GEOLOGICAL SCIENCES 100
CORAL REEFS MINI-COURSE
KYGER C LOHMANN
1. Sexual Reproduction
2. Asexual Reproduction
3. Interesting Facts
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.
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
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 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.