I am very knowledgeable with dealing with the sexual reproduction of corals having written articles about this interesting concept. With over 30 years of saltwater reef biotope involvement either through the many tanks I maintain, or as a guru, so to speak, in the hobby while working at two pet stores where I can honestly say that my credentials as an expert, regarding anything dealing with this fascinating hobby and way of life.
Genetically heterogeneous population of corals produces the greatest diversity of characteristics and resistance to environmental stressors. This same progressions holds true in the human population as well. The only way of inducing these optimal traits is through sexual reproduction. Because of this, corals possessing the adroit ability to carryout sexual reproduction, are most likely to procure superlative results, not only in the wild but in an aquarium as well. Sexual reproduction is nature’s way of ensuring that survival of the fittest can best be achieved.
In part one of this series, I explored the various forms of asexual reproduction and what it entails. Having the exact same genetic make-up or ramet as the parent colony, also known as a clone, often times, provides undesirable results. Diminished tolerance and resistence qualities and the inability to promote a diverse array of characteristics and offspring doesn’t do much to enrich the gene pool. Moreover, corals displaying this type of sexual behavior tend to succumb to many monstrosities, not evident in corals with the ability to fuse gametes, or eggs and sperm.
Of course, sexual reproduction is not fool proof. The passing on of genetic defects, weaknesses and unfavorable traits is not out of the realm of possibilities. However, it is these same discommodious results that tend to get weeded out in favor of more congenial results ensuring the success and perseverence of the particular species of coral.
Tim Wijgerde’s findings in his article are fascinating to say the least and backs up my knowledge in the marine hobby. Let’s take a look at some of the sexual reproduction strategies that various corals partake in. Some of these strategies will be clear cut to most marine aquarists, while others will surely raise an eyebrow. In Tim Wijgerde article entitled ” Coral Reproduction- Part 1: Biology” sexual reproduction is a very fascinating process that each, in their own specialized way, provide the foundation and and framework for the birth of a reef and the assurance that the species survives.
Gonochorism is just a fancy name for individual corals that are either male or female. Many fish, reptiles, mammals, birds and amphibians display this form of sexual reproduction where the parents brood their eggs.
Hermaphroditism is a sexual strategy performed by most stony corals where each individual polyp contains both male and female gonads meaning testes and ovaries, respectively. Eggs are released into the water followed by the sperm to fertilize these eggs and allowed to drift from the parent colony to adhere elsewhere allowing for the continuation of the species. This process is called either broadcast spawning or brooding. The only difference is that in brooding the ova is fertilized internally. Brooding produces fewer eggs and larger larvae hence quality over quantity is the patern. Broadcast spawning represents the other end of the spectrum where thousands of gametes are released into the water column in hopes of being fertilized. This provides a greater chance of fertilization and food for small fish and other corals and filter feeders. In the aquarium the same results can be ascertained, however, protein skimmers tend to go on overload. Brooding corals produce large larvae which tend not be suspended in the pelagic areas of a tank very long often settling and attaching quickly to reduce the likeliness of being removed by skimming and other filtration means.
Now, it gets more eccentric as we enter the twilight zone of the undersea world. Many fish and coral species possess the unique ability to change from one sex to the other. This is called sequential hermaphroditism and can be employed in two distinct ways. Those transferring from male to female is called protandry while switching from female to male is termed protogyny. Clownfish represent protandry whereas, Wrasses personify the protogyny process. These sex strategies are uniquely relevant and critical to many fish and coral species because they ensure that the particular species perseveres. Case and point, say their is a community of mushroom coral all same sex species, sexual reproduction would not be carried out. Now enter squential hermaphroditism. This same species is now able to release eggs and sperm, interchangeably, into the pelagic areas of reefs and aquariums to initiate self fertilization. This form of sexual reproduction enables coral, during times of stress, the ability to produce sperm, instead of eggs, which takes much more time and energy to produce. To take this one step further, many of these animals are able to switch back and forth depending on the population and ratio of male to female animals at the current time.
Parthenogensis is yet another form of sexual reproduction. With this, a coral is able to produce eggs and undergo mitosis and initiate embryonic development without self fertilization. This form of sexuality is a double edge sword. Corals are able to undergo sexual reproduction in the absence of a males sperm to ensure the success of the coral in the gene pool. The drawbacks to this are that genetic variation dwindles and hereditary defects can become more prominent such that that the ability of corals to change and adapt to environmental conditions, suffer greatly.
Symbiotic corals require light for the nutrients needed to survive. The reason for this is due to the zooxanthellae that resides in the tissue of these corals. Zooxanthellae is a single celled algae that receives protection from its host while returning the favor in the form of nutrients consisting of glucose, amino acids and carbon.
I know that when it comes to transferring zooxanthellae from the mother coral to its offspring, two sequences can take place depending on whether the coral is a brooding or broadcast spawner. Having witnessed these occurrances in brooding corals, where zooxanthellae is transferred from the mother to its young through its ova. This is called vertical transmission. In broadcast spawners, the eggs are aposymbiotic meaning they cant receive nutrients from light. Because of this, the larvae needs to incorporate zooxanthellae into its tissue another way. The process by which aposymbiotic corals receive zooxanthellae into their tissue is through the water around the larvae. Horizontal transmission is the process used to enable asymbiotic larvae to accumulate zooxanthellae into their tissue after beginning their life without it.
Vertical transmission has a disadvantage in that larvae can only receive the strain of zooxanthellae that the mother has in its tisse. This hinders the young corals ability to develop other strains that may be better suited to help the coral during changing environmental factors. Horizontal transmission allows the larvae a much greater chance of developing various strains of zooxanthellae to make the coral more prepared and have the advantage towards selecting the right strain of zooxanthellae at the time.
These two types of transmission give the coral larvae the nutrients they need to survive and grow to maturity. Ultimately, through sexual or asexual reproduction the coral colonies larvae will be able to establish and ascertain that the coral line will be healthy and able to live to reproduce one day, itself.