What was the role of the freshwater layer in the sea stars' experiment?

• A. It served as a potential barrier affecting the sea stars' climbing behavior.
• B. It served as an indicator of water quality.
• C. It measured temperature changes.
• D. It determined the oxygen level.

Answer: (A)

A freshwater layer sitting on top of seawater creates a salinity boundary that can act as a barrier for sea stars. Marine animals like sea stars are adapted to a relatively stable, salty environment, and crossing into freshwater would cause osmotic stress—water tends to move into their tissues when exposed to a hypoosmotic (less salty) environment. This physiological challenge makes crossing the boundary risky and uncomfortable, so it can alter their climbing behavior, causing hesitation or avoidance of the layer. In this experiment, the freshwater layer mainly influences movement by presenting a salinity-related barrier rather than signaling water quality, tracking temperature changes, or measuring oxygen levels.