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Coral Sex

Author: Isabela Rios

Figure 1. Aerial view of coral reproductive material floating over the Great Barrier Reef, Australia. 

Do corals have sex? The answer is YES! Corals can sexually and asexually reproduce.

Sexual reproduction occurs in the form of broad spawning. All male and female corals throw their gametes in the ocean simultaneously with the hopes of them clashing and making coral babies.

Figure 2. Female , Fungia, releasing

eggs. Photographer: Peter Harrison

Figure 3. Male coral, Fungia, releasing

sperm cloud. Photographer: Peter Harrison

Asexual reproduction occurs through fragmentation. When a coral breaks into pieces, those fragments can grow into a genetically identical, mature coral.

Figure 4. Marina fragmenting coral,

A. cervicornis, for coral farming. Photographer: Fran Reina.

Figure 5. Coral fragments, A. cervicornis, growing

in tree nursery. Photographer: Fran Reina.

Coral farming typically uses asexual reproduction

Scientists commonly utilize asexual reproduction in coral restoration by cutting mature corals into multiple fragments. These fragments are later grown in a coral nursery until they have an appropriate size to grow on the reef. The process is repeated, ultimately creating denser reefs through coral framing.

Figure 6. Nursery grown corals, A. cervicornis, returned to the reef. At the base of the fragments the tissue has fully adhered to the rocks, indicating a healthy transition.Photographer: Fran Reina

Reef degradation inhibits sexual reproduction.

Sexual reproduction is required to increase genetic diversity. Natural spawning is beautiful but does not have a high success rate. Nowadays, corals are too spread out on the reef for the eggs and sperm to meet and fertilize due to reef degradation.

Figure 7. Coral, A. millepora, releasing about 130,000 eggs during a spawning event in Horniman Museum laboratory. 

To give reefs a fighting chance, scientists deciphered a way to do in vitro fertilization (IVF). The gametes are captured in large containers and combined in the lab to produce new, genetically distinct coral babies. Some restoration laboratories utilize sexual reproduction through IVF; however, it is less common due to its complex nature.

A Guide to Coral In-vitro Fertilization:

1. Decide WHEN the massive spawning event will occur:

Scientists do not know what causes broad spawning. Therefore, deciding WHEN the massive spawning event occurs is complicated. Spawning tends to vary by species and location; thus, many scientists around the globe study coral's sexual tendencies. Broad spawning only happens for about 15 minutes at night and has been linked to the lunar cycle, narrowing it down.

Figure 9. Coral, A. cervicornis, prior to spawning. The eggs are bright pink, indicating full maturity.

Photographer: Valerie Taylor

Figure 8. Cracked stem of coral, A. cervicornis. The eggs within are white, indicating spawning is distant.

Nova Southeastern University's (NSU) Coral Reef Restoration Assessment and Monitoring Lab (CRRAM) uses a strategy called cracking. This entails cracking the stem of coral, A. cervicornis, and looking inside at the coral's eggs. The coral eggs lie within the coral skeleton, and their color indicates how close the spawning event is. The eggs range from white to pink, and spawning is near when the eggs are bright pink.

2. Capture the gametes (eggs and sperm):

Figure 10. Collection net for gamete extraction of Elkorn coral, A. palmata. Photographer: Paul Selvaggio/SECOREInternational

During massive spawning events, scientists night dive with giant nets to capture as many gametes as possible. Since the gametes float, they will be caught by the net and held in the collection jars. The secure container is quickly taken to the lab for in-vitro fertilization.

Figure 11. Gametes floating into collection container

Photographer: Paul Selvaggio/SECOREInternational

3. Lab time: In-vitro Fertilization (IVF):

Figure 12. (A) SExual COral REproduction (SECORE) Laboratory breeding (B) Singular polyp of Oculina searching for substrate to adhere to to start a coral colony. Photographer: Amikam Shoob (C) Planula larvae of Acropora. Photographer: Peter Harrison. 

4. Let them GROW!

Figure 13. (A) Coral in early life stages, asexually reproducing into multiple polyps (B) AIMS marine scientist Dr Kate Quigley. Photographer: Justin Gilligan

Once the planula larvae have found their new home, they will grow in an aquarium. Growth speed ranges between species, but once the baby coral has multiple polyps and healthy size, it will be returned to the reef.

5. The last step: Coral Restoration!

Lastly, the new corals are returned to the reef. While natural reproduction would be ideal, anthropogenic degradation has made it nearly impossible for coral babies to be born. With international efforts scientists, and environmentalists are reviving the reefs.

Coralisma's efforts have mainly used asexual reproduction (fragmentation) for reef restoration. In the near future we hope to restore Playa del Carmen's reefs with novel methodologies such as IVF.

Figure 14. Dr. Claudia Padilla restoring INAPESCA's aquarium grown corals back onto the Playa del Carmen reefs. 
Moral of the story: Corals have sex!

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