Page 226 - Canadian BC Science 9

Because sea urchin eggs are fertilized outside the body of the female, not all of the eggs will be fertilized. Often, egg cells do not survive ocean storms that disturb the tide pools and coral reefs in which sea urchins live. The sea urchin also has many predators, so even if the eggs are fertilized the developing embryos or developing young are frequently eaten. Figure 6.19 shows the life cycle of a sea urchin, which begins with external fertilization.
egg and sperm
External fertilization: sperm cell enters egg cell.
four-cell stage
zygote
Male and female sea urchins release gametes.
adult sea urchins
hollow ball of dividing cells
208 MHR • Unit 2 Reproduction
young sea urchin
Figure 6.19 The life cycle of the sea urchin. Many water-dwelling animals that rely on external fertilization have a similar life cycle.
External fertilization for salmon takes place in the gravel beds of rivers and streams (Figure 6.20A on the next page). With sweeping movements of her tail, the female salmon digs out a gravel nest. The male swims by and releases his sperm as the female deposits her eggs. Both the male and female salmon die after spawning (Figure 6.20B on the next page).
free-swimming stage