How do you know the sex of a seahorse?
Seahorses are famous for their eccentric sexual behaviour: it is the male who incubates the eggs, and gives birth to baby seahorses that look like tiny adults.
Thus, in theory, it is quite simple :
• adult males have a ventral pouch, located under the tail just behind the abdomen,
• females have none.
But in practice, the male's pouch may not be very easy to see in winter time. Paradoxically, it is often much easier to make sure of the sex of a female!
Females always have a marked angle between abdomen and tail.
The (small) anal fin usually is well visible.
Female long-snouted seahorses from Thau lagoon ; photos P. Louisy.
Males normally retain an area of soft skin making transition between abdomen and tail unclear, even when the pouch is not developed at all.
The anal fin is not visible unless the pouch is fully retracted.
Male long-snouted seahorses from Thau lagoon (winter, reduced pouch) ; photos P. Louisy.