In the sea stars experiment, what specific environmental condition differed between the two tanks?

• A. Temperature
• B. Light exposure
• C. Salinity
• D. Presence/absence of a freshwater layer on top

Answer: (D)

The difference between the tanks was whether a freshwater layer sat on top of the seawater. That creates a surface boundary where the salinity is much lower in one tank than in the other. For sea stars, surface salinity affects their water balance and physiology because osmotic gradients drive how water moves across their tissues. Having a freshwater layer at the top changes the environmental exposure at the interface, making it the key variable being tested. The other factors—like how warm or bright the tanks were or the overall seawater salinity—were kept the same, so the presence or absence of that freshwater layer is what distinguished the two conditions.