Why does the sailfish retract its towering dorsal fin when cruising at high speeds?

Answer

If the fin remained upright, the resulting drag would inhibit its speed.

The massive dorsal fin folds down into a groove along the back when the fish is cruising fast. This mechanism is critical because if the fin stayed upright at top velocity, the drag created would likely inhibit the speed the fish is famous for.

Why does the sailfish retract its towering dorsal fin when cruising at high speeds?
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