Background
Citing Freiwald et al. (2004), Wild et al. (2008) and Roberts et al. (2009), the authors write that "Scleractinian cold-water corals (CWCs) are principally known from cold-water deep-sea environments," where some of them act as "ecosystem engineers and architects of marine biodiversity hot spots," and where many of them "are presumed to survive at the uppermost end of their temperature distribution range (4-13°C)," which thermal limits are largely congruent with the general distribution of CWC Dendrophyllia cornigera and D. dianthus throughout the Mediterranean, as described by Peres and Piccard (1964), Orejas et al. (2009) and Vertino et al. (2010). However, Naumann et al. say that the occurrence of a few isolated records in temperate shallow waters ranging from 13.5 to 14.3°C "suggest that both species, at least temporarily, tolerate elevated ambient seawater temperature," citing Castric-Fey (1996), Forsterra and Haussermann (2003), Palanques et al. (2006) and Freiwald et al. (2009).

What was done
Determined to explore this situation in more detail, Naumann et al. collected live specimens of D. cornigera and D. dianthus of similar skeletal dry weight south of Malta in the Mediterranean Sea with the help of a manned submersible. They then transferred them to two identically equipped and darkened 100-L flow-through aquaria through which Mediterranean subsurface seawater was continuously pumped from a 50-meter depth and supplied to the tanks at a rate of about one liter per minute. This water was maintained at a temperature of 12.5 ± 0.1°C for approximately 30 months prior to initiating the primary phase of their experiment, which was to increase the temperature of one of the aquariums by 0.5°C per day until a value of 17.5 ± 0.1°C was achieved and thereafter maintained for a further 87 days, during which time "daily visual assessments of coral health (i.e. tentacle protrusion, suspension feeding and mortality/survival) and monthly growth measurements by the buoyant weight technique (Davies, 1989) were conducted," the latter or which were ultimately translated into coral dry weight data."

What was learned
In the words of the three researchers, "over the entire experimental period, both CWC species showed neither differences in tentacle protrusion and suspension feeding nor mortality at ambient (12.5°C) or elevated (17.5°C) seawater temperatures." And they say that "D. cornigera specimens developed a non-quantified number of new polyps at both temperatures suggesting efficient thermal acclimatization." In addition, they report that "D. dianthus exhibited growth rates for ambient and elevated temperatures of 0.23 ± 0.08% per day and 0.19 ± 0.06% per day, whereas D. cornigera grew at 0.05 ± 0.01% per day under ambient and 0.14% ± 0.07% per day under elevated temperature conditions."

What it means
In the final sentence of their paper's abstract, Naumann et al. say their findings "suggest that D. dianthus and D. cornigera may be capable of surviving in warmer environments than previously reported, and thus challenge temperature as the paramount limiting environmental factor for the occurrence of some CWC species." And who knows but what other species may be able to beat the heat as well.

References
Castric-Fey, A. 1996. The scleractinian Dendrophyllia cornigera in shallow water, at Ushant (Brittany, NE Atlantic), related to the absence of a thermic barrier. Oceanologia Acta 19: 665-671.

Davies, P.S. 1989. Short-term growth measurements of corals using an accurate buoyant weighing technique. Marine Biology 101: 389-395.

Forsterra, G. and Haussermann, V. 2003. First report on large scleractinian (Cnidaria: anthozoa) accumulations in cold-temperate shallow water of south Chilean fjords. Zoologische Verhandelingen Leiden 345: 117-128.

Freiwald, A., Fossa, J.H., Grehan, A., Koslow, T. and Roberts, J.M. 2004. Cold-Water Coral Reefs. UNEP-WCMC, Cambridge, United Kingdom.

Freiwald, A., Beuck, L., Ruggeberg, A., Taviani, M. and Hebbeln, D. 2009. The white coral community in the central Mediterranean Sea revealed by ROV surveys. Oceanography 22: 58-74.

Orejas, C., Gori, A., Lo Iacono, C., Puig, P., Gili, J.M. and Dale, M.R.T. 2009. Cold-water corals in the Cap de Creus canyon, north-western Mediterranean: spatial distribution, density and anthropogenic impact. Marine Ecology Progress Series 397: 37-51.

Palanques, A., Durrieu de Madron, X., Puig, P., Fabres, J., Guillen, J., Calafat, A., Canals, M., Heussener, S. and Bonnin, J. 2006. Suspended sediment fluxes and transport processes in the Gulf of Lions submarine canyons. The role of storms and dense water cascading. Marine Geology 234: 43-61.

Peres, J.M. and Piccard, J. 1964. Nouveau manuel de bionomie benthique de la mer Mediterranee. Recuil des Travaux de la Station Marine d'Endoume 31: 1-137.

Roberts, J.M., Wheeler, A., Freiwald, A. and Cairns, S. 2009. Cold-Water Corals: The Biology and Geology of Deep-Sea Coral Habitats. Cambridge University Press, New York, New York, USA.

Vertino, A., Savini, A., Rosso, A., Di Geronimo, I., Mastrototaro, F., Sanfilippo, R., Gay, G. and Etiope, G. 2010. Benthic habitat characterization and distribution from two representative sites of the deep-water SML Coral Province (Mediterranean). Deep-Sea Research II 57: 380-396.

Wild, C., Mayr, C., Wehrmann, L.M., Schottner, S., Naumann, M., Hoffmann, F. and Rapp, H.T. 2008. Organic matter release by cold water corals and its implication for fauna-microbe interaction. Marine Ecology Progress Series 372: 67-75.