This crazy monogamous fish changes sex 20 times per day. Why?

Monogamy and hermaphroditism are not all that uncommon in the animal kingdom, but put both of these traits together and ramp them up to 11 and you have an unusual type of fish that changes its sex role more than 20 times per day while remaining faithful to its life partner.

This unusual species is known as the chalk bass (Serranus tortugarum), and according to a new study published recently in the journal Behavioral Ecology, this tiny reef fish lives in Panama, is neon green in color, grows to more than three inches in length, and enters a lifelong relationship with a mate that requires it to rapidly switch between male and female roles.

According to National Geographic, chalk bass utilize a reproductive technique known as “egg trading” in which they split their daily egg clutch into small groups, then alternate sex roles in repeated spawning bouts to ensure that they can fertilize as many eggs as they make. Switching roles encourages reciprocation and faithfulness from their mates, the study said.

“Our study indicates that animals in long-term partnerships are paying attention to whether their partner is contributing to the relationship fairly – something many humans may identify with from their own long-term relationships,” lead author Mary Hart, an adjunct biology professor at the University of Florida, said in a statement.

Unexpected behavior appears to provide a reproductive advantage

Switching sex roles is key to the survival of the chalk bass, Hart explained, because it is the only way that a member of the species can make sure its partner is contributing equally. Basically, if a member of the species wants its mate to produce more eggs, it first has to lead by example.

Hart told Nat Geo that, during her observations of the species, she found that individuals would rarely produce more than two egg parcels at one time before changing sex roles and encouraging their mate to follow their example. Her team monitored the chalk bass for six months and found that all of the couples remained together the entire time that both were living at the site.

“I found it fascinating that fish with a rather unconventional reproductive strategy would end up being the ones who have these long-lasting relationships,” said study co-author Andrew Kratter, an ornithologist with the Florida Museum of Natural History and Hart’s husband of 10 years. He noted that the species “live in large social groups with plenty of opportunities to change partners, so you wouldn’t necessarily expect this level of partner fidelity.”

Based on Hart and Kratter’s findings, the chalk bass looks to be a unique creature. Only three to five percent of all known species are monogamous, the researchers said, and only two percent of fish are estimated to be hermaphroditic, Nat Geo added. Unlike most of those species, however, the chalk bass can simultaneously produce both male and female gametes (sperm or eggs), which is a trait possessed by only a handful of subfamilies of primarily deep-sea fish.

Hart told Nat Geo that the frequency with which the carp bass switch sex roles is particularly unusual, and that she and Kratter are still uncertain exactly why they do change so many times. However, she hypothesized that the behavior likely provides the creatures with some sort of a reproductive edge, and that assuming both male and female roles ups the odds that the fish will be able to pass on their genes to their offspring.


Image credit: Mary K. Hart