Well, I would be happy to discuss the matter with you and explain the general idea behind sexing juvenile seahorses, but I have to warn you that it’s very tricky to accurately assign gender to immature seahorses and the younger they are, the more daunting the challenge becomes.
It’s possible to sex immature seahorses with no indications of an incipient pouch using the position of the anal fin and a few other subtle indicators as a guide, but the younger the seahorses are the more difficult this is to accomplish with any degree of accuracy. Suffice it to say, sexing juvenile seahorses that are much younger than 5 months can be a very challenging adventure. This is typically done by noting the position and shape of the anal fin as well as the curvature of the abdomen. In immature females, the anal fin is situated right at the very base of the abdomen where it meets the tail and points more or less straight downward, almost flush up against the tail. In immature males that lack a brood pouch, the anal fin is located higher up on the abdomen, allowing room for the brood pouch to subsequently develop, and protrudes outward at an angle from the tail. This makes it appear as if there is more space between the tail and anal fin in juvenile males.
The difference in the position of the anal fin is due to the way the vent is situated in males and females. The seahorse’s vent is the cleft formed by the combined openings of the anus and urogenital pore (Seahorse Anatomy, 2004). It is the simple recessed passage located just above (cranial to) the anal fin in females; in males, the anal fin is located in the middle of the vent where it separates the anal opening from the urogenital pore. The male’s anus is therefore situated above the anal fin while the genital opening of the male is located below the anal fin at the mouth of the brood pouch. In females, however, both the anus and the urogenital pore are located above the anal fin. The presence of the brood pouch makes the anal fin less conspicuous in males, whereas the anal fin is often more obvious on females.
Adolescent males will also sometimes show a thick dark line near their vent where their pouch will eventually form as they mature.
That’s the quick rundown on sexing young seahorses, Chris, but the differences in gender are really rather subtle in the juveniles and it may be very challenging to determine the sex of three month old seahorses.
Best wishes with all your fishes, Chris!