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		Brief description of sea urchin fertilization Fertilization is the union of two gametes, the sperm and the egg to create a new organism. Although some unicellular animals reproduce asexually, sexual reproduction is the preferred method of propagation in most multicellular animal species. The resulting zygote contains genetic information from both parents. Sea urchins and other echinoderms have long been favorite subjects for the study of fertilization and early development. They produce large numbers of gametes which can be combined to create embryos which rapidly develop in real or artificial sea water. The embryos are transparent, allowing the direct observation of internal and external structures. The union of sperm and egg presents several challenges for sea urchins: Fertilization is external.They live and spawn in tide pools and reefs in the ocean, where there is a tremendous amount of water rushing about. To prevent the sperm and eggs from being washed away and diluted, sea urchins have evolved mechanisms to bring the gametes together, including synchronizing spawning and chemotaxisof the sperm towards the egg (Gilbert, 6th edition, Figure 7.9). At the same time, many animals, even many echinoderms coexist in the same habitats, so that there need to be safeguards to ensure species-specificityand prevent association wih different species. Both the sperm and the egg have specific receptors for the other that must bind and transduce a signal sequentially for fertilization to occur. In addition, all those attractive forces can work too well and bring many sperm to each egg. Two is good, but more is not better; fusion with multiple sperm will bring in multiple genomes and multiple centrioles and result in the death of the embryo. Therefore, there have to be mechanisms to prevent polyspermy (fertilization with more than one sperm). The sperm is basically a stripped-down vehicle for transporting genes. It contains a haploid set of chromosomeswhich have been ultracompacted by the addition of a set of DNA-binging proteins, a flagella to supply motility and mitochondriato provide energy. The basal bodyof the flagella in sea urchin sperm, but not mammalian sperm, forms one of the asters for cell division. Finally, the sperm contain a very large exocytic vesicle known as the acrosome at the very front, just under the plasma membrane.