Sea Urchin Development

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The following images shows the growth and development of the sea urchin, Lytechinus pictus. These sea urchins are found off the coast of California. The adult female releases close to a million unfertilized eggs directly into the ocean. Some of them will become fertilized by sperm released by a male sea urchin. The resultant fertilized egg will develop into an embryo while powerlessly floating through the ocean waters. After a few days the embryo will gastrulate and form a primitive digestive system (early prism stage). At this stage ciliated bands are formed that allow the larva to form feeding currents by which to concentrate particles in the water for ingestion. As the larva develops the structure becomes more complicated and larval "arms" are generated to aid in locomotion and feeding (pluteus stage larvae). Eventually this feeding larva will grow and develop enough so that it can metamorphose into a juvenile sea urchin and begin to look like the sea urchin most people are familiar with (i.e., a pin cushion). During metamorphosis most of the larval body shown here is castoff or used to supply the energy that fuels the dramatic transition from a free living, bilaterally symetrical larva, to a sphere-like, pentaradial sessile juvenile sea urchin.