...for Marine Aquarium Cultured Fishes and Invertebrates
This article appeared in Volume 1, Number 3, Summer 1993 of The Breeder's Registry newsletter. The article is reproduced with the acknowledgment and permission of the author. All rights reserved, 1993.
Modern captive coral reef aquaria can support long term healthy maintenance of many tropical reef corals. Asexual reproduction has occurred for many corals in modern reef aquariums. Captive propagation of corals is proceeding slowly. Asexual reproductive methods include fragmentation, budding, fission, and colonial proliferation. Corals currently being asexually propagated consist of numerous soft species (Orders Actiniana, Corallimorpharia, Zoanthiniaria and the entire Subclass of Octocorallia) and a few stony corals (Order Scleractinia). When captive ecosystems include a simulated natural environmental variables, some stoney and soft corals sexually reproduce. Examples of sexual reproduction are:
brooded planulae,
broadcast egg and sperm release,
egg and sperm bundles,
planulae bundle releases.
This article contains a review of coral reef ecosystem parameter research and describes successful captive spawning induction techniques. A literature cited listing provides references to additional information. A glossary of scientific terms is also included.
On a natural marine reef, variables may affect the spawning behavior of corals. Juvenile scleractinian coral recruitment on the Great Barrier Reef have seasonal peaks which are greatest during the local spring-summer (Oct to Feb) season. The percentage of coral recruitments occurring there in spring-summer can be 89%, in summer-winter (Feb to Jan) 8% and in winter-spring (Jun to Oct) 3% (Wallace, 1985).
The periodicity in the breeding patterns for marine species can be analyzed as three distinct rhythms. The annual rhythm determines the length of the breeding season. Temperature is probably the most important seasonal inducer. Breeding seasons can extend all year or be as short as a couple of days per year.
A monthly rhythm can exist. This could be caused by tides, moon luminance, or biological factors. Recent studies have shown that moon luminance may be the most dominate monthly rhythmic trigger.
The daily rhythm establishes when marine species actually release gametes. Certain repeatable patterns suggest that each species has an optimum daily time for releasing (Korringa, 1947).
The simulation of annual sea temperature variations, annual diel light cycles, and monthly lunar or tidal cycles might induce spawning in a captive tropical reef ecosystem. Sea water temperature patterns may influence gametogenic cycles. Lunar brightness might induce spawning (Babcock et al., 1986).
Some parameter variances, simulating natural rhythms, have been used to induce corals to sexually spawn in captivity. Within this article, these parameters are all described only with reference to their value in inducing captive spawning. Information detailing the routine healthy maintenance of reef species can be found in more species specific articles. Always use caution when changing any parameter in your captive reef and use the suggested guidelines or establish safer variances by closely monitoring the condition of your specimens. The exact duplication of natural parameters might have negative implications towards the healthy maintenance of captive reef species. The longer photoperiod and higher temperatures which occur during the summer months can put severe stress on corals and possibly cause bleaching (Yap et al., 1992). Winter's seasonally low temperatures and shorter photoperiods can depress coral growth (Kojis, 1986). Reef flat tidal variations can cause incredible daily extremes in temperature, luminance and salinity.
Sunlight diel time represents the total time that the ecosystems main lighting is at full intensity. The period or "cycle" time for daylight reef lighting should vary in an annual natural rhythm corresponding to a natural reef's seasonal daylight cycle. This natural daylight cycle is relative to a tropical reef's latitudinal location. For example, on the eastern side of Cangaluyan Island, Pangasinian, Philippines (16 deg 22 min N) (120 deg 00 min E) the highest recorded value of mean day length in 1983 was 12.97 hours and the lowest was 11.22 hours (Yap et al., 1992). The suggested captive system photoperiod range is 10 hours simulated day length during winter and up to 13 hours during summer. This day length is separate from twilight and moonlight periods which are discussed later in this article. On the Great Barrier Reef, when the photoperiod increases to 11.2 to 11.4 hours, oocytes (immature eggs) increase in size and begin proliferating in Acropora palifera (Kojis, 1 986b). Captive coral spawns have been reported to occur with photoperiods of 11 to 12 hours. The maximum suggested rate of photoperiod change is 1 hour per month with 30 minute diel length shifts.
Sunlight luminance is normally simulated using metal halide bulbs or VHO florescent tubes with color temperature values greater than 5000° K. The heat generated from metal halide bulbs needs to be considered.
Most annually spawning corals develop and release gametes during the late spring or early summer season. One possible seasonal factor might be the energy requirements for planula production. Corals aquire energy from products of photosynthesis (Rinkevich, 1989). The longer summer diel times probably cause an increase in the creation of algal photosynthesis nutrient products (Tomascik and Sander, 1987). Nutrients appear to be consumed more rapidly during planulae production. Perhaps longer diel times stimulate gamete production and induce planulae brooding corals to spawn more regularly.
Suspended particulate matter can limit the amount of surface illumination which penetrates the water. This lower light intensity has been shown to affect coral growth rates (Tomascik and Sander, 1985). Captive coral reef ecosystems require special design, regular maintenance, and adequate filtration to limit suspended particulate matter and to prevent microalgae overgrowths. Micro and macro algaes can overgrow a coral colony and restrict the illumination the coral receives.
The captive reef's support and maintenance systems must be operating superbly if diel times of 12 to 13 hours are to be achieved without hair algae growths. An abundant growth of coralline algaes helps prevent microalgae blooms.
Each coral requires a specific light intensity and wave length for duplication of its natural environment. The main difficulty with duplication attempts is that natural ecosystem lighting is not known by the reef breeder. Soft and hard corals can modify their light gathering abilities by morphological changes in growth, algal symbionts population changes and daily physical form manipulation. This ability has probably evolved due to the dynamic nature of the coral reef environment and can be used by corals in captivity if the coral is properly acclimated.
Twilight period is equivalent to the time in which the sun is setting or rising on the horizon. A twilight, or low light intensity, occurs for a short duration. Many corals and fishes use the night twilight as a trigger for spawning activity. The majority of spawn releases in captive reef aquaria occur during the last few minutes of night twilight and for a few minutes afterward. On natural reefs, scientists use hours from sunset as a standard spawn time measure; this should equate with hours from start of twilight for a captive ecosystem. During mass synchronous spawnings of scleractinian corals on the Great Barrier Reef, most releases of gametes begin after sunset. Species spawn during specific times which range from 10 minutes after sunset to 4 hours after sunset (Babcock et al., 1986). The onset of darkness might induce spawning after other seasonal and monthly triggers have occurred.
Captive reef ecosystems will operate more naturally if twilight periods are simulated. The suggested twilight duration is 30 minutes to 2 hours. The author's reef contains corals and fish which spawn regularly. The fish benefit from extending night twilight. The extended twilight permits the fish to spend more time in the prespawning ritual dance. The extended twilight might provide extra coral bundle setting time.
The morning twilight period is 30-60 minutes and the evening twilight period is 60-120 minutes. Twilight can be simulated with actinic florescents or a dimmed daylight bulb. The twilight allows observation of gamete releases. Spawn releases which occur after the twilight period can be observed if the simulated moonlight has enough luminance, otherwise occasional searches with a flashlight have to be performed.
The reflected light from the moon completes one full cycle in approximately 29.5 days long. The full moon is 180 degrees out of phase with the sun; when the sun sets the moon rises with maximum luminance. The new moon is in phase with the sun and rises with the sun. At first and last quarter the moon is 90 degrees out of phase with the sun and remains in the sky for half the night.
Since corals usually receive moonlight and starlight at night an ambient luminance should exist. A suggested full moon simulation would be equivalent to twilight or 1/2 to 1/4 the luminance of twilight. Moon light can be reproduced with 15-25 watt incandescent bulbs positioned over the top of the reef. Moonlight on a natural reef is a very small fraction of maximum daylight luminance. Night irradiance measurements show full moon photosynthetic photon-flux density was 0.01 µE/m², (microvolts per square meter), while midday readings showed 2,000 µE/m² (Jokiel et al., 1985).
The reproductive traits for 210 of the 600 known species of scleractinian corals were detailed. Most species were hermaphroditic broadcast spawners (131), while some were hermaphroditic brooders (11) and gonochoristic brooders (7) and a few gonochoristic broadcasters. Broadcast spawners outnumbered brooders in the Pacific regions and the Red Sea. Brooding appears to be the dominate form of reproduction in the Caribbean.
Over 80% of spawns occurred following the full moon while less than 15% followed the new moon phase (Richmond and Hunter, 1990). Nocturnal illumination might be the fine tuning or forcing function for determining timing of spawning.
An accelerated lunar cycle experiment was conducted on the author's reef. The full moon periodicity was shortened to a 15 day cycle instead of the normal 29.5 days. The experiment attempted to produce a synchronous mass spawn within a 3 month test but was not successful. While this experiment was running an annual broadcast egg spawner released eggs and 2 brooders released planulae bundles. The egg broadcaster was a branching Euphylli ancora coral that released eggs after an artificial new moon on July 8, 1992. Planulae bundles of green and brown color were also released during this experiment.
1992: June 6 new moon June 9 green planulae bundle released June 11 brown planulae bundle released June 14 full moon June 21 new moon June 29 full moon July 6 new moon July 8 approximately 100 eggs broadcast July 9 approximately 200 eggs broadcast July 9 green planulae bundle released July 10 approximately 1500 eggs broadcast July 11 approximately 50 eggs broadcast July 11 brown planulae bundle released July 14 full moon July 21 new moon July 21 approximately 250 eggs broadcast July 22 approximately 1000 eggs broadcast July 23 approximately 250 eggs broadcast July 24 approximately 200 eggs broadcast July 25 constant full moon interruption till August 4. August 5 new moon August 9 approximately 100 eggs broadcast.
The recordings illustrate that the monthly planulae brooders were probably not able to produce mature planulae in half normal time. The egg broadcaster released eggs after the new moon on July 6 and fifteen days later after the July 21 new moon. This demonstrates that once the eggs are mature the spawning release will sync into the lunar phase even if this phase is twice as fast. This experiment shows the importance of using a rhythmic moon luminance in spawning captive corals. Since most spawns occur following the full moon and new moon, it appears that the actual triggers may be the lowering of luminance intensity from full and the increasing luminance from new. The author's reef system is currently synced into a natural 29.5 day lunar rhythm.
GLOSSARY OF TERMS
- brooding
- developing eggs within the body cavity or on external surface.
- diel
- the number of daylight hours best suited to the growth and maturation of an organism.
- embryogenic
- the formation and development of the embryo.
- fecundity
- fertile, productive, prolific.
- gametes
- a reproductive cell that is haploid and can unite with another gamete to form the cell that develops into a new organism.
- gametogenic
- process of consecutive cell divisions and differentiation by which mature eggs or sperm are developed.
- gonads
- an organ in animals that produces reproductive cells; esp., an ovary or testis.
- gonochoric
- separate sexes, male reproductive organs in one individual and the female organs in another.
- haploid
- an organism or cell having only one complete set of chromosomes ordinarily half the normal diploid number.
- hermaphroditic
- animal with sexual organs of both male (testes) and female (ovaries).
- hermatypic
- see hermaphroditic.
- hydrodynamic
- having to do with the motion and action of water and other liquids; dynamics of liquids.
- morphological
- form and structure, as of an organism, regarded as whole.
- oocytes
- an egg that has not yet undergone maturation.
- oogenesis
- the process by which the ovum is formed in preparation for its development
- parameter
- a variable.
- photoperiod
- the number of daylight hours best suited to the growth and maturation of an organism.
- planulae
- the ciliate, free swimming larva of a coelenterate.
- propagules
- a structure that propogates an organism.
The full moon is 180 degrees out of phase with the sun; when the sun sets the moon rises with maximum luminescence. The new moon is in phase with the sun and rises with the sun with its illuminated side away from the earth.
REFERNCES CITED (IN ORDER OF CITATION):
Wallace C. C. (1985): Seasonal peaks and annual fluctuations in recruitment of juvenile scleractinian corals. Mar. Ecol. Prog. Ser. 21:289-298
Korringa P. (1947): Relations between the moon and periodicity in the breeding of marine animals. Ecol. Mongr. 17:345-381
Babcock R.C., G.D. Bull, P.L. Harrison, A.J. Heyward, J. K. Oliver, C.C. Wallace and B.L. Willis (1986): Synchronous spawnings of 105 Scleractinian coral species on the Great Barrier Reef. Marine Biology 90:379-394
Yap H. T., P. M. Alino and E. D. Gomez (1992): Trends in growth and mortality of three coral species (Anthozoa: Scleractinia), including effects of transplantation. Mar. Ecol. Prog. Ser. 83:91
Kojis B.L. (1986b): Sexual reproduction in Acropora(Isopora) (Coelenterata: Scleractinia) II. Latitudinal variation in A. palifera from the Great Barrier Reef and Papua New Guinea. Marine Biology 91:311-318
Rinkevich B. (1989): The contribution of photosynthetic products to coral reproduction. Marine Biology 101:259-263
Tomascik T. and F. Sander (1987): Effects of eutrophication on reefbuilding corals. III. Reproduction of the reef building coral Porites porites Marine Biology 94:77-94
Tomascik T. and F. Sander (1985): Effects of eutrophication on reefbuilding corals. I. Growth rate of the reef building coral Montastrea annularis. Marine Biology 87:143-155
Jokiel P. L., R. Y. Ito and P.M. Liu (1985): Night irradiance and synchronization of lunar release of planula larvae, in the reef coral Pocillopora damicornis. Marine Biology 88:167-174
Richmond R. H. and C. L. Hunter (1990): Review - Reproduction and recruitment of corals: comparisons among the Caribbean, the Tropical Pacific, and the Red Sea. Mar. Ecol. Prog. Ser. 60:185-203
EQUATIONS:
°F to °C: C = (5/9) * (F - 32)
°C to °F: F = (C * (9/5)) + 32
Liters to US Gallons: G = L/3.8
US Gallons to Liters: L = G*3.8
mg/liter of water:
ml/liter of water =~ PPM
Liter to ml: 1 ml = L/1000
ml to µliter: 1 µliter = 1000 ml
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