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Why does a cornetfish have a long filament extending from its tail?
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gasterosteiform
Lined seahorse (Hippocampus erectus).
The cornetfish, a member of the gasterosteiform order, possesses a long filament that extends from the end of its backbone beyond the tail fin. This filament, which can reach a length equal to that of the fish's body, is believed to contain sensory pores that assist the animal in detecting prey.1