Coral Reproduction: A Range of Strategies to Ensure Long-Term Survival
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Most organisms reproduce either sexually or asexually. However, some organisms employ numerous reproductive strategies. Among these organisms are corals, tiny creatures that are the foundation of the ocean’s coral reefs. Reefs are made up of many individuals called polyps. Large collections of polyps form what is known as a colony.
Sexual Reproduction: Broadcast Spawning
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Roughly 75 percent of all stony coral species reproduce sexually through broadcast spawning. This method allows new colonies to be established that are genetically different from either parent colony. Male and female corals cannot move out of place to make reproductive contact as can fish, crabs, and other marine animals. Broadcast spawning allows them to overcome this hurdle to sexual reproduction.
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Broadcast spawning events can occur one or several times each year. During broadcast spawning, corals found in many reefs and colonies release huge numbers of female and male gametes (eggs and sperm) into the water at precisely the same time, usually at sunset. Once the gametes are released, they float toward the surface of the water. If a male and a female gamete join together, a larva is produced called a planula. If a planula survives to settle and mature into a single coral polyp, it can start a new colony. Each polyp is genetically different from both its parents. Fifty percent of their genes come from one parent, while fifty percent of their genes come from the other parent.
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Broadcast spawning has both disadvantages and advantages. Perhaps the biggest disadvantage is that the chances that an individual egg and sperm will eventually form an entirely new coral colony are very low. Many gametes never fuse together to form a planula. Ocean predators consume many of the larvae before they find a suitable place to settle, attach to a solid surface, and mature. Finally, there is no guarantee that a planula will find a suitable place to establish a new colony.
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Producing gametes requires energy. This helps explain why broadcast spawning occurs in colonies that are well established. Such colonies can devote the energy required for broadcast spawning, which also has important advantages. The successful formation of a new colony that is meters or even kilometers away from the parent colonies enhances the genetic diversity of coral reefs and helps increase their numbers. The fact that corals in different colonies have different genes also increases the overall chances that corals as a species survive. One colony may not have a genetic makeup that allows them to adapt to an environmental change. However, another colony with different genes might.
Asexual Reproduction: Budding
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As a result of broadcast spawning, there may be a solitary polyp attached to a hard surface. Coral reef colonies, however, consist of many polyps. Therefore, this single polyp must find a way to increase its numbers. A form of asexual reproduction known as budding is used to establish new colonies and increase the size of existing ones. Once a polyp reaches a certain size, it simply divides into two, producing a pair of identical polyps. This process is repeated many times. All of the polyps produced are exact genetic copies of the original polyp. Regardless of how many polyps eventually make up a colony, they all have the exact same genes. In other words, 100% of the genetic information is passed from each parent polyp to its offspring.
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Budding has several advantages that make it an ideal strategy for increasing polyp numbers and establishing a new colony. The process is efficient, since successful reproduction does not require both male and female gametes. None of the energy used during reproduction is wasted; all of it is used to create additional colony members. Members of a newly formed colony may eventually reproduce sexually through either broadcast spawning or another method called brooding.

Other Reproductive Strategies
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Stony corals that do not broadcast spawn reproduce sexually by brooding. In brooding, only male gametes are released into the water. If a male gamete encounters a female coral polyp with egg cells, the male gamete is taken in, and it fertilizes the egg inside the female coral. Later, an almost-fully mature planula is released into the water.
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Fragmentation, like budding, is another way that corals can reproduce asexually. A portion of a coral colony may break off during a storm or boat-grounding. The individual corals that have broken off can start new colonies that are genetically identical to the parent colony.
For coral species, the use of each different reproductive strategy has its own advantages. Based on the text and what you know about reproduction, drag each advantage into the most appropriate category.
Advantages of Broadcast Spawning
Advantages of Budding
increases the number of polyps in a colony
helps eliminate harmful mutations from the coral gene pool
increased chance for survival of coral species
efficient use of reproductive energy
coral able to reproduce without producing gametes at the same time as other coral
genetic variation in offspring
1 answer
Advantages of Broadcast Spawning
Advantages of Budding
