How do male sea horses become pregnant?

Seahorses display a kind of reversed pregnancy – after fertilisation, eggs are transferred into the male’s brood pouch to develop. The father makes quite an investment in terms of time and energy.

The eggs receive oxygen and protection, and gestation lasts from 14 to 28 days, after which the male gives birth to live young known as fry. These diminutive offspring are at the mercy of predators for the first few weeks of their lives: only a tiny fraction will survive to adulthood.

Why do male sea horses become pregnant?

By passing the eggs to her mate, the female can invest her energies in producing more eggs immediately after transferring the previous batch – so almost as soon as the male gives birth he becomes pregnant again. Thus, each mating pair can produce and brood eggs continuously throughout the breeding season, maximising the number of offspring that might survive.

Brooding is an ideal reproductive strategy for fish that produce small numbers of eggs, resulting in greatly reduced predation. At the other extreme, ‘broadcast spawners’ each release tens of thousands of eggs into the water column for external fertilisation. Few will survive, but meaningful parental care of such large numbers of eggs would be impossible.

Main image: Short-snouted male seahorse with eggs. © Getty Images