The seahorses at Portobello Aquarium have been mating recently and in their peculiar underwater society, it is the males who give birth.
The courting process, between the nine female and eight male seahorses, involves males inflating their bellies to attract females.
If a female likes what she sees, she performs a courtship dance and then passes her eggs into the male's brood pouch, where they are fertilised and carried by him for four weeks.
New Zealand Marine Studies Centre manager Tessa Mills said it was "quite a sight" to see the seahorse males showing off their bellies, but the males giving birth was even more remarkable. The baby seahorses looked "like a bunch of cotton threads coming out of the belly", she said.
Up to 800 eggs were laid at one time and it was common for about 400 babies to be born.
"Some of our visitors have even been lucky enough to witness the males giving birth," she said.
Baby seahorses were perfectly formed and able to survive on their own as soon as they were born - at the same time their father's paternal instincts ended.
Offspring are released into Otago Harbour shortly after birth, because specialist food and tanks would be required to raise them in captivity.
• In a celebration to mark Father's Day entry to the aquarium will be half price this Sunday.
