Coral Reef Science:  Development Highlights

Habib Sekha

S. K. Davy, S. G. Burchett, A. L. Dale, P. Davies, J. E. Davy, C. Muncke, O. Hoegh-Guldberg and W. H. Wilsone. 2006. Viruses: agents of coral disease? Diseases of Aquatic Organisms Special Issue, Vol. 69, nr. 1.

Abstract:

The potential role of viruses in coral disease has only recently begun to receive attention. Here we describe our attempts to determine whether viruses are present in thermally stressed corals Pavona danai, Acropora formosa and Stylophora pistillata and zoanthids Zoanthus sp., and their zooxanthellae. Heat-shocked P. danai, A. formosa and Zoanthus sp. all produced numerous virus-like particles (VLPs) that were evident in the animal tissue, zooxanthellae and the surrounding seawater; VLPs were also seen around heat-shocked freshly isolated zooxanthellae (FIZ) from P. danai and S. pistillata. The most commonly seen VLPs were tail-less, hexagonal and about 40 to 50 nm in diameter, though a diverse range of other VLP morphotypes (e.g., rounded, rod-shaped, droplet-shaped, filamentous) were also present around corals. When VLPs around heat-shocked FIZ from S. pistillata were added to non-stressed FIZ from this coral, they resulted in cell lysis, suggesting that an infectious agent was present; however, analysis with transmission electron microscopy provided no clear evidence of viral infection. The release of diverse VLPs was again apparent when flow cytometry was used to enumerate release by heat-stressed A. formosa nubbins. Our data support the infection of reef corals by viruses, though we cannot yet determine the precise origin (i.e., coral, zooxanthellae and/or surface microbes) of the VLPs seen. Furthermore, genome sequence data are required to establish the presence of viruses unequivocally.

Comments:

Corals are showing a decline in health in certain parts of the world, which is readily visible to the naked eye. This decline may be due to changes in the water's physical or chemical parameters but can also be caused by an excessively high density of multicellular parasites, ciliates, bacteria and perhaps also viruses.

Until recently, viruses as disease-causing agents in corals did not get much attention. The above article's authors subjected corals and zoanthids, as well as their isolated zooxanthellae, to heat stress. Heat stress was found to induce virus-like particles either by the hosts or the zooxanthellae, or both (depending on the species tested). Transmission electron microscopy could not prove that these particles were viruses and genome sequence data would possibly be required to determine the exact nature of the virus-like particles.

Nevertheless, their data do, so far, support the idea that coral infection by viruses is possible, but they don't supply conclusive proof. It is striking to note that the virus-like particles from heat stressed A. formosa were able to induce cell lysis in freshly isolated, non-stressed, zooxanthellae from that coral. Perhaps a similar mechanism would have resulted in some of the "chain-reaction" disasters witnessed by some aquarists, regardless of the exact nature of these virus-like particles.

L.E. Wysocki, J.P. Dittami and F. Ladich. 2006. Ship noise and cortisol secretion in European freshwater fishes. Biological Conservation, Vol. 128, Issue 4, 501-508.

Abstract:

Underwater noise pollution is a growing problem in aquatic environments and as such may be a major source of stress for fish. In the present study, we addressed the effects of ship noise and continuous Gaussian noise on adrenal activity in three European freshwater species. Underwater ship noise recorded in the Danube River and two Austrian lakes was played back to fish at levels encountered in the field (153 dB re 1 µPa, 30 min). Post exposure cortisol secretion was compared with control situations. Cortisol was measured with enzyme immunoassay techniques (EIA, ng cortisol/l water/g fish) in extracted aquarium water with corrections for fish mass. In the first series, two hearing specialists, the common carp (Cyprinus carpio) and the gudgeon (Gobio gobio) and one hearing generalist, the European perch (Perca fluviatilis) were exposed to ship noise. The noise level was well above hearing thresholds in these species. In a second series, fish were exposed to continuous Gaussian noise at a similar level (156 dB) which is known to induce temporary hearing loss in hearing specialists. All three species responded with increased cortisol secretion when exposed to ship noise. Interestingly, no elevation was observed when fish were exposed to continuous Gaussian noise. Our results indicate that ship noise characterized by amplitude and frequency fluctuations, constitutes a potential stressor in contrast to continuous noise. Surprisingly, the data also demonstrate no apparent differences between species possessing excellent hearing abilities (hearing specialists) and species with poor hearing abilities like perch.

Comments:

Although the above abstract does not deal directly with a marine-related topic, it might still be interesting to marine aquarists. The authors found that fluctuating (in amplitude and frequency) sound typical of that produced by ships was able to stress freshwater fish, regardless of their hearing ability.

Stress was determined by measuring the secretion of cortisol, which allows the monitoring of stress (and its quantification) even before it is visually apparent, something most aquarists would not be able to perform. Sure, the sound was very loud (156 dB) and the fish were not marine species, but it raises questions about how far the fish we keep are stressed by the sound of aquarium equipment such as powerheads, various other pumps and skimmers.