Media Review: Review of the Literature
In the last few months a couple of articles/letters appeared both online (http://reefkeeping.com/issues/2002-05/eb/feature/index.htm) and in the printed media (FAMA, August 2002) where the author was questioning the claim that bacterial disease(s) were to blame for the problem with poor survival of imported Catalaphyllia (Elegans coral) colonies in the last year or so. This author brought into question the probability that bacteria could be held accountable for any coral or invertebrate disease, pointing out the lack of published work in this area.
This month I would like to bring to your attention a recent article that appeared in the journal Marine Biology. The abstract of the article can be viewed online here, so I will not go into great detail on the entire text of the article but I would like to bring to your attention the following key points.
Ben-Haim, Y. and E. Rosenberg. 2002 A novel Vibrio sp. pathogen of the coral Pocillopora damicornis. Marine Biology 141: 47-55.
In this paper Ben-Haim and Rosenberg describe how they were able to isolate a previously unknown pathogenic bacterium Vibrio coralyticus YB from colonies of Pocillopora damicornis collected from the coast of Zanzibar (in fact they found 15 strains of Vibrionaceae that were dominant in diseased colonies but not healthy ones, but V. coralyticus YB was the most virulent). They were able to inoculate healthy colonies with this bacterium (by adding it to the water and by putting an affected coral into direct contact with a healthy piece) and produce the same symptoms of rapid tissue loss resulting in death within a few weeks. They were also able to infect other colonies of P. damicornis collected from the Red Sea (ambient water temperature of collection area was 22-26°C) with the same bacterium.
In experiments of disease transmission at differing temperatures it was found that no symptoms appeared after inoculation at 20 and 25°C after 20 days (68-77°F), but that 100% of the tested fragments showed disease and died at 27 and 29°C (80.6-84.2°F) after just 16 days (the rate was slightly faster at 29°C than 27°C). It is interesting to compare the photos shown in Fig. 3a-d in this paper and those published in The Reef Aquarium volume two pages 444-445 (Sprung and Delbeek, 1997).
Although no studies have been conducted on the mechanism by which this bacterium results in tissue loss, it is felt that a mechanism similar to that found with the well-studied Vibrio shiloi/Oculina patagonica bleaching disease in the Mediterranean (see references in the above article) may be at work where virulent genes only become expressed with increases in temperature. For example, the V. shiloi adhesion, which is required for binding the bacterium to a ß-galactose-containing receptor on the coral surface, is only produced at higher temperatures. The mechanisms by which V. coralyticus YB causes tissue damage to P. damicornis are, at present, unknown. Preliminary data indicate that the bacterium adheres to the coral surface and then penetrates into the tissue. Ben-Haim and Rosenberg are currently examining the specificity of this adhesion and the effect of temperature on the process. In addition, they are studying the host specificity of the infection and the production of potential V. coralyticus YB virulence factors, such as extracellular proteinase and toxins similar to those produced by V. shiloi. They have also recently isolated a strain of Vibrio that is very similar to V. coralyticus YB, from diseased Red Sea P. damicornis.
So what does this mean for aquarists? Well, it brings home again the point that running aquaria with corals close to their thermal maxima can be a risky business. A mere 2°C (3.6°F) increase in temperature was enough to cause complete mortality in the P. damicornis used in this study. There is of course a great deal of variability between corals and their thermal tolerances, but without the data on where the corals were collected, what the prevalent water temperatures were and/or what particular clade of zooxanthellae a coral is carrying, why take the risk?
Some V. coralyticus YB facts:
- Growth Rate (doubling time in MBT media)
- 20°C - 140 min
- 25°C - 36 min
- 30°C - 25 min
- Antibiotic sensitivity: Sensitive to: erythromycin, tetracycline, chloramphenicol, gentamycin
- Resistant to: kanamycin, ampicillin, penicillin
It's informative to see that V. coralyticus is sensitive to chloramphenicol, an antibiotic first suggested by Craig Bingman as a treatment for RTN in aquaria. Unfortunately, chloramphenicol is not a very pleasant drug to deal with and is difficult to get, as you need a prescription from a veterinarian. Doxycycline is more readily available and has also been shown to also be effective in treating RTN.
For further information on coral diseases found in the wild take a look at:
- Drs. McCarty and Peters' website detailing what is known about coral diseases from the Caribbean, a very useful and informative site: http://ourworld.compuserve.com/homepages/mccarty_and_peters/coral/dis- list.htm
- Fitt, W.K., Brown, B.E., Warner, M. E. and R. P. Dunne. 2001. Coral bleaching: interpretation of thermal tolerance limits and thermal thresholds in tropical corals. A useful and informative review on what is known about coral bleaching, includes definitions and an extensive reference list. http://link.springer.de/search.htm
- Kuffner, I.B. 2002. Effects of ultraviolet radiation and water motion on the reef coral, Porites compressa Dana: a transplantation experiment. Journal of Experimental Marine Biology and Ecology 270:147-169.
As many hobbyists know, most corals, marine invertebrates and algae are exposed to significant amounts of biologically effective ultraviolet radiation (UVR, 280-400 nm) in nature. To help them cope with this they manufacture chemicals that act like a natural sunscreen. This class of chemicals known as mycosporine-like amino acids (MAAs) are contained in the corals' tissues where they absorbed UVR energy and release it as heat. What many hobbyists don't know is that these compounds are colorless and can only be measured by chemical extraction and liquid chromatography techniques. In other words, a coral's color and any changes therein, have nothing to do the presence or absence of MAAs. When looking at factors that control MAA production the most obvious one is the presence of UVR. Numerous studies in a wide range of marine organisms have shown that MAA concentrations are strongly correlated with UVR. However, few studies have looked at other factors that may act in conjunction with UVR such as water motion and overall light levels. It is well known that water motion plays a major role in zooxanthellae rates of photosynthesis, calcification rates and nutrient uptake, with these factors increasing with increasing water motion, presumably due to the reduction in the thickness of the boundary layer surrounding coral tissue, resulting in greater diffusion rates of nutrients and wastes into and out of the coral. Kuffner's study was conducted to determine if water motion would also have an effect on MAA production.
Kuffner used Porites compressa nubbins collected 14 nubbins from each of 9 colonies on the windward side of Coconut Island in Kaneohe Bay, Oahu, Hawaii. She transplanted half of these nubbins to the opposite (leeward) side of the island and placed some under UV blocking acrylic and some under UV transmitting acrylic. The rest were left on the windward side and some were placed under UV blocking acrylic and some under UV transmitting acrylic. Water flow on the windward side averaged 5 cm/s and on the leeward side less than 2 cm/s. She then measured MAA concentrations, calcification rates (by weighing the nubbins) and photosynthetic pigment concentrations initially and then at three week and six week intervals for a total of six weeks.
MAA concentration was found to behave much as expected; when UVR was present in the control site (windward) MAA levels remained about the same, but those under UVR blocking acrylic dropped by 29%. However, those nubbins on the leeward side dropped 20% with UVR present and 36% when UVR was blocked! Coral nubbins on the windward side showed no decrease in photosynthetic pigment concentration or calcification rate with or without UVR (this indicates that MAA production does not exact a metabolic cost on this coral as far as growth and photosynthetic pigment production are concerned), but those on the leeward side (lower water motion) showed declines of 22% and 11.6 % respectively, irrespective of UVR presence.
The reasons for these declines are not fully understood but it may be that the thicker boundary layer that results when water motion decreases, slows the uptake of the necessary chemical precursors for MAA production by the zooxanthellae.
Interestingly, differences were found in MAA concentration patterns between the various colonies. Some colonies had higher levels or different combinations of MAAs. This indicates that there is a genetic component to MAA production and may explain why some colonies exhibited bleaching in the summer of 1996 when water temperatures were elevated and doldrums caused unusually calm waters for several weeks in Kaneohe Bay. Since MAAs are produced by zooxanthellae, it may be that different strains/clades of zooxanthellae also produce different amounts and combinations of MAAs.
The implications of this study for aquarium culture of corals are interesting. The significant differences in MAA concentrations, calcification rates and photosynthetic pigment concentrations between high and low water motion sites suggest that corals may have specific flow requirements and could suffer stress (short term?) when moved or when water motion parameters are changed. What is not shown by this study is whether or not corals can eventually adapt to new water flow regimes either by altering their growth form and hence the diffusion boundary layer, or by altering their zooxanthellae make-up. When introducing newly collected or imported corals it would be useful to know how long those corals have been in transit or holding before arriving in systems with ideal conditions of water motion and lighting. Given that many of the newer and/or popular lamps on the market also give out measurable UVR and most hobbyists implement the ill-advised practice of not using shielding under their lights fixtures, chances are good that UVR shock would be a likely result.
Interesting Citations from the Periodical Literature
The following are citations for some of the articles that might also be of interest to aquarists, which were published in the spring and summer of 2002.
- Barneah, O., Malik, Z. and Y. Benayahu. 2002. Attachment to the substrate by soft coral fragments: desmocyte development, structure and functions. Invertebrate Biology 121(2):81-90.
- Battaglene, S.C., Seymour, J.E., Ramofafia, C. and I. Lane. 2002. Spawning induction of three tropical sea cucumbers, Holothuria scabra, H. fuscogilva and Actinopyga mauritana. Aquaculture 207(1-2):29-48.
- Buck, B.H. Rosenthal, H. and U. Saint-Paul. 2002. Effect of increased irradiance and thermal stress on the symbiosis of Symbiodinum microadriaticum and Tridacna gigas. Aquatic Living Resources 15(2):107-118.
- Grover, R., Maguer, J.F., Reynard-Vaganay, S. and C. Ferrier-Pages. 2002. Uptake of ammonium by the scleractinian coral Stylophora pistillata: effect of feeding, light and ammonium concentrations. Limnology and Oceanography 47(3):782-790.
- Jompa, J. and L.J. Mcook. 2002. Effects of competition and herbivory on interactions between hard corals and a brown alga. Journal of Experimental Marine Biology and Ecology 271:25-39.
- Smith, L.D., Rees, M. and S.N. Field. 2002. Enhancement of coral recruitment by in situ mass culture of coral larvae. Marine Ecology Progressive Series 230:113-118.
- Bythell, J.C., Barer, M.E., Cooney, R.P., Guest, J.R., O'Donnell, A.G., Pantis, O. and M.D.A. LeTissier. 2002. Histopathological methods for the investigation of microbial communities associated with diseases lesions in reef corals. Letters in Applied Microbiology 34(5):359-364.
- Diaz-Pulido, G and L.J. McCook. 2002. The fate of bleached corals: patterns and dynamics of algal recruitment. Marine Ecology Progressive Series 232:115-128.
- Dube, D., Kim, K., Alker, A.P. and C.D. Harvell. 2002. Size structure and geographic variation in chemical resistance of sea fan corals Gorgina ventalina to fungal infection. Marine Ecology Progressive Series 231:139-150.
- Perante, N.C., Pajaro, M.G., Meeuwig, J.J. and A.C.J. Vincent. 2002. Biology of a seahorse species, Hippocampus comes in the central Philippines. Journal of Fish Biology 60(4): 821-837.