Many tropical zooplankton have elaborate spines. These have been interpreted as either a defense against predation or a means to increase drag and prevent sinking. How would you determine which was the likely reason for these structures

Means to increase drag and prevent sinking is the correct answer.

Explanation:

Plankton avoid sinking by increasing their surface area and decreasing their density. Spines slow down sinking by adding surface area without increasing density. Defense against predation would be to sing below. Not the opposite.