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