Asexual Reproduction in Coral Reef Systems

By Ashley Gustafson


coral-spawning-the-great-barrier-reef-australiaReproduction is an incredible, fascinating and fundamentally crucial component of every living organism. As life itself is immensely diverse, the methods of reproduction can vary greatly from organism to organism. From budding bacteria reproduction to pathogenic lizards to classical sexual reproduction, it seems as though reproduction knows no bounds. While reproduction has been a necessity of life from the beginning, scientists still continue to learn and discover new aspects and methods every year. Coral reefs have a very specialized asexual and sexual reproductive process adapted just for them.

Asexual Reproduction vs Sexual Reproduction in Coral

In order to understand how coral reproduce it is important to understand how both asexual and sexual reproduction work. While reproduction varies greatly from organism to organism, most forms of reproduction can be neatly put into two categories. The first category is that of asexual reproduction. Asexual reproduction generates offspring that is identical to that of the parent, a clone. That is there is no meiosis or combination of genetic material. Thus the offspring has the exact same genetics as its parent. This can have some perks for the organism such as low energy cost and convenience (no mate necessary!) but it can create problems for an organism since cloning creates low to no diversity making them vulnerable to fatal pathogens or even genetic mutations. The second category of reproductive methods is sexual reproduction. Sexual reproduction is the combination of two parents supplying genetic material to produce a unique offspring. There are many benefits to reproduction such as genetic diversity which can allow organisms to adapt and change over time. While sexual reproduction may seem like the best option, finding a mate can be costly in terms of energy and the fusion of gametes is complicated and can malfunction causing nonviable offspring. Like I previously stated many animals do combinations of both categories of reproduction. Think of it as a “designer” sort of reproduction where organisms have developed and adapted the ability to tailor reproductive methods to best fit their lifestyles.

Corals are examples of organisms that have parts of their reproductive cycles that are asexual and parts that are sexual. While coral reefs have many parts, the living and reproductive part of the coral is the polyp. In the asexual part of the reproductive cycle, the young clonal polyps will bud directly off the parent polyp. This process is called budding and allows the parent polyp to grow and expand and thus begin a new colony of coral. This can only happen in mature polyps that are a certain size. Coral organisms can also reproduce through an asexual method called fragmentation where the coral simply fragments or breaks and is re-settled somewhere else. This can happened naturally, by outside force, or for growing purposes.

The sexual part of the reproductive coral is naturally a bit more complicated as sexual reproduction is fundamentally more complex than asexual reproduction. While each coral species is unique, the majority of stony coral species produce male and/or female gametes. That’s right: some corals will produce both male and female gametes depending on conditions. Most species of coral are broadcast spawners, meaning that each coral releases massive numbers of male (sperm) and female (egg) gametes into the water in order to distribute their offspring throughout the water. As one can imagine, the timing of these mass spawning events must be timed perfectly and synchronized to be successful due to the stationary nature of mature corals. The goal is for the egg and sperm from the individual corals to meet in the water, fusing to form free-floating or planktonic larvae. These larvae are called planulae. To compensate for the hazardous mode of reproduction, coral reproduce incredibly large amounts of planulae. Within the coral life cycle, the highest mortality rate is in between that of the planulae and the stage of settlement. Environmental cues allow for the success of these reproductive strategies particularly over wide ranges. These cues can include but are not limited to light, temperature, day length, and lunar activity. Successful planulae exhibit positive phototaxis meaning they swim towards light. This is very important for survival because coral organisms thrive in shallow (more light), nutrient rich water.

What other organisms have reproductive cycles?

Many other types of organisms reproduce in combinations of asexual and sexual cycles. One example is bacterial reproduction. The majority of single-celled bacteria produce in an asexual method called binary fission. This is the process in which a single, mature bacterium divides or splits into two identical, clone daughter cells. Eukaryotic Bacteria can also reproduce as coral do through the process of budding. Finally, some bacteria can even reproduce sexually. While the concept of sexual reproduction of bacteria is the same, the process is very different than what normally comes to mind with sexual reproduction. Instead of gametes, bacteria exchange plasmids which are essentially packets of the bacterium’s DNA. Plasmids are capable of transfer of DNA from cell to cell and don’t necessarily have to be from the same species of bacteria. This transfer is called is called bacterium conjugation. Like sexual reproduction bacterial conjugation creates new combinations of genetic material and unique offspring. It is important to remember that this is just a glance at the vast capacities of reproduction in bacteria reproduction.

Why don’t all organisms reproduce sexually?

It may seem that I would be a no brainer for organisms to reproduce sexually. It provides genetic diversity and allows organisms to change, adapt, and grow much quicker than producing asexually allows. However, many animals are able to reproduce exclusively asexually and are very successful. One crucial organism on our planet often reproduces asexually: plants. It is without a doubt plants have found success at reproducing many different ways both sexually and asexually. Asexual plant reproduction includes many methods I have already discussed including budding. Other forms of asexual plant reproduction include vegetative propagation and fragmentation. Vegetative propagation, often seen in strawberry plants, occurs when the adult plant sends out horizontal stems called stolons which find their way into the ground and grow into roots which eventually form a new plant. Fragmentation is a method of asexual reproduction exhibited in not only plants but corals too. Fragmentation is a method often used in plant nurseries and greenhouse to produce plants quickly. This happens by a breaking off a portion of the stem, creating a clone, and placing it is soil or water. This can often happen naturally, as seen in many corals.

As you can now see, reproduction is so much more than two categories or a defined set of processes. Reproduction is a vital part of survival and success in any organism from bacteria to plants to coral to human beings. In fact according to Darwin’s theory, the single most important act an organism must do to max out its fitness is to pass on its genes to as many offspring as possible or simply reproduce as frequently as possible before it dies. This overview of asexual and sexual reproduction is only the beginning of all the methods and theories of reproduction because there is so much more information to not only learn but discover as well.