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If you are familiar with corals, you may be shocked by these bright colors. These corals are stressed by warm water and increased UV exposure. They have expelled their symbiotic algae, responsible for their normal green and brown colors, and soon these coral will be white, indicating they are close to death. The bright colors come from fluorescent pigments, used as sunscreen now that the algae are no longer shading the coral. See photos and videos of a bleaching event here.
Credit: Derek Manzello
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Reproduction is the process of creating offspring. Organisms must reproduce in order for their species to survive. How do corals reproduce? Remember that corals are sessile so they have to be creative when it comes to reproduction. In this unit, we will learn about different strategies that coral use to reproduce.
Corals reproduce by one of two methods:
Let’s begin with sexual reproduction, the production of a new organism from two others of the opposite sex. This requires the production of sperm and eggs, which are often referred to as gametes. Gametes are mature sexual reproductive cells. The majority of coral species are hermaphroditic, meaning that they produce both sperm and eggs. The rest consist of separate sexes (male or female) meaning that they produce either eggs or sperm. When a sperm and egg combine to form a new organism it is called fertilization.
Humans can do it, and now corals can too – freeze their eggs, sperm, and embryos for later usage. In 2010, scientists set up the first ever frozen bank for corals. Using techniques learned from human fertility, scientists have successfully figured out a way to preserve coral’s reproductive cells for many years to come. Currently, corals are under threat and their future is uncertain, probably dire. These scientists are planning ahead by cryopreserving different coral species just in case they need restore reefs in the future.
Check out the video about Dr. Mary Hagedorn’s work: https://www.youtube.com/watch?v=VUZMKLo-VoE
The majority of corals are considered hermaphroditic broadcast spawners, a type of external fertilization. Broadcast spawners, sometimes referred to as mass spawners or synchronous spawners, release both sperm and eggs into the water at the same time (figure 5-1). Some corals release buoyant egg and sperm bundles. These do not self fertilize, but instead, they float to the water’s surface where they break apart, releasing gametes, which then combine with those from other corals, completing the process of fertilization. Other corals fertilize the eggs internally in the gastrovascular cavity (see Unit 3: Coral Anatomy), and allow them to grow into planulae (see Unit 6: Life Cycle). These corals are called brooders.
Both of these are different reproductive strategies that have various outcomes. Can you think of the benefits and disadvantages of each of these strategies?
Figure 5-1. Mass spawning event where the round objects are eggs and the white clouds are sperm.
|Benefit to Spawners||Benefit to Brooders|
|More sperm and eggs released; safety in numbers||Fewer but better developed planulae|
|Carried in currents over greater distances; greater genetic diversity||Can settle immediately; less chance of getting eaten|
|Requires less energy||Can release planulae at any time because it is already in its planktonic stage|
|Disadvantage to Spawners||Disadvantage to Brooders|
|Longer distance = less chance of survival||Less genetic diversity|
|Have to get the synchronization correct for gametes to be able to reproduce||Requires more energy|
Some corals can actually use a combination of brooding and spawning in order to benefit from each reproductive strategy and ensure the survival of their species.
A mass spawning occurs when many different coral species synchronize the release of their eggs and sperm. Mass spawnings have been observed on coral reefs throughout the world. Usually the event occurs once a year; however, two spawnings have been documented. Additionally, in some areas, minor spawning events occur throughout the year. Generally, spawning events take place after a full moon during certain times of the year, dependent on location.
Here are a few examples:
Other non-coral invertebrates have been observed releasing their gametes during the mass spawning as well, including sea stars, sea urchins, sea cucumbers, sponges, marine worms, and molluscs. Again, this reproductive behavior is beneficial by allowing a large amount of gametes to be present; therefore, the likelihood of an individual being eaten is decreased. This is sometimes referred to as safety in numbers. Even though there is a feeding frenzy by fish and invertebrates (figure 5-2), there are many gametes present and many will survive.
Figure 5-2. Brittle star feeding on coral’s gametes
In areas like the Great Barrier Reef, the spawn is so large that a pink slick of unfertilized eggs and embryos (fertilized eggs) can be seen on the surface of the ocean (see top banner).
Can you imagine getting smashed by a wave and then you produce a clone? Scientists have found that after a mass spawning event, once embryos have formed, they do just that. The coral embryos are fragile, lacking a protective outer layer, and when they are hit by a large wave, they can break apart. Instead of dying, the broken embryo continues to produce more cells normally. They may be a little behind the curve in terms of size and development, but they appear to be able to settle and grow just like normal embryos. This is just one more reproductive advantage that corals can use to ensure survival.
Check out the planulae and settled coral polyps here: https://www.youtube.com/watch?v=9WCBOhBW2Tk
Source: Heyward, A.J., and Negri, A.P. 2012. Turbulence, Cleavage, and the Naked Embryo: A Case for Coral Clones. Science (335) 6072: 1064.
Some corals are asynchronous spawners meaning that they can spawn during the mass spawn, but they are more likely to spawn before or after it. Different species spawn at different times.
There are certain environmental cues that tell corals when to spawn. It is unclear exactly which cues affect spawning events, though it is believed that corals respond to multiple environmental cues. What environmental cues do you think could affect spawning?
Here are some potential environmental cues:
As corals become more threatened by anthropogenic, or human produced effects, it is likely that successful coral reproduction will also be at risk (see Unit 19:Threats, coming soon).
The second way that corals can reproduce is via asexual reproduction. Asexual reproduction is a means of reproduction where a new organism arises from a single organism. This new organism will only have the genes of the parent organism and it is an identical clone of the parent. Corals use several different methods of asexual reproduction.
Figure 5-3. A coral nursery in Vava’u, Tonga
Photo Credit: Andrew Bruckner
Figure 5-4. Coral budding
Figure 5-5. a) Intra-tentacular budding; b) Extra-tentacular budding
Photo Credit: Ken Marks
Figure 5-6. Mushroom coral undergoing fission
Figure 5-7. Polyp bailout of a staghorn coral
Figure 5-2. By Haplochromis [Public domain], 16 February 2007 via Wikimedia Commons. https://commons.wikimedia.org/wiki/File%3AStony_coral_spawning.jpg.
Figure 5-7. For use of staghorn coral vector. By Tracey Saxby, Integration and Application Network, University of Maryland Center for Environmental Science (http://ian.umces.edu/imagelibrary/).
Babcock, R. C., Wills, B. L., & Simpson, C. J. (1994). Mass spawning of corals on a high latitude coral reef. Coral Reefs 13: 161-169.
Great Barrier Reef Marine Park Authority (GBRMPA). (2011). Coral Reproduction. Retrieved May 16, 2014 from http://www.gbrmpa.gov.au/about-the-reef/corals/coral-reproduction.
National Ocean Service. (10 February, 2014). Coral Spawning. Retrieved May 16, 2014 from http://flowergarden.noaa.gov/education/coralspawning.html.
Sammarco, P. W. (1982). Polyp bail-out: An escape response to environmental stress and the new means of reproduction in corals. Marine Ecology Progress Series 10: 57-65.
Szmant, A. M. (1986). Reproductive ecology of Caribbean reef corals. Coral Reefs 5: 43-54.