Seahorse Frequently Asked Questions / How can I tell the difference between a female and a male seahorse?
Male seahorses have a pouch, while the females do not. The pouch is at the base of the tail, and slopes from the body down to the tail, so the transition is smooth. A good description I heard is if you look at the overall shape of the seahorses body and tail, males look like a D while females look like a P. Male seahorses also have a ridge on their chest called a keel, and most males have shorter snouts than females, though the differences are subtle and can be difficult to see.
Immature seahorses look like females.
This illustration shows a male and female side-by-side:
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