What was the outcome of the sea stars in the tank with only seawater?

• A. They climbed to the top of the tank.
• B. They remained at the bottom.
• C. They sank and died.
• D. They showed increased activity at mid-depth.

Answer: (A)

The main idea here is that dissolved oxygen in water is usually higher near the surface because air keeps diffusing into the water there. Sea stars breathe using dissolved oxygen in the water, so they’ll move toward the area with more oxygen to meet their metabolic needs. In a tank with only seawater, the surface has more available oxygen than the bottom, so sea stars tend to move upward, climbing toward the top where oxygen is richer. This behavior reflects their drive to access a better oxygen supply for respiration. The bottom would be more oxygen-poor, and sinking or dying wouldn’t be the immediate outcome if they can reach more oxygen by moving upward. Moving to mid-depth isn’t as consistent with the typical surface-to-bottom oxygen gradient.