Love is in the Water: Coral Spawning Explained

Written By Ayla Sage

The biological world is a weird and mysterious place. Animals and plants reproduce in a multitude of ways, but all with one common goal: to further produce life and pass on their genes to the next suitable offspring. Corals specifically have different modes of reproduction, some sexual and others asexual. Now, let’s explore what they are.

Types of Reproduction

Most coral species are hermaphrodites, meaning they can produce both male and female reproductive cells called gametes. However, not all corals need to produce both cells to reproduce. They instead utilize asexual reproduction methods. The three modes of asexual reproduction all result in clones and include: budding, fragmentation, and polyp bailout.

Budding occurs when a daughter polyp forms from a parent polyp one of two ways: as an intratentacular bud or an extratentacular bud [2]. Intratentacular budding involves the parent polyp dividing itself into two or more daughter polyps. Differently, extratentacular budding occurs when daughter polyps form adjacent to the parent colony, external to the tissue. This polyp is generally smaller than its surrounding neighbours but grows over time [2].

**Figure 1.** a) Intratentacular budding; b) Extratentacular budding
Living Oceans Foundation

Growth from fragmentation can be both intentional and unintentional. If it is intentional, a coral will begin to kill off some polyps in order to release a damaged party of the colony that would otherwise use too much energy to rebuild [2]. Unintentional coral fragmentation commonly occurs when corals are exposed to physical disturbances such as storms. If fragmentation is successful, a portion of one colony will establish a new coral colony that is genetically identical to the parent colony. Success rates are increased when a fragment lands on a favorable substrate, like on top of a living colony, compared to an unfavorable area like bare sand [2].

Lastly is the rarer form of asexual reproduction, polyp bailout. This coral bailout happens when a single coral polyp splits from an adult polyp and begins to drift off and settle elsewhere. This is most likely to occur if the colony is in a state of stress. After the single polyp settles, it grows and begins to create its own colony [2].

Sexual reproduction includes two different modes that can be used to produce larvae. Typically, abundant reef-building species are broadcast spawners. Broadcast spawners release their eggs and sperm into the water column and let them float to the surface [3]. Eggs and sperm from different species begin to mix together and the subsequent process of fertilization and larval development begins. Other species of corals are called brooders. Brooders undergo internal fertilization and release their offspring from the adult colony as a singular larvae. An advantage to being a brooder is that the larvae is already at a relatively advanced stage of development and solely needs to find a place to settle in order to survive [3]. However, the disadvantage is that they do not interact with other colonies making them less genetically diverse.

**Figure 2.** Overview of sexual coral reproductive cycles.
Rivera et al.

Mass Coral Spawning

Coral spawning events are actually quite phenomenal. What makes them so spectacular is that this mass reproduction only happens once a year [1]. Colonies and different species of corals simultaneously release tiny egg and sperm bundles from their gut cavities into the water. By expelling these bundles at the same time, the corals increase the likelihood of fertilisation taking place. The mass spawn occurs after a full moon, at night, and only after rising water temperatures have stimulated the maturation of the gametes within the adult coral [1]. Other factors that contribute to the timing of the event include day length, tide height, and salinity levels [1].

The spawning event as a whole can last between a few days and up to a week so that different species can release their eggs and sperm at alternate times to avoid the production of hybrids (the fertilization between corals of different species). In a way, they like to be pure breeds. Spawning can also be dependent upon location across the totality of the reef [1]. Corals inshore usually begin to spawn one to six nights after the first full moon in October (for Great Barrier Reef corals), whereas those in the outer reefs spawn during November or December [1]. Once an egg has successfully become fertilized, it can develop into a coral larva called a planula. Some planulae have the ability to survive for months before settling due to their symbiotic relationship with zooxanthellae, even at this early stage in life [2]. However, once the planula has found a suitable place to settle on the ocean floor, it can begin to bud, and the new coral colony can develop.

Some of these mass spawning events are so large that they can be seen from space. The eggs and sperm released cover the surface of the water and typically leave it an orange/pink colour. The large numbers of egg and sperm are congruent with a mass reproductive strategy [2]. Since many planulae get eaten or die in the process of looking for a place to settle, it is beneficial to produce as many as possible.

**Figure 3.** Overview image of coral spawning at Ningaloo Reef in Western Australia

Research

This past year on August 6/7th, Staghorn corals grown in a nursery and replanted at a reef restoration site off of Key Biscayne in Miami, FL have spawned for the first time [4]! This is a sign of hope that shows that the process of fragmenting corals and outplanting them to reefs is a viable approach to restoring some of Florida’s most valuable marine ecosystems. Scientists from University of Miami’s Rosenstiel School and collaborators from SECORE International were able to collect eggs and sperm which they later fertilized in order to be regrown and replanted to continue the cyclical approach in helping reefs become more resilient [4]. Having outplanted corals spawn is a gold standard for a successful restoration program. Coral restoration programs still have a long way to go, but this is a beginning sign of success!

Resources

[1] “Coral Reproduction.” GBRMPA, http://www.gbrmpa.gov.au/the-reef/corals/coral-reproduction. 

[2] “CoralReproduction.” Coraldigest, coraldigest.org/index.php/CoralReproduction.

[3] “Sexual Reproduction.” Reef Resilience, reefresilience.org/restoration/coral-populations/larval-propagation/biology-of-reproduction/.

[4] “Restored Corals Observed Spawning for the First Time in Waters off Miami.” University of Miami News and Events, 1 Feb. 2021, news.miami.edu/rsmas/stories/2020/08/restored-corals-observed-spawning-for-first-time-in-waters-off-miami.html.

Cover Photo: Blue Ocean Network