Introduction: Sea Urchins
Sea urchins are spiny marine organisms; many different species are found in oceans around the world. Male and female individuals produce eggs and sperm that are shed freely into seawater. Scientists have long used the urchin as a model for studying fertilization and development.

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Species-specific sea urchin fertilization
Sperm from the sea urchin species Lytechinus pictus are added to a mixed population of eggs from L. pictus (large eggs) and Strongylocentrotus purpuratus (small brown eggs). Only eggs from L. pictus are fertilized by the sperm.

Elapsed time about 3 minutes.
Sperm from the sea urchin species Lytechinus pictus were added to a mixed population of eggs from L. pictus (large eggs) and S. purpuratus (small brown eggs). Eggs were then immediately mounted in seawater between a slide and coverslip using a silicon spacer. Images were taken at room temperature on a compound inverted microscope using a 20x DIC objective and digital camera.