Why do sea urchins have external fertilization? [Discuss the life style of the developing sea urchin embryo in the water column compared to a very different ecological niche of the adults on the ocean floor. Besides, would you want to give live birth to a sea urchin! ouch!]
Then why don’t the parents take care of the young? [In fact the adults do in a way. After the embryo has grown large enough and metamorphosed into a young urchin they hide in the spines of the adults, living off of the scraps of food produced from the messy eating of the adults]
Because the urchin life involves two or more ecological niches, they are more susceptible to predation and exposure to environmental toxins. What environmental conditions, predators and toxins might they be exposed to at each stage?