Fertilization

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Coral gametes are viable for several hours after spawning occurs. Thus, it is critical that the fertilization phase of larval propagation occurs as soon as is feasible, where gametes from different parents are combined. In addition to timing, concentration of the gametes is another important factor to consider. Sperm concentration must be high enough to ensure efficient egg/sperm encounters, but not too high that sperm cause anoxia. Gametes should be combined in approximate volumetric proportion from as many parents as feasible (at least 5 parent colonies). After most of the bundles have broken up, additional seawater can be added to adjust the sperm solution to the appearance of a weak lemonade (sperm visible but not milky).

Close-up of fertilized eggs of an elkhorn coral. Photo © Paul Selvaggio/SECORE International

Close-up of fertilized eggs of an elkhorn coral. Photo © Paul Selvaggio/SECORE International

Fertilization containers should be sized so that just a few layers of eggs are at the surface. Gentle and frequent agitation is often used to promote fertilization. After fertilization, early stage embryos are fragile and easily broken (these partial embryos can continue to develop normally but will result in ‘runts’, small sized larvae and settlers). At this stage, cleaning the water is important but care must be taken not to damage these fragile embryos.

Fertilization of coral gametes in the laboratory. The eggs are rinsed with fresh seawater until the water changes its color from milky to clear. Photo © Paul Selvaggio/SECORE International

Fertilization of coral gametes in the laboratory. The eggs are rinsed with fresh seawater until the water changes its color from milky to clear. Photo © Paul Selvaggio/SECORE International

Excess sperm should be rinsed from the fertilized eggs after a couple of hours. Fat-separating pitchers, which pour from the bottom are excellent tools in that they allow you to pour away the sperm solution from below the floating layer of eggs/embryos. New seawater can then be poured slowly back into the container through the spout to perform three or four rinses, until the water appears completely clear. Alternatively, a siphon tube can be used to suck the sperm solution from below the egg layer, and new rinse water added by pouring carefully along the wall of the container.
 

Critical Populations

In areas where parent colonies are rare or sparsely distributed, collecting and concentrating gametes for just the fertilization stage may yield substantial subsidy to the larval supply. Even if a full sexual propagation effort is not feasible (including larval culture, settlement, and outplanting), fertilized eggs can be released to disperse and settle naturally.

 

Even if proper procedures are followed, poor fertilization may still occur. Low fertilization rates will result in batches that need constant attention, because any unfertilized eggs (which at this point cannot be distinguished from fertilized eggs) will break down and reduce the water quality. These batches need more frequent water changes. Gametes that fertilize poorly may be produced by parent colonies that are stressed by heat or pollutants.

Fertilization is visible approximately four hours after fertilization using a dissecting microscope. Fertilized coral embryos will then be cultured for rearing larvae.

KEY CONSIDERATIONS

  • Combine gametes from at least 5 genetically-distinct parent colonies (not clones).
  • Combine gametes within 2 hours of spawning (immediately is best).
  • Target sperm concentration is when water mixture is a lemonade color.
  • Be gentle during the fertilization phase as early-stage embryos are fragile.
  • Fertilization may be impaired in stressed populations.

 

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This content was developed with SECORE International. For more information, contact info@secore.org or visit their website at secore.org.

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