Reproduction of a Sea Urchin
Sea urchins are echinoderms, in the same family as sea cucumbers and starfish. There are more than 800 species of sea urchin inhabiting the ocean floor, scavenging for plants and animals. The aquaculture of sea urchins, a popular delicacy in some countries that had led to overfishing in many areas, has allowed scientists to closely study the reproductive and growth cycles of sea urchins. Making baby sea urchins is a rather complicated procedure.
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Male and Female
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Though sea urchins can be male or female, the gonads of either sex are indistinguishable by observation alone. Both eggs and sperm are released from five gonads, which are on the side of the sea urchin's mouth. Sea urchin gonads have long been considered a delicacy in Japan, where they are known as "uni," consumed annually by the ton.
Mass Spawning
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Sea urchins are mass spawners. Within a given area, spawning occurs simultaneously, possibly triggered by chemical cues in the water such as a growth of plankton suitable to support the food needs of newly hatched larvae. Sea urchins are external fertilizers: First, the males release their sperm. Then, the female sea urchins release several million tiny, jelly-coated eggs. When sperm and eggs touch, the sea urchin egg becomes fertilized. According to ScanAqua, a large farmer of sea urchins, between 60 percent and 90 percent of the total gonad contents are lost during spawning.
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Larval Stages
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According to Jean-Marie Cavanihac, who has watched the process in "Microscopy U.K.: Sea Urchin," the cells of the fertilized egg quickly begin to divide and multiply. After about six divisions taking place within three hours, a ciliated larva called a blastula hatches. Blastulas are one of the many creatures that make up zooplankton. The anus is the first organ that will appear on sea urchin larvae, followed by a mouth at the other end of the gut. At this stage, the larva is called a gastrula. In about another week, a rudimentary skeleton begins to appear and two long oral arms used to push food into the mouth. The larva begins to feed on single-cell algae. At this stage, the larva is called a pluteus.
Metamorphasis
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About a month into the Pluteus stage, the larva will sink to the ocean floor and bury itself, where it undergoes a dramatic metamorphosis. The sea urchin will emerge having all the features of the adult, only smaller, and will continue to grow in size for the next few years.
Sexual Maturity
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Most species of sea urchin reach sexual maturity after about five years. According to researchers at Bowdoin college, they may mature slower in colder water temperatures.
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- Photo Credit sea urchin image by avaz from Fotolia.com
