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Do Seahorses Change Sex?

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There's a reason talks with kids about the birds and the bees don't include seahorses. Fish with unusual methods of reproduction or that change sex can add to the challenges of a home aquarium just as to explaining the facts of life. Seahorses evolved an extremely rare approach to sex and reproduction.


Seahorses come in a variety of textures, with certain species sporting knobs or even growths along the spine that resemble a horse's mane. Identification of seahorses -- and locating them if you're diving -- is complicated by their skill at camouflage. These creatures change colors and can blend in so well with their environment as to be nearly invisible. They also remain still for long periods, typically wrapping their long tales around a support. Seahorse species range from the tiny 2-inch pygmy seahorses to the 1-foot-long big-belly ones, according to Dr. Ellen Prager in her book "Sex, Drugs and Sea Slime."

Males and Females

It can be difficult to tell female and male seahorses apart. When you watch a pair, the female may be slightly larger than the male. The ability to change color isn't specific to one gender. Seahorse pregnancy may confuse you if you're new to observing seahorses. A pregnant seahorse you see while diving or watching an aquarium is a male. Male seahorses of all species have a brood pouch. The brood pouch protects the developing offspring.


Seahorses are monogamous during each breeding cycle, and certain pairs may mate for life. The mating dance can continue for hours. The couple swims together, twining tails in a water ballet. A female has an ovipositor for depositing her eggs in a male's brood pouch. It's normal for a female to deposit her eggs in a male when she becomes mature; no sex change is involved. Females may compete for males, which some observers consider a sex-role reversal.


The male's role in reproduction begins with producing sperm to fertilize the eggs in his pouch. His brood pouch keeps the young at a comfortable temperature and may provide nourishment. The male has contractions and expels the young seahorses into the water. The birth can result in hundreds of defenseless seahorses. The male will soon accept another batch of eggs. Although this may seem like a change in sex roles, it's natural for all types of seahorses.


The reasons for male pregnancy in seahorses are still under investigation. Continuing research suggests male seahorses may have more influence on mate selection than previously believed. Male pot-bellied seahorses prefer large females, possibly because the resulting offspring are larger, according to Swiss researchers who reported their findings in the 2009 issue of "Behavioral Ecology and Sociobiology." Seahorse babies fend for themselves from birth. The parents don't feed or care for them once they're born. Larger offspring may have a survival advantage.