Jellyfish reproduction is caught in an evolutionary state between direct genetic exchange and full-blown asexual reproduction. An egg is fertilized sexually, but the offspring are produced asexually, which is a solution unique to the cnidaria phylum (jellyfish, corals, sea anemones) and a few other invertebrates.
Like many marine animals, jellyfish have a clear demarcation between male and female genders that is upheld for the entire life of the organism, but they do not necessarily have the same complex sexual organs as vertebrates. Instead, eggs or sperm, depending on the gender, are couched within the gonads just inside of the body wall. Fertilization occurs externally, meaning that the genders do not engage in any sexual acts in order to exchange genetic material through direct contact.
After a jellyfish chooses a mate, the male releases sperm from the stomach into the water, which enter the female and fertilize the egg so that the process of a fertilized egg leaving the organism and developing on the outside of the body begins. Some species of jellyfish carry the egg within a pit of the oral arm, which is an arm around the mouth that contains the venomous cells dangerous to other forms of life.
When the egg is released into the water, it begins the normal process of cell division. However, at the embryo stage it begins to transform into a planula, which is a flattened, larval creature with hair-like cilia on its body that allow it to move. The planula is carried by the ocean current for a few hours or days until it finds a hard, rocky place on the ocean floor, where it attaches itself.
The planula then grows into a polyp, which is a cylindrical organism that can live for several years in a transitional stage on the rocky floor, with its mouth and tentacles facing upward in order to capture small marine organisms for food. New polyps "bud" from the old polyps, much like the budding that occurs with plants. Budding is an asexual act fundamental to the jellyfish life cycle and requires no additional sperm. Essentially, the polyp is producing separate embryonic lifeforms that will continue to grow to their own maturity. This usually happens around spring.
Ephyra and Medusa
Over time, grooves begin to appear in order to differentiate the polyps into individual lifeforms. The polyps eventually form disc-shaped, maturing jellyfish called ephyra that begin to break away. Within a few weeks, they begin their lives as fully mature, bell-shaped jellyfish called medusas, which may be male or female. Tens of thousands of new jellyfish may be produced in this way from a single planula.
- Photo Credit jellyfish image by cherie from Fotolia.com
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