Does Dark-Spot Syndrome Experimentally Transmit among Caribbean Corals?

Abstract

Over the last half-century, coral diseases have contributed to the rapid decline of coral populations throughout the Caribbean region. Some coral diseases appear to be potentially infectious, yet little is known about their modes of transmission. This study experimentally tested whether dark-spot syndrome on Siderastrea siderea was directly or indirectly transmissible to neighboring coral colonies. We also tested whether open wounds were necessary to facilitate disease transmission. At the completion of the experiments, we sampled bacterial communities on diseased, exposed, and healthy coral colonies to determine whether bacterial pathogens had transmitted to the susceptible colonies. We saw no evidence of either direct or waterborne transmission of dark-spot syndrome, and corals that received lesions by direct contact with diseased tissue, healed and showed no signs of infection. There were no significant differences among bacterial communities on healthy, exposed, and diseased colonies, although nine individual ribotypes were significantly higher in diseased corals compared with healthy and exposed corals, indicating a lack of transmission. Although our experiments do not fully refute the possibility that dark-spot syndrome is infectious and transmissible, our results suggest that in situ macroscopic signs of dark-spot syndrome are not always contagious.

Fig 1. Donor coral colonies.

Representative donor colonies (a and b) sampled for use in experiments testing the transmission of dark-spot syndrome on Siderastrea siderea. Inset images show disease lesions in detail.

Fig 2. Cascading experimental design.

The design had unidirectional water flow (arrows) from upper chambers (2.7 L each) to lower chambers (2.7 L each), which allowed the healthy corals in the lower chambers to be exposed to water from the upper-chambers housing diseased corals. All upper chambers were supplied with UV sterilized and mechanically filtered seawater, and all effluent water was treated with UV sterilization and bleach, before allowing the water to drain into a nearby canal.

Fig 3. Thermal history of the corals tested in a series of laboratory-based transmission experiments.

a. Mean monthly sea-surface temperature (dotted line) and the minimum and maximum of the 10-year monthly mean (gray range) obtained from HadI sea-surface temperature records for the site of collection [44]. b. Mean daily sea-surface temperature from a Hobo® data logger deployed near the site of collection (data logged every 15 minutes) and the standard deviation around the mean (gray range). c. Mean daily experimental temperatures measured with a standard laboratory thermometer for the duration of the transmission experiments. Gray range = standard deviation around the mean; W = waterborne transmission, D = direct-contact transmission.

Fig 4. Time-series images of two healthy-coral colonies (~16 cm²) of Siderastrea siderea in a direct-contact transmission experiment.

The first column of images is a close-up of the fragments (~1 cm²) that were placed in direct contact with the coral colonies. The first row of images show a healthy-coral colony (~16 cm²) in direct contact with a healthy fragment (~1 cm²) (control). The second row of images show a healthy-coral colony in direct contact with a fragment with dark spot lesions (~1 cm²) (experimental). Note that Day 1 images were taken prior to the addition of the direct-contact coral fragment. Note also the direct-contact lesion scars on S. siderea on Day 5.

Fig 5. Time-series images of Siderastrea siderea from waterborne-transmission experiments.

The top row represents diseased or healthy-coral colonies (~16 cm²), depending on the treatment, from the upper chambers. The bottom row represents healthy-coral colonies (~16 cm²) from the lower chambers.

Fig 6. Photochemical yield on corals tested in a waterborne disease-transmission experiment.

Mean photochemical yield of 3-hour dark-adapted colonies of Siderastrea siderea tested in a dark-spot transmission experiment on two days. Measurements of diseased-coral colonies were taken directly on tissue with dark spots. Where n = 7, and error bars denote standard deviation.

Fig 7. Relative abundance of bacteria in coral tissue.

Mean bacterial community structure on healthy, exposed, and dark-spot tissue of Siderastrea siderea, including the 10 most abundant taxonomic classes from each experimental condition (totaling 12 classes). None of the most abundant classes were found exclusively in any one treatment.