Some fish use the lunar cycle to manipulate the gender of their offspring

The moon has fascinated astronomers and poets alike for centuries, but did you know that it also influences the life-and-death struggles of some reef fish? As it turns out, for the six-striped wrasse, the lunar cycle plays a crucial role in determining its gender and survival.

Jeff Shima, a distinguished professor of ecology at Te Herenga Waka, Victoria University of Wellington, delved into the depths of reef life to understand the spawning patterns of the six-striped wrasse.

His latest study sheds light on how this species of fish manipulates the lunar cycle to give its offspring the best chance of survival, opening the door to some intriguing biological strategies.

Cracking the code of the lunar cycle

It’s no secret that Sixbars appear most often during the new moon. But here’s the kicker: offspring born at this time have the lowest chance of survival. A complete paradox, right?

Looking more closely, Shima’s research revealed that Sixbar offspring born during the new moon required a different rate of development.

“We know that sixbars spawn most frequently around the new moon, but this pattern is particularly puzzling because offspring born at this time have the lowest chance of survival. In contrast, offspring born closer to the full moon fare much better.

“We found that the Sixbar offspring born during the new moon had to change their developmental speed in order to survive. This ultimately influenced their sexual maturation and meant that they were more likely to become male,” said Professor Shima.

Moon game and the reef fish

That may sound like a risky gamble, but the study suggests it may be a calculated risk. Surprisingly, the few babies that survive tend to be male. And in the six-striped wrasse’s unique mating system, males have the advantage, as they can father many more offspring.

“Most of these babies will not survive, but the few that do are more likely to become male, and that’s like winning the lottery because males in the six-bar mating system can produce many more offspring.

“However, we believe the spawning patterns of these fish may be designed to spread their bets, with low-risk bets (spawning on a full moon) paired with some high-risk bets – in this case, spawning on a new moon.”

Reef fish in the company of the moon

The influence of the moon does not end there. Its cycle is also related to the growth rate of young Sixbars.

“As larvae, six-striped wrasse can extend or delay their development to arrive at the reefs at the most opportune time under the cover of darkness,” Shima adds.

The mystery of the lunar cycle and marine life continues to be unraveled, but this delicate balance presents many challenges.

“Moonlight is affected by artificial light from coastal areas at night, as well as dramatic changes in the distribution and intensity of cloud cover – a consequence of climate change,” notes Shima.

“These human-induced changes in nighttime lighting may have major and largely unrecognized consequences for the growth, survival and reproductive success of organisms in some areas.”

Importance of moon phases for reef fish

The spawning timing of the six-striped wrasse is an elegant dance marked by the rhythmic pulse of the moon phases.

As Shima’s research underlines, this synchronization is not a mere coincidence; it reflects an evolutionary strategy finely tuned over generations.

By timing their reproductive cycle to the new moon, crescent moon hybrids increase the likelihood that their eggs will be released during the ebb and flow of ocean currents, thus being distributed over large areas.

This strategy increases genetic diversity, which is paramount for the species’ resilience to environmental change and predation. The challenge is to ensure that enough offspring survive to maintain this delicate balance, as the numbers are heavily skewed against them.

Impacts on reef ecosystems

The implications of Shima’s findings extend beyond the six-striped wrasse and touch on broader questions about the conservation of marine ecosystems.

Given the extreme brightness of the moon and the introduction of artificial lighting, there is an urgent need to preserve the natural darkness of coastal habitats. This artificial lighting can disrupt the delicate synchronization between spawning and survival, potentially having cascading effects on reef biodiversity.

The resilience of the six-striped wrasse that Shima’s research uncovered is a powerful reminder of how closely the lives of marine animals are tied to their ecological context.

Therefore, understanding these interactions is crucial for the effective protection not only of the six-striped wrasse, but also of the diverse biodiversity in reef ecosystems.

The results lead us to an intriguing question: Have some species evolved reproductive strategies that use the moon as a guide to determine the sex of their offspring? Has nature evolved a celestial ballet between the moon and marine life?

While researchers work to unravel these mysteries, we may have to content ourselves with admiring the intelligence of species like the six-striped wrasse, which has found a way to dance with the moon.

The study was published in the journal Proceedings of the Royal Society B: Biological Sciences.

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