Barbara Harrison, age 13, of Fort St. James, B.C., Canada, for her question:
How do starfishes reproduce?
The parents do no more than cast their egg cells into the water, which is one reason why infant mortality rate among the starfish is at least 99.9999 per cent. Among millions of male and female cells, just a few thousand unite to form fertilized embryos. Of these, perhaps a dozen or so, if that, survive to become adult starfish. The rest are devoured by hordes of hungry creatures that populate the sea.
The starfishes are not true fishes and nowadays a lot of people call them sea stars. After all, they do live in the sea and many of the common types are shaped like five pointed stars. However, there are at least 2,000 species and some have ten or more tapered tentacles. Most recognizable animals are two sided, or bilateral. But not the sea stars. They are remarkable because they have a radial body plan, with a minimum of five sides.
The development of this unusual body plan can be traced through the stages of the sea star's reproductive cycle. The great event begins when spring warms the oceans. In the angles between the arms, there are pockets where the females develop egg cells and the males develop sperm cells. When temperature and other conditions are just right, they simply release millions of these opposite cells into the water. That is the limit of the sea stars' reproductive role and also the limit of their parental responsibilities.
The cells are microscopic and finding each other while drifting in the hungry sea is very chancy. But now and then a swimming sperm cell manages to find and fertilize a female egg cell. The swarms of unfertilized cells can survive only a short while, The rare fertilized cell immediately becomes a thriving embryo.
The egg cell duplicates itself, then divides into four, eight and sixteen and so on. This multiplication by twos continues and by the end of the first day, the embryo has enough cells to build a tiny hollow ball. The outer surface is ciliated, or hairy. The tiny hairs wave and the tiny ball goes swimming through the sea.
This free swimming embryo in no way resembles an adult sea star. What's gyre, during the next few hours changes occur that make the young larva even less like its parents. The surface of the hollow ball forms dents and hollows that indicate its future features. But for a time, the immature sea star is a bilateral animal with a left and right side.
The free swimming larva keeps this two sided body plan until it is ready to settle down. Then for a time, it remains fixed to some solid object on the sea floor. Meantime, its body is completely remodeled. Anew mouth opens on the left side, a rear opening forms on the right side. The former left and right sides become the top and bottom and the radial arms form around the center. When the remodeling is complete, the young adult sea star sets forth to hunt for food. With luck, it may live about five years.