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External Anatomy
Dorsal Fin
Pinna dorsalis
The fin on top of a fish - stabilizes against rolling, may be used for defense (spines) or communication (color displays).
Region: Trunk Externally visible Universal in fish Unique to: all
📖 Overview
The dorsal fin is a fishs keel. Its primary role is roll prevention during swimming, but it has been repurposed hundreds of times across evolution: erectile venomous spines in scorpionfish and lionfish, mating-display colors in bettas and sunfish, camouflage in the leaf fish, and even predatory lures in anglerfish (a modified dorsal spine forms the illicium).
🐠 Variations Across Species
Single (perch), two-part (bass, some catfish), soft-only (many carps), heavily spined (scorpionfish, lionfish), sail-like (sailfish - 1.5m tall). Some fish erect and lower the dorsal fin dynamically for signaling.
✨ Fun Fact
The sailfishs dorsal fin can reach nearly its full body length - functionally a giant sail that helps herd baitfish schools.
🔗 Related External Structures