Eyes That Fade, Eyes That Move: Fish Embryos in the Lab

Two Strange Fish in the Lab

1Wilhelmina Kipketer, an eighth grader, spent two weeks of her summer in a youth program at a university fish lab. The lab studies how fish bodies take shape before and after hatching, and it raises two unusual fish in its tanks. The lab's director, Dr. Iñaki Valdés-Moreau, gave Wilhelmina a microscope with a tiny measuring scale built into the eyepiece. The staff placed each egg, embryo or larva on a slide in a drop of tank water, so the young fish could be returned to the tanks after a quick look.

2The first fish comes from rivers and limestone caves in northeastern Mexico. Small silvery fish called tetras live in the rivers, and adult tetras have large dark eyes. Deep in some caves, where it has been completely dark for many thousands of years, lives a fish of about the same size and shape. Adult cave fish, however, are pale pink and have no eyes that can be seen. Smooth skin covers the spots where eyes would be, and the fish find food mostly by touch and by sensing movement in the water.

3Wilhelmina measured the eyes of embryos and young fish of both kinds, all raised in tanks at 25 °C. Both kinds hatch about one day after the eggs are fertilized. On day 1, each cave fish embryo had an eye cup with a clear lens, just as each river tetra embryo did. By day 3, Dr. Valdés-Moreau explained, many cells in the cave fish lens were dying, and soon afterward the eye stopped growing. Over the next weeks it slowly sank into the head while skin grew over it. Table 1 shows Wilhelmina's measurements.

4The second fish is the summer flounder, a flatfish from the Atlantic coast. An adult flounder lies flat on the sea floor on its right side. Both of its eyes are on its left side, which faces up, and that upper side can change color to match the sand. Its lower side is white.

5The lab hatches flounder eggs in cool seawater. Wilhelmina looked at larvae of different lengths under the microscope and wrote down where their eyes were and how they swam (Table 2, in Part 2). Most other fish, from tetras to trout, keep one eye on each side of the head all their lives and swim with their backs up.