(Photo from The Guardian.)
What is it about seahorses?
(Photo from the Monterey Bay Aquarium's exhibit The Secret Life of Seahorses, running through 2012.)
First there's their unlikely physiology. From U Michigan's Museum of Zoology Animal Diversity Web:
"The seahorse has a prehensile tail, which it uses to hold onto seaweed and coral. The scales have been replaced by rings of about 50 rectangular bony plates, encasing the body in a semi-rigid skeleton. The eyes can swivel independently or converge to achieve binocular vision. The most distinguishing feature between the male and the female seahorse is the kangaroolike pouch that the male has on its ventral side, used for reproduction."
Courting seahorses holding tails.
(Three photos above from The Guardian.)
A pregnant male, complete with kangaroolike pouch. This pouch is the centerpiece of the seahorse's unimaginably gender-bending life. From Arkive:
"Perhaps the most unique and unusual feature of seahorse biology is the fact that it is the male and not the female who becomes pregnant. When mature, males develop a pouch on the belly, known as the brood pouch. Breeding takes place in spring and summer; the female inserts her ovipositor into the male’s pouch and lays her eggs. The male then fertilises them and they become embedded into the wall of the pouch. The pouch is very similar to the womb found in female mammals; a placental fluid removes waste products and supplies the eggs with oxygen and nutrients. As pregnancy progresses, this fluid gradually becomes similar to the surrounding seawater, so that when the young seahorses are ‘born’ the change in salinity is not too great a shock. After 20 to 28 days of pregnancy the male goes into labour, typically at night when there is a full moon. After hours of thrusting, the miniature seahorses, which look exactly like the adults, are released from the pouch. The offspring are fully independent after birth and must fend for themselves. They are pelagic in the first stage of life, or hold onto floating debris at the surface with their tail."
(Photo by Silverseahorse, aka Terri Renne, from here.)
A male seahorse in labor, giving birth.
Of course there's a twist in the plot, as described in this unusually good short science video from Nature (the science journal Nature, not the PBS program).
You can read the research paper behind the video: Paczolt KA, Jones AG (2010) Post-copulatory sexual selection and sexual conflict in the evolution of male pregnancy. Nature 464:401-404. doi:10.1038/nature08861
(Photo from the Monterey Bay Aquarium's The Secret Lives of Seahorses.)