World Oceans Day 2019 Part 2: Female Parrotfish Do Math for Better Reproductive Odds

Illustration by Missy Chimovitz

Illustration by Missy Chimovitz

This is part 2 of a three-part series honoring World Ocean Day 2019’s Theme of Gender and Oceans. See Part 1 here

Like many fish species, parrotfish form harems.  Because sperm is cheap to make, one male can produce plenty of it—enough to fertilize all the eggs of many females.  And like many harem-forming fish species, parrotfish sex changers, starting life as a female and then transitioning to male.  This strategy allows an individual to reproduce when small (as a female), and then when big enough to fight for and win a harem of females, they can transition to male and boost their reproductive output.

That’s usually what happens. But in bucktooth parrotfish, the largest female doesn’t always change to male when given the opportunity. Why?  Because sometimes, the transition just doesn’t add up when it comes to potential offspring output.

Bucktooth parrotfish, S. radians.

Bucktooth parrotfish, S. radians.

For fish, the bigger you are the more eggs you make. A very large female may produce ten times more eggs than a female half her size (we mammals are very different in this sense—all females have roughly the same number of eggs to start with, and that number goes down as we age). And somehow, female bucktooth parrotfish get this math…and act on it.

 If the largest female is so big that she produces more eggs than all the other females combined, she won’t transition to male.  A smaller female then takes up the male role, willing and able to fertilize all the eggs of the biggest female, plus the other females in the bunch.  

 Just how the big female conducts this calculation remains a mystery. But for this species, a little math goes a long way towards successful sex in the sea.For more details on this amazing strategy, see here.

Stay tuned for Part 3 of Sex in the Sea’s World Ocean Day: Gender and Oceans series.