The hermaphroditic Serranus tortugarum is nothing if not progressive, a fish à la mode if ever there was one. Arguably, not since swallowing live goldfish was a thing among 20th-century college students has a gill-breathing aquatic chordate expressed more poignantly the spirit of the time.

We leave it to others to elaborate the cultural meanings, as indeed other news sources have done, directly or indirectly:

• “Relationship advice from a gender-bending fish that mates for life” (Science Daily)

• “Cooperative fish take turns with gender roles” (Phys.org)

From the National Geographic story:

New research published in Behavioral Ecology suggests that the small reef fish, no more than three inches long, may switch sex roles with their partner up to 20 times each day.

Chalk bass use a reproductive strategy known as “egg trading,” wherein they subdivide their daily egg clutch into “parcels” and alternate sex roles with their mating partner throughout a sequence of spawning bouts.

The fish demonstrated a remarkable commitment to varying their sex roles, explained Mary Hart, an evolutionary ecologist at the University of Florida and the lead author on the study.

…

Most hermaphrodites transition from one sex to another at some stage in their development, a strategy known as sequential hermaphroditism. The transformation is usually prompted by a social or behavioral trigger, like the loss of a dominant male from the social group. The chalk bass, however, is capable of producing both male and female gametes (sperm or eggs) simultaneously.

Though simultaneous hermaphroditism is not unique to chalk bass, it is rare, particularly because the fish do not self-fertilize. The frequency at which the fish switch sex roles is especially uncommon.

In the research, Dr. Hart collaborated with her husband Andrew Kratter. The couple, along with news writers at the University of Florida, emphasize the lessons for humans. From the press release:

“I found it fascinating that fish with a rather unconventional reproductive strategy would end up being the ones who have these long-lasting relationships,” [Kratter] said. “They live in large social groups with plenty of opportunities to change partners, so you wouldn’t necessarily expect this level of partner fidelity.”

That said, “The chalk bass… is not opposed to the occasional fling.”

If one partner has more eggs than the other, it may share the extra with other couples. Hart said this infrequent option, which happened only 20 percent of the time in the study group, may add stability to the system of simultaneous hermaphroditism paired with monogamy.

But the fish always returns to its mate at the end of the day.

This is more personal and revelatory than we’re accustomed to from a press release:

Hart said the loyal chalk bass offers humans in relationships this simple wisdom: You get what you give.

…

Hart and Kratter said delving into what drives the bond between monogamous animals has had an impact on their marriage.

Dr. Hart and Dr. Kratter are sufficiently discreet as to not go into additional details.

From an evolutionary perspective, why develop such a bewildering and presumably costly routine? It’s a “mystery,” Mary Hart concedes. National Geographic again:

However, she hypothesized that as long as the benefits outweigh the costs, this form of reciprocity may yield a reproductive advantage for the chalk bass. The sex switching offers each fish a return on their investment on eggs by allowing them to fertilize their partner’s eggs. Acting as both male and female improves their chances of passing on their genes to the next generation.

In Darwinian terms, the “benefits” would have to “outweigh the costs,” yielding a “reproductive advantage.” Otherwise how would the behavior evolve?

But in that case, if so wise and adaptive, why is it also supremely “rare”? Darwinism, we’re reminded again, is a theory that can accommodate any evidence, nearly as a flexible and easygoing as the chalk bass itself.

Photo credit: Mary Hart, University of Florida.

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