Coral Reef Science:  Development Highlights

Eric Borneman

This month, I cover an article on the digestion of microalgae in the soft coral, Dendronephthya sp. and another on tentacle expansion and contraction in corals...

Widdig, Alexander, and Dietrich Schlichter. 2001. Phytoplankton: a significant trophic source for soft corals? Helgol Mar Res 55:198-211.

Abstract:

Histological autoradiographs and biochemical analyses show that ¹⁴C-labeled microalgae (diatoms, chlorophytes and dinoflagellates) are used by the soft coral Dendronephthya sp. Digestion of the algae took place at the point of exit of the pharynx into the coelenteron. Ingestion and assimilation of the labelled algae depended on incubation time, cell density, and to a lesser extent on species-specificity. ¹⁴C incorporation into polysaccharides, proteins, lipids and compounds of low molecular weight was analysed. The ¹⁴C-labelling patterns of the four classes of substances varied depending on incubation time and cell density. ¹⁴C incorporation was highest into lipids and proteins. Dissolved labelled algal metabolites, released during incubation into the medium, contributed between 4% and 25% to the total ¹⁴C activity incorporated. The incorporated microalgae contributed a maximum of 26% (average of the four species studied) to the daily organic carbon demand, as calculated from assimilation rates at natural eucaryotic phytoplankton densities and a 1h incubation period. The calculated contribution to the daily organic carbon demand decreased after prolonged incubation periods to about 5% after 3h and to 1-3% after 9h. Thus, the main energetic demand of Dendronephthya sp. has to be complemented by other components of the seston.

Highlights:

Nannochloropsis was offered at a natural cell density of 600-6,000 cells ml^-1. Exposed for one hour, this contributed a maximum of 34% of the daily carbon needs for this coral. But, if exposed for longer times (3 hours and 9 hours), the amount fell to 5% and 1-3% respectively. This seemed to occur because phytoplankton cells plugged the pharynx and prevented further ingestion and digestion. Even at 60,000 cells ml^-1, the total carbon supplied was only about 60%. The authors conclude that other sources of nutrition must supply the balance, and suggest that perhaps dissolved organic material and other micro-particulate organic matter may be the sources. For aquarists, perhaps pulsed periodic feeding of phytoplankton would maximize the potential of this food source in contributing to the energy demands of azooxanthellate soft corals like Dendronephthya species.

Levy, O., Z. Dubinsky, and Y. Achituv. 2003. Photobehavior of stony corals: response to light spectra and intensity. J Exp Biol 206: 4041-4049.

Abstract:

Tentacle expansion and contraction were investigated in four zooxanthellate coral species and one azooxanthellate coral (Cladopsammia gracilis). Favia favus, Plerogyra sinuosa and Cladopsammia gracilis expand their tentacles at night, while tentacles in Goniopora lobata and Stylophora pistillata are expanded continuously. Light at wavelengths in the range 400-520·nm was most effective in eliciting full tentacle contraction in F. favus and in P. sinuosa. Higher light intensities in the range 660-700·nm also caused tentacle contractions in F. favus. Tentacles in C. gracilis did not respond to light. Zooxanthellar densities in tentacles were significantly higher in G. lobata, which has continuously expanded tentacles, than in F. favus and P. sinousa, where tentacles are expanded at night. Photosynthetic efficiency in F. favus and P. sinuosa was lower in specimens with contracted tentacles. However, in the dark, no differences were found in the maximum quantum yield of photochemistry in PSII (Fv/Fm) of the expanded versus the contracted tentacles of any of the four species. This work suggests that species whose tentacles remain continuously expanded have either dense algal populations in their tentacles, as in G. lobata, or minute tentacles, like S. pistillata. Dense algal populations in tentacles allow harvesting of light while small tentacles do not scatter light or shade zooxanthellae in the underlying body of the polyp.

Highlights:

The expansion and contraction of different species of corals is often a behavior used to moderate the light environment to maximize photosynthetic performance of the zooxanthellae. Factors controlling tentacle expansion include flow speed, irradiance level, light spectrum, the presence of prey, zooxanthellae density, polyp size, and diurnal behavior. For aquarists, this means that the degree of polyp or tentacle expansion may not be a good indicator of coral health or "happiness," but rather may be a species- or environment-specific attribute.