Weird & Wild

These Fish Swap Their Sex Up to 20 Times a Day

The hermaphroditic chalk bass, a Caribbean native, switches from female to male and back again, multiple times a day.

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A chalk bass explores the coral reef near the Smithsonian Tropical Research Institute in Panama. This fish has been marked with an individualized color code so that its behavior can be recorded during spawning.

The natural world offers many curiosities, but hermaphroditism—the presence of both male and female reproductive organs—may be among the most peculiar.

Take the chalk bass (Serranus tortugarum), for instance. 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.

Hart found that individuals would rarely produce more than two egg parcels consecutively before switching roles to ensure reciprocation from their partner. This attention to reciprocity helps to maintain cooperation among the partners and reduces the temptation of cheating.

Simultaneous Hermaphrodites

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. Hart said it still remains a mystery why they switch so many times.

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.

It is estimated that about 2 percent of fish species are hermaphroditic, but simultaneous hermaphrodites are uncommon and are limited to only a handful of subfamilies, said Eric Fischer, an expert in evolutionary ecology now with the Congressional Research Service. Additionally, many of the other simultaneous hermaphroditic fish are deep-sea species and difficult to study, he added.

This study is one of the first to quantify the mating habits of the chalk bass and other simultaneous hermaphrodites, and also provides evidence to corroborate some long-held theories on sexual cooperation among fish.

Tight Bonds

Hart spent six months researching the chalk bass at the Smithsonian Tropical Research Institute in Panama, observing the fish through daily dives on coral reefs. During the course of the research, she was also surprised to learn that the chalk bass displayed admirable devotion to their partners.

“All of the fish that I marked in the first month were together for the entire six months until one or both of them disappeared from the social group,” said Hart.

Though not fully monogamous—mating is often interrupted by male streakers that try to shoot between the mating couple—the fish returned to their partners day after day, for months at a time.

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