Heterotrophic feeding on picoplankton and nanoplankton by two scleractinian corals from Okinawa: response under healthy versus bleached conditions and elevated seawater temperature
Beatriz E. Casareto
International Journal of Marine Science, 2019, Vol. 9, No.
Received: 01 Jan., 1970 Accepted: 01 Jan., 1970 Published: 31 Mar., 2020
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The objective was to understand the grazing strategies of the corals Montipora digitata and Porites lutea, which are dominant in Okinawan reefs and have been subjected to recurring bleaching events. The study focused on their grazing efficiencies on pico-nanoplankton that represent the most important organic matter sources for scleractinian corals. Coral nubbins were incubated 6 h at normal (27 ºC) and stressful (33 ºC) seawater temperatures and supplied with natural assemblages of pico-nanoplankton concentrated via tangential flow system. Bacteria (BA), picocyanobacteria (PCY), picoflagellates (PF), nanoflagellates (NF), and dissolved organic carbon (DOC) were monitored. At 27 °C, M. digitata consumed 71-87% more pico-nanoplankton cells and 81-87% more organic carbon than P. lutea. Under bleaching conditions and 33 °C, both coral species consumed similar cells numbers. In terms of carbon, M. digitata consumed mainly NF, which was its major carbon source. In contrast, P. lutea consumed PCY and PF, which provided relatively less carbon. Coral secrete mucus (mainly DOC) to trap plankton from the seawater. A negative DOC flux in bleached M. digitata at 33 °C showed decreased plankton capture by reducing mucus secretion, however P. lutea continued to release DOC. Therefore, mucus release in P. lutea could serve another purpose besides plankton trapping. M. digitata was highly dependent on heterotrophy and very efficient at food capture. Nevertheless, its plankton capture performance was substantially diminished by the combination of bleaching and heating. On the other hand, P. lutea was comparatively less dependent on heterotrophy and was able to catabolize stored materials.
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International Journal of Marine Science
• Volume 9