Some fish use the moon cycle to manipulate their offspring's sex

The moon has fascinated astronomers and poets alike for centuries, but did you know it also holds sway over the life-and-death struggles of some reef fish? As it turns out, for the sixbar wrasse, the moon’s cycle plays a pivotal role in deciding their sex and survival.

Jeff Shima, an esteemed professor of ecology at Te Herenga Waka—Victoria University of Wellington, trekked into the depths of reef life to understand the spawning patterns of the sixbar wrasse.

His latest study illuminates how this fish species manipulates the moon’s cycle to give their offspring the best chance at survival, and throws open the doors to some mind-boggling biological strategies.

Cracking the moon cycle code

It’s no secret that sixbars spawn most often on a new moon. But here’s the clincher: offspring born at this time are least likely to survive. A downright paradox, right?

Drilling deeper, Shima’s research revealed that sixbar offspring born on a new moon needed to change their developmental rates.

“We’ve known sixbars spawn most often on the new moon, but this pattern has been particularly puzzling because offspring born at this time are least likely to survive. In contrast, offspring born closer to the full moon do much better.

“What we found is that in order to survive, sixbar offspring born on a new moon needed to alter their development rates, which ultimately shaped their sexual maturation and meant they were more likely to become male,” said Professor Shima.

Moon gambles and the reef fish

It may sound like a high-stakes gamble. However, the study suggests that it might be a calculated risk. Surprisingly, the few babies who manage to survive are more likely to become male. And in the unique mating system of the sixbar wrasse, males have the advantage as they can produce many more offspring.

“Most of these babies won’t survive, but the few that do are more likely to become male and that’s akin to winning the lottery as males can produce many more offspring in the sixbar mating system.

“However, we think the spawning patterns of these fish may be calculated to spread their bets, with low-risk wagers (full moon spawning) coupled with some high-risk ones – in this case, spawning on the new moon.”

Reef fish in the company of the moon

The moon’s influence doesn’t end here. Its cycle has also been linked to the growth rates of young sixbars.

“As larvae, sixbar wrasse can lengthen or delay their development to arrive at the reefs at the most opportune times, under cover of darkness,” adds Shima.

The mystery of the lunar cycle and sea-life continues to unfold. Yet, the challenges in this delicate balance are many.

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

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

Importance of moon phases for reef fish

The timing of spawning events for the sixbar wrasse is an elegant dance shaped by the rhythmic pulse of the moon’s phases.

As Shima’s research underscores, this synchronization is not merely coincidence; it reflects an evolutionary strategy fine-tuned over generations.

By aligning their reproductive cycles with the new moon, sixbars increase the chances of their eggs being released amid the ebb and flow of ocean currents, dispersing them across vast areas.

This strategy enhances genetic diversity, which is paramount for the species’ resilience against environmental changes and predation. The challenge lies in ensuring that enough offspring survive to continue this delicate balance, as the numbers are heavily skewed against them.

Implications for reef ecosystems

The implications of Shima’s findings extend beyond the sixbar wrasse, touching upon broader conservation concerns within marine ecosystems.

With the moon’s luminance swinging between extremes and the introduction of artificial light, there is an urgent call to preserve the natural darkness of coastal habitats. This artificial illumination can disrupt the delicate synchrony between spawning and survival, potentially leading to cascading effects on reef biodiversity.

The resilience of the sixbar wrasse, as unveiled in Shima’s research, serves as a poignant reminder of how intricately connected the lives of marine species are to their environmental context.

Thus, understanding these interactions is critical for the effective preservation of not only the sixbar wrasse but also the rich tapestry of life within reef ecosystems.

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

While researchers work to unravel these mysteries, we might just have to be content with marveling at the intelligence of species like the sixbar wrasse, who’ve found a way to dance with the moon.

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

—–

Like what you read? Subscribe to our newsletter for engaging articles, exclusive content, and the latest updates. 

Check us out on EarthSnap, a free app brought to you by Eric Ralls and Earth.com.

—-