By: ANN TARRANT
Over the past decade aquarists have made remarkable advances in rearing scleractinian (hard or stony) corals (Siegel 1997). This has been possible as a result of a better understanding of nutrient cycling and improvements in technology, such as the development of protein skimmers and improved filtration methods. At the Hawaii Institute of Marine Biology, where I conduct my research, we are very fortunate in being able to maintain corals in an open system. Essentially, the water chemistry is the same as in the natural environment, with little effort on the part of the scientist. These corals have been observed to spawn consistently, and the timing, as best we can determine, is identical to that in the field.
Dr. Bruce Carlson, the director of the Waikiki Aquarium, has been extremely successful at maintaining Indo-Pacific reef-building corals. The aquarium has recently been reported to grow and maintain 57 species, including several species of the genus Acropora (Atkinson et al. 1995). Only one of these species, Sandolitha robusta, was observed to spawn during several years of observation. These corals have grown rapidly, and dozens of fragments have been distributed to collectors, researchers and educators over the years. Why then are corals in aquarium environments so rarely observed to spawn or planulate? How are these environments different from the natural habitats of the corals, and what could be done to make them more similar? The answers to these questions will ultimately be of great use to biologists in understanding coral reproduction. Maintaining corals that reproduce sexually and raising the offspring to maturity may present the ultimate challenge to reef aquarists.
Coral reproduction is actually a common phenomenon in aquariums. Every time a branch or fragment is broken from the parent colony to grow independently, we can say that asexual reproduction has occurred. In some species, such as Pocillopora damicornis in the eastern Pacific Ocean, asexual reproduction appears to account for most of the colonies on the reef (Richmond and Hunter 1990). These corals are most prominent organisms on many reefs in Panama, where they form dense thickets â€” apparently by continually growing and fragmenting to form new colonies.
The distinction between growth and asexual reproduction becomes somewhat fuzzy in colonial organisms like corals. Some would argue that any time new polyps are added to a colony, a new organism has formed and reproduction has occurred. Others would limit the term reproduction to the formation of new colonies through processes, such as fragmentation, polyp bailout and partial mortality.
Regardless of how one defines the terms, an improved ability to grow corals in aquaria and to produce new colonies from fragments of the parent colony have already decreased the impact of aquarists and scientists on reefs. Over the years, aquarists have assumed increasing responsibility for maintaining reef environments. Many of the corals in aquariums around the world today originated as fragments from colonies grown in other aquariums. The Geothermal Aquaculture Research Foundation in Boise, Idaho, even offers reef farming courses and seminars to teach cultivation techniques to others.
Similarly, scientists have begun to take advantage of opportunities for rearing fragments. Bob Richmond, at the University of Guam, has raised hundreds of corals from fragments for use in toxicology bioassays and impact assessment studies. The colonies produced from a single parent colony are genetically identical and should respond in the same way to a given treatment, making results easier to interpret. By raising the corals in captivity, Dr. Richmond can minimize his own impact on the reefs he is trying to protect.
In order for a coral to propagate in this way, it needs to be able to meet its physiological requirements well enough to grow and calcify. But, does this mean that the coral is healthy? Perhaps this is a bad question, because even the experts cannot agree on what â€œhealthyâ€ means for a coral polyp, colony or community. A better question might be: What sort of factors limit coral reproduction and govern the trade-off between sexual and asexual reproduction?
Sexual Reproduction in Scleractinian Corals
Sexual reproduction in corals takes two major forms: brooding, and broadcast spawning. Some corals have been observed to develop eggs that are fertilized internally and released as planula larvae. Others broadcast their eggs and sperm into the water column, where they are fertilized and develop into larvae. Corals may be of separate sexes or hermaphroditic â€” containing both ovaries and testes in the same colony. Recent research indicates that the picture is even more complicated, as some species appear to have a variety of forms of sexual reproduction.
Corals from the Indo-Pacific kept at the Waikiki Aquarium have been shown to grow fast, produce lightly calcified skeletons, but not generally to reproduce sexually (Atkinson et al. 1996). Linear growth rates reported for Acropora elseyi and A. pulcra were 12.7 and 20.6 centimeters (5 and 8 inches) per year respectively. These growth rates are comparable to the highest measured rates in the field. It seems that the corals are devoting their resources to rapid tissue growth and linear extension, not to building a dense skeleton or gametes. It seems that they arenâ€™t reproducing because they are growing too well!
Does growth in corals occur at the expense of reproduction? There is some evidence to support this idea. Several studies have shown that corals recovering from tissue damage are significantly less fertile than undamaged colonies (Rinkevich 1996). Also, in some coral species it has been observed that polyps near the actively growing edges of a colony tend to be less fertile or sterile. Finally, there appears to be some minimum size and/or age for coral sexual reproduction â€” a coralâ€œpuberty.â€ (Incidentally, this might explain why some aquarium corals have not been observed to spawn.) Scientists do not entirely understand the regulation of coral growth and reproduction or what resources might be limiting to each. Some possibilities might be energy, nutrients or production of specialized cell types. Part of the answer might lie in the cues corals use for gamete (sperm and eggs, just like in other animals) development and spawning.
Environmental Cues for Sexual Reproduction in Corals
The main environmental cues for spawning and planulating appear to be photoperiod (day length) and nocturnal illumination (related to phases of the moon and time of night). Some of these controls on spawning were demonstrated in a series of experiments on two Hawaiian species of the genus Montipora (Hunter 1988). Changes in nocturnal illumination â€” created by maintaining corals in total darkness overnight (to simulate a constant new moon) or under constant nighttime illumination from lamps adjusted to full moon irradiances â€” showed dramatic changes in spawning synchrony in the second month of the experiment.
Many other factors are related to changes in light cues in nature. For example, temperature, food and nutrient availability, and the amount of light available to zooxanthellae for photosynthesis, are related to the time of year and photoperiod. The size of the tides is controlled by the phase of the moon and is correlated with nocturnal illumination. It is logical that all of these factors would be important to coral metabolic processes. We donâ€™t yet understand how all of these factors interact.
Exactly how corals respond to cues, such as light and temperature, depends on the species and region of the world. For example, most coral species (of those that have been studied) on the Great Barrier Reef spawn during the austral spring. The same species in other areas, such as the Red Sea or Central Pacific, tend to spawn in the summer (Richmond and Hunter 1990).
Current Research on the Controls of Coral Reproduction
Steve Kolinski, a graduate student at the University of Hawaii, is studying the tradeoffs between sexual and asexual reproduction in the coral Montipora verrucosa. His research will show how energy and nutrients are divided between tissue growth and gamete production, and how changes in the environment can cause changes in this allocation. Some of the factors he is considering include nutrient levels, turbidity, phytoplankton concentrations and light levels.
Selina Ward and other scientists in Australia have recently completed the ENCORE (Elevated Nutrients on Coral Reefs) project to investigate the effects of high nutrient levels on many different aspects of coral reef ecology and coral biology. Ward studied the effects of nutrients on several different aspects of coral reproduction, including gamete development, fertilization, larval development and settlement. She found that the interactions between corals and nutrients might be very complicated and can cause decreased fertilization rates and changes in fecundity and egg size (Ward, personal communication).
Another possibility, not necessarily exclusive, is that coral gamete development and spawning are under hormonal control. It is possible that corals respond to the environmental cues by producing chemical signals that lead to the development of eggs and sperm and to spawning. Researchers have reported that estrogen-like compounds are released during a multi-species mass-spawning event in Western Australia (Atkinson and Atkinson 1992). My own research involves measuring these estrogens or estrogen-like compounds in corals eggs and coral tissues at different times during the year. Through this work and other experiments I hope to gain insight into how these compounds might regulate coral reproduction.
So, What Does It All Mean?
In the past 10 years, great advances have been made in coral cultivation and the understanding of asexual reproduction. I predict and hope that the next 10 years will bring similar advances in the understanding of the control of coral sexual reproduction. In this way, it may be possible to produce corals with more desirable traits for aquaria and provide a steady source of specimens without depleting the natural environment. It may even be possible to use this technology to reseed damaged reefs.
There is still a great deal to be learned about coral reproduction, but scientific studies can lend some insight. If you are interested in observing spawning or planulation in aquaria, the following tips may prove helpful:
1) Know what to look for. In some cases, corals may be reproducing unobserved. Richmond and Hunter (1990) provide a good review of known spawning times.
2) Maintain a lighting environment as similar to the natural environment as possible, including moonlight and changes in day length. Delbeek (1995) recommends fluorescent or metal halide lights with sufficient blue light and moderate levels of red and yellow. Corals do seem to be sensitive to changes in day length, so it may be possible to stimulate coral reproduction by adding a timer to your light system. Moonlight has also been demonstrated to affect coral spawning and planula production. Full moon irradiance in Hawaii has been reported as 0.01ï¿½E m2 s-1 (Jokiel et al. 1985)*what is this unit of measurement – microEinsteins per square meter?*. Corals appear to be sensitive to even these extremely low light levels.
3) Keep fewer large (as opposed to a greater number of small) specimens to help to ensure that your colonies are mature. Also, separate different species of both hard and soft corals as much as possible to minimize competitive interactions, which could drain resources away from other uses, such as reproduction. References