongue is not good for the sea fan, disease is killing sea fans at a much greater rate and poses a much larger threat to the existence of the Gorgonian ventalina.
The most prevalent disease affecting the purple sea fan today is caused by a fungal pathogen, Aspergillus sydowii. According to H.B. McCarty and E.C. Peters Aspergillus sydowii causes a band or patch of white filamentous material. Specifically McCarty and Peters describe the disease as:
ÒÉShowing one or more irregularly-shaped patches or extensive loss of the purple tissue, with fine white filamentous material and sediment on the black axial skeleton that supports the tissue. Often, tissue adjacent to the patch is colored more deeply purple. Disintegration of the axial skeleton also occurs, leaving behind holes or complete loss of the sea fan blade structure. Sea fan tissue can also exhibit small dark purple spots and protuberant masses of purple tissue.Ó
Aspergillus is a known pathogen of humans and other animals and transmission experiments have confirmed the ability of this fungus to cause the disease in the sea fan. The problem with prevention, is that there is no conclusive evidence regarding distribution of the disease, the source of the fungus, the characteristic of the sea fan that allows the fungus to kill it and whether the death of sea fans is strictly caused by Aspergillus or if there are other factors that either make the fan especially susceptible to infection or if they contribute to the death of the sea fan.
Interestingly enough, the question, Òwhat can we do?Ó is actually preceded by Òwhat are the colonies doing?Ó Apparently the sea fans donÕt have much faith and have begun to defend against this pathogen on their own. A study by K. Kim, Harvell, D. Kim, G.W. Smith and S.M. Merkel, published in the journal of Marine Biology investigates the causes associated with the disease and the response from the sea fan. Interestingly, the scientists looked at specimen from Sal Salvador and from Alligator Reef in the Florida Keys. The crude extracts from both species were tested to determine concentrations inhibiting germination of A. sydowii spores. According to these scientists, crude extracts from both species inhibited spore germination at concentrations as low as 1.5mg ml^-1; most samples were active at 5 to 10 mg ml^-1. These concentrations were found to be higher in healthy colonies suggesting their role in mediating disease susceptibility (K. Kim, et. al). The most fascinating finding was that in healthy communities, resistance was highest at colony edges and lowest in medial and central regions of the colony (K. Kim, et. al)! Even more telling were colonies that had fans with lesions in the center of the colony and an increased level of resistance on the edges of the colony, indicating a deliberate response from the group of sea fans (K. Kim, et. al).
The impact that we are having is difficult to measure because the data is so new and the effects are difficult to measure. There are some factors like the temperature in the water that appear to effect the proliferation of the disease, but it is not conclusive and the increase in temperature in the water is such a broad explanation, that it leaves us searching. So how can humans help purple sea fans? H.B. McCarty and E.C. Peters have some suggestions:
Record the location where disease is seen (reef name, latitude longitude and depth)
Record date of observation
Make careful observations of the diseases appearance and any factors that might be affecting the sea fan
Take photos if possible
Sea Fans are one of the many treasures found underwater. They add to the beauty of the sea and at the same time, their destruction is evidence of mans footprint on our environment. While the evidence is inconclusive because we have not gathered enough, we can start by respecting the environment and not take sea fans as souvenirs. We can refuse to buy sea fans from local who are poaching them and we can report any diseased sea fans that we encounter and provide scientists with the most detailed observations we can make.Bibliography:
Alker, Alisa P., Smith, Garriet W. and Kim, Kiho. Characterization of Aspergillus sydowii (Thom et Church), a fungal pathogen of Caribbean Sea fan corals. Vol 460.. 105-111.
Baker, Tabitha A. and Wood, James B. Bermuda Institute of Ocean Studies. .
Kim, K., Kim, P.D., Alker, A.P. and Harvell, C.D. 2000. Chemical resistance of gorgonian corals against fungal infections. Vol 137. Journal of Biomedical and Life Sciences. 393-401.
Kim, K., Harvell, C.D., Kim, P.D., Smith, G.W. and Merkel, S.M. 2000. Fungal disease resistance of Caribbean Sea fan corals (Gorgonia spp.). Vol 136. Journal of Marine Biology. 259-267.
McCarty, Harry B. and Peters, Esther C. The Coral Disease Page. 2000.
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