GEOLOGICAL SCIENCES 100
CORAL REEFS MINI-COURSE
KYGER C LOHMANN

 
4. Reproductive Modes 

Outline:
1. Sexual Reproduction
2. Asexual Reproduction
3. Interesting Facts

SEXUAL REPRODUCTION

    One interesting feature of the corals (and other coelenterates) is the method of reproduction, either sexual or asexual.  As we discussed in class, sexual reproduction involves the splitting of sexual cells within gonads of the coral through a process called Meiosis. In this case, the chromosome number of the resulting gamete is one-half (haploid)of the number require for a viable cell (diploid).  Through a process of fertilization, two haploid cells (egg and sperm) unite to form a complete number of chromosomes (diploid) which is then followed by cell division (Mitosis) and differentiation of cells into tissues with the ultimate formation of organs.  As discussed in class, this process of fertilization occurs outside of the organism, free floating within the water column of the ocean, and thus is a random process of combination of chromosomes.  The timing of egg and sperm release into the overlying water is triggered by progessive warming of waters and by lunar influences such that within a period or a week or two, all of the corals synchronize their release of gametes.  The synchroneity of release dramatically increases the likelihood of a successful unification.  Despite the co-incidence of timing, this method of reproduction is highly inefficient with regard to the biomass of the coral.  Often times,during gametogenesis (egg/sperm production and release), the individual polyps undergoe a net loss of mass and do not grow.  Moreover, many of the release cells are consumed by other organisms feeding around the reef.

    As is the case of human beings, you must certainly agree the the pursuit of sexual reproductive junctures is particularly inefficient in terms of energy.  Why then, do organisms undergo sexual reproduction?  Because sexual reproduction leads to genetic diversity.  The random process of fertilization between genetically differing coral individuals maintains and creates diversity in the genetic code of the coral population, a feature which in turn allows for evolution and adaptation as the earth's environment evolves.  Moreover, as part of the development of the adult coral polyp, sexual reproduction leads to the formation of an intermediate life stage, a small swimming polyp called a planulae. This swimming juvenile provides for mobility such that new areas can be colonized.  Upon finding a suitable surface for colonizing, such as a hard rock surface, the planulae attaches and grows into an adult polyp. This is where asexual reproduction becomes important (see below).

   Differences between Coral and Jellyfish Reproductive Cycles
      Note that in the case of the coral,  the polyp is the adult form; this differs for other members of the phylum Coelenterata.  In the case of the jellyfish, the adult form is the medusa (think snakes for hair).  During sexual reproduction these also develop a planulae stage, and a juvenile polyp stage, however, this polyp undergoes asexual budding to produce the adult medusae.
 
 

ASEXUAL REPRODUCTION

Asexual reproduction is a means of reproducing and replicating an adult by "budding". Tissues of an adult coral polyp differentiate to form a genetically identical copy of the parent (these are clones). Still attached by a connective tissue, these budded polyps then comprise colonial reef structures.  Clearly, in constrast to the inefficiency of sexual reproduction, asexual replication is the most effective means of increasing biomass without wasting energy of the production and dispersion of sexual gametes.
 
 
 
 
 
 
 
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