Fertilization: When sperm meets egg – WS Mexico
Reproduction is planned on the long run. More than two months before the magic night, each mature Acropora polyp switches into reproduction mode. That means it stops growing, starts to eat more and collects the fats included in the zooplankton it is feeding on. Later on, it takes all this storage to produce its one and only gamete bundle of the year.
The bundle contains 10 to 15 eggs and a small white portion of sperm―all tightly packed into a pink or orange colored ball. Since the eggs are made of a lot of lipids, the bundles float up to the ocean’s surface as soon as they are released. The reason behind this phenomenon is physics: lipids have a lower density than water and are much lighter. Thus they float.
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The floating ability serves two main purposes. First, “the bundles have to get caught in the waves, because wave action usually is what breaks up the sperm and egg bundles,” says SECORE coral expert Mark Schick. Secondly, the wave action brings sperms and eggs from different colonies together, mixes them and makes fertilization happen.
However, due to the severe decline of elkhorn corals at the Mexican Caribbean coast, eggs and sperm from colonies with different genotypes hardly ever meet. This is where SECORE and its methods of sexual reproduction of corals comes in. “Our divers collect the gamete packages released by the colony in nets, each of them having a collection jar attached to its end,” says SECORE coral scientist Dr. Dirk Petersen.
When the collection jars are filled, the divers close the lids and hand them upside down to snorkelers. They bring them to the boat, where a biologist gently turns them right way up and empties them into a small container. “During the whole fertilization process, we have to make sure that the gametes come from unrelated colonies. The reason is that the eggs have to be fertilized by sperm from a colony with another genotype. Otherwise fertilization will not happen,” says Dirk Petersen.
At the reefs close to Puerto Morelos, the diversity of elkhorn corals is high. “Of the 20 colonies we work with, there are 15 different genotypes and 6 different genotypes of the symbionts,” says workshop leader and UNAM scientist Dr. Anastazia Banaszak.
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When all available jars are emptied, the biologist starts to stir the egg and sperm soup gently―mimicking the wave action and breaking up the gamete bundles in order to help eggs and sperm to meet. “As soon as a sperm and an egg from genetically different colonies meet, fertilization can take place,” says Anastazia Banaszak. A new life begins, forming a tiny, swimming planula larva within the next two days.
By the way, elkhorn coral Acropora palmata belongs to the so-called broadcast spawners, which release eggs and sperms into the water column. This allows the stationary animals to mix genetically and to disperse offspring over great distances. Another group of tropical corals that develop planula larvae internally are called brooders―one of them is the golfball coral Favia fragum.
Tonight, our team collected millions of Acropora eggs. Their actual number is hard to count, as one milliliter of seawater may contain up to 50.000 - 55.000 eggs.