Coral Reef Science:  Development Highlights

Eric Borneman

This month, I cover an article on the effect of feeding corals...

Ferrier-Pagès , C., J. Witting, E. Tambutté and K. P. Sebens. 2003. Effect of natural zooplankton feeding on the tissue and skeletal growth of the scleractinian coral Stylophora pistillata Coral Reefs 22: 229-240.

Abstract:

Laboratory experiments were designed to estimate the ingestion rates of the scleractinian coral Stylophora pistillata under varying prey concentrations and feeding regimes and to assess the effect of feeding on the tissue and skeletal growth. Six sets of corals were incubated under two light (80 and 300 µmol photons per square meter per second) and three feeding levels (none, fed twice, and fed six times per week) using freshly collected zooplankton. Results showed that the number of prey ingested was proportional to prey density, and no saturation of feeding capability was reached. Capture rates varied between 0.5 and 8 prey items per 200 polyps per hour. Corals starved for several days ingested more plankton than did fed corals. Fed colonies exhibited significantly higher levels of protein, chlorophyll a, and chlorophyll c2 per unit surface area than starved colonies. Feeding had a strong effect on tissue growth, increasing it by two to eight times. Calcification rates were also 30% higher in fed than in starved corals. Even moderate levels of feeding enhanced both tissue and skeletal growth, although the processes involved in this enhancement remain to be determined.

Comments:

For many years, I have been trying to emphasize the importance of feeding corals and many of you may have heard my presentations on the subject. Yet, I am still amazed at the number of aquarists (and scientists) who seem to think that corals are mostly living by light alone. Mostly, this is because of the rationale of aquarist efforts. If one doesn't purposely and directly feed corals, it is assumed they are not feeding. However, much of coral prey is not visible, or barely so. Among those aware of this fact, the comment frequently becomes, "there must be enough in the water for them, because they are doing great and growing." Yet, when I have looked at water samples from aquaria under a microscope, I am awed by the consistent lack of living organisms in tank water by comparison with a same volume of sample water taken from a reef or coastal area. Even very transparent oligotrophic reefs will teem with life in a 100ml sample, while 100ml samples of reef tanks take some amount of manipulation under a microscope to find something moving. This implies that either there is very little being produced, or a whole lot being eaten as it is produced. In effect, the result is the same…a barren water column for which the results have been speculated. Here is a paper that examines the potential of such a depauperate water column.

While there are many papers that have covered the subject of heterotrophic acquisition of nutrients in corals (soft, stony, and others), few recent studies have directly examined the effects of feeding on calcification and tissue growth.

To expand on the abstract, this work found that starved coral ingested more than regularly fed corals, and both ingested more than intermittently fed corals. But, fed corals grew faster. So, on a practical note, regular daily feedings will likely result in the fastest growth and the best compromise of prey items captured. In this study, flow rates varied between 30 and 60 mm/second, The prey items captured somewhat reflected the relative seasonal abundance, but some prey items were rarely found in gut contents even when available. Fed corals contained 30-49% more protein than starved corals in both high and low light treatments, but the protein increase was not significant between corals fed two or six times per week. A strong effect of feeding and light occurred for tissue growth rates. Fed corals also contained 35-60% more chlorophyll per square centimeter than starved corals. The number of zooxanthellae in tissues was also higher in fed corals. Skeletal growth rates in fed corals experienced 50-73% more growth under both light treatments in January, and 46% more growth under low light in July (no difference between fed and starved under high light in July in skeletal growth, although tissue growth was still limited). In the conclusion, the author's state, "These results indicate that, even under optimal light conditions, photoautotrophy cannot satisfy the maximal daily energy and nutrient requirements of both maintenance and growth….It is also clear that calcification can be enhanced by feeding under a variety of ambient conditions."

Ronald L. Shimek, Ph. D.

This month, I will discuss an interesting article on toxicity of coral reef décor...

Ho, K. T, A. Kuhn, R. M. Burgess, M. Pelletier, D. G. McGovern, J. Charles, and L. Patton. 2003. Use of marine toxicity identification and evaluation methods in determining causes of toxicity to fish in a marine aquarium facility. North American Journal of Aquaculture 65, 14-20.

Abstract:

We obtained a water sample containing broken pieces of a tropical coral reef décor that was suspected of causing fish toxicity in a major aquarium. A toxicity identification and evaluation (TIE) was performed using three species: a mysid shrimp, Americamysis bahia; inland silverside Menidia beryllina; and an amphipod, Ampelisca abdita. Initial tests indicated that only the shrimp was sensitive to the unknown toxicant. The first phase of the TIE indicated that the toxicity to the shrimp could be eliminated by either the addition of EDTA or manipulation of the cation exchange column. Elevated concentrations of cadmium were detected by inductively coupled plasma analysis, and metal toxicity was confirmed when the cation exchange column treatment successfully isolated the toxic metal. Analysis of affected fish tissue indicated cadmium levels ranging from less than 0.3 ng/g (ppb) in the muscle to 200 µg/g (ppm) in the liver. This study demonstrates a unique application of TIE methods to diagnose toxicity problems in aquaria and other aquaculture situations.

Comments:

Interestingly, while the fish in the aquarium were being killed by the cadmium leached out of the reef décor, the "test" fish were unaffected. The main question is, of course, where the cadmium in the "tropical reef décor" came from initially. It either originated when 1) the "reef décor" was alive, as live coral or organisms on a reef rock incorporating materials into themselves, or 2) subsequent to collection by contamination of the "reef décor" either in the aquarium in question or prior to its use in the aquarium. Unfortunately, the mode of contamination was unclear. However, what is clear is that heavy metal contamination leaching from materials is a real problem for which aquarists must contend.