Reproduction
Although Purple Sea Urchins are dioecious, meaning they have separate male and female sexes, it is hard to distinguish the two because they are very similar. For example, male and female Sea Urchins possess the same reproductive organs. Regularly, each Sea Urchin has 5 gonads, while the irregular forms have only 4. Each gonad has a duct to open at a gonopore, a pore found it many invertebrates that in this case is located in one of the plates surrounding the anus. There are muscles lining the gonad so that the Urchin may squeeze out the gametes through the duct and out of the organism. Purple Sea Urchins reach sexual maturity after about two to three years. They do not show courtship or special behavior towards their mate. Instead, they spawn, meaning that they release their eggs and sperm into the water, reproducing sexually. In particular, Sea Urchins leave reproduction to chance. They do not select a mate, but rather release their gametes and leave it to chance that they will mix and get fertilized. Only 10% to 40% of eggs get fertilized. The gametes of Sea Urchins are similar to our own in many ways. Sperm are composed of 3 parts. The first part is the head which contains chromatin and an acrosome that releases an enzyme that helps penetrate the egg. Next is the mid-piece which contains the mitochondria needed for the ATP used to generate the flagellum and the bases of microtubules which make up the third part, the flagellum or the tail. An egg completes meiosis in the ovary and, because of this, it is a haploid. It is made of a nucleus and cytoplasm which contains ribosomes, mRNAs, mitochondria, and yolk that is used as energy for the embryo until it can feed on its own. A Sea Urchin haploid cell contains 21 or 22 chromosomes, which contain the genetic information that is passed on to offspring. They use external fertilization in reproduction. The male releases sperm and the female releases eggs. When the gametes meet, the sperm releases ancrosomal enzymes that digest the egg's jelly coat. The plasma membrane of the sperm fuses with the egg and the sperms nucleus enters the egg. With the nucleus inside, the haploid nuclei of the sperm and egg fuse together forming a diploid nucleus.
Development
The Purple Sea Urchin forms in the egg for about two months. It takes them about 3 to 5 years to reach full maturity. They can live anywhere from a few years to 200 years although the average life expectancy is 30-70 years. The Purple Sea Urchin starts out as a zygote. However, mitosis begins and therefore the cells double. Once 32 cells have been made, there are many stages of development that the embryo goes through the main ones being blastula, gastrula, and pluteus. The Sea Urchin goes through three main stages. The first is cleavage in which the cells divide into many smaller cells. The second is gastrulation which is when the cells migrate to form 2 or 3 germ layers. The third and last stage is organogenesis in which the primary germ layers form into tissues and organs. Purple Sea Urchins have many offspring, averaging from a few thousand to a little over a million. However, this is less than half the eggs released (several million). Because Purple Sea Urchin spawn, the offspring receive very little or no parental care. A few Sea Urchin have tried to keep their eggs safe either by putting them in a special pouch or under their bodies, but most give no parental care at all. Even the few who do rarely stay till the egg hatches.
Meiosis and Mitosis
Meiosis is a type of cell division in which a sex cell divides into four daughter haploid cells. This division is used only in sex cells(the gametes) and is essential to reproduction. Mitosis is the division of body cells which results in results in two daughter diploid cells. It is through mitosis that organisms grow, develop, and heal.
Sources
http://www.buzzle.com/articles/sea-urchin-reproduction.html
http://tolweb.org/treehouses/?treehouse_id=4879
http://tolweb.org/treehouses/?treehouse_id=4881
http://www.indiana.edu/~l113/independent_projects/SeaUrchFert_Dev.pdf
http://link.springer.com/article/10.1007%2FBF00993645?LI=true
http://www.stanford.edu/group/Urchin/gametes.htm
http://tolweb.org/treehouses/?treehouse_id=4879
http://tolweb.org/treehouses/?treehouse_id=4881
http://www.indiana.edu/~l113/independent_projects/SeaUrchFert_Dev.pdf
http://link.springer.com/article/10.1007%2FBF00993645?LI=true
http://www.stanford.edu/group/Urchin/gametes.htm