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.

For some sex-changing parrotfish, math is really sexy

While we've all been busy crunching numbers for tax time, the female bucktooth parrotfish has been doing a little math, too. Only, instead of calculating write-offs, she's deciding whether or not to change sex.  In the world of fish, the right math can make or break your sex life.

Fish are some of the few vertebrates that have the ability to regularly change sex (other verts, such as some frogs, may do it under extreme circumstances). And while this is a pretty nifty party trick, changing sex takes a lot of energy- transforming one's genitals and gonads (external and internal sex parts) is no simple task. So why do fish bother? The answer is: reproductive success. It all comes down to how many offspring an individual can produce. Here's how it works:

The most common way fish change sex is from female to male. This is called (take a breath) sequential (one sex after another) protogynous (first female, then male) hermaphroditism (one individual, two sexes in a lifetime). In general, for species where big males can control access to females (think harem), it pays to be female when small (you get to reproduce with the dominant male) and then turn into a male when you are big enough to duke it out with a competing male to win access to a group of females.  There are also protandrous hermaphrodites, who start male and turn to female (brace yourselves, Finding Nemo fans, the folks at The Fisheries Blog don't mince fins on this one)—more on that strategy later.

Back to why switching to male can make sense: sperm are less expensive to manufacture than eggs, so one male will be able to make lots more sperm than a similar sized female can produce eggs—as long as there are enough females churning out enough eggs for the male to fertilize, he'll make more offspring. The female-to-male sex switching strategy allows for maximum reproduction during all stages of life, when small and when larger...if the conditions are right.

Bucktooth parrotfish. Photo credit:  Kevin Bryant .

Bucktooth parrotfish. Photo credit: Kevin Bryant.

And here's where a little math goes a long way.

Unlike us, the bigger the fish, the more eggs or sperm they can produce. For our species, every woman is born with a set number of eggs, no matter if she grows to be a towering 5'11" or petite 5'0". And a man's sperm count has little to do with body size.  Not so for fish. The bigger the female, the larger the number of eggs she produces (and often, the higher quality).  A fish that doubles in size may gain over 10x as many eggs. This fact alone has major implications for management: catching one large female fish can reduce the reproductive output of a school by the same amount as several smaller females. Accounting for that in setting quotas and size limits can be really complicated, but it is critical to effective management. Saving these Big Old Fat Fertile Female Fish (BOFFFS) has become a focused effort of some conservation initiatives, including design of marine protected areas.

Which takes us back to the arithmetic those bucktooth parrotfish females are constantly conducting.  As they grow larger, they have to weigh which choice will bring more babies: changing to male and gaining access to multiple females or staying female, and having your eggs fertilized by the dominant male.  If the sum total of all the eggs produced by all the other females is more than your total number of eggs, it pays to switch and be the male. But, if as a large female you make more eggs than all the other females combined, than you should stay female.

Photo credit: Flickr Creative Commons

Photo credit: Flickr Creative Commons

If (female 1 + female 2 + female 3) < eggs than female 4, female 4 stays female.

If (female 1 + female 2 + female 3) > eggs than female 4, female 4 will likely switch to male and take over the harem (and thus begin fertilizing all those eggs).

A large female is especially likely to skip sex change if there are other males around crashing the love nest. In some species, the dominant male is not as dominant as he looks: "sneaker" males that hide nearby can dash in and add their own sperm to the mix (remember, fish have external fertilization, casting sperm and eggs into the water) right when the dominant male spawns. These "sneakers" create high levels of sperm competition that dilute the dominant male's success rates.  This is good for females, whose eggs are bathed by plenty o'sperm (and a greater diversity of genes), but it is bad news for the male. Studies have shown that in species where this happens, large females often choose not to turn into males. Such is the case with bucktooth parrotfish.

Instead of the standard large size/status-triggered sex change of most protogynous hermaphrodites, large female bucktooth parrotfish are constantly sizing up the other females in the harem, noting the sneakers streaking in, and doing a little mental math to figure out whether its worth it to change sex. And you thought figuring out your taxes was hard.