Fish develop their sex based on phases of the Moon

The phase of the Moon influences the sex of new-born fish, according to a new study.

The study, published in Proceedings of the Royal Society B: Biological Sciences, finds that sixbar wrasse are more likely to develop as male if spawned at the new moon.

Sixbar wrasse (Thalassoma hardwicke) are small fish that live on coral reefs in the Indian and western Pacific oceans.

These fish develop their sex after they’re born, with different social environments influencing their sexual maturation.

“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,” says lead author Professor Jeff Shima, an ecologist at Te Herenga Waka—Victoria University of Wellington, New Zealand.

Shima, along with colleagues from the USA and Australia, studied a population of sixbar wrasse in Mo’orea, French Polynesia.

“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,” says Shima.

Fish spawned on the new moon were more likely to find the best quality sites, settle them at an older age and with a larger body, and be part of social groups, all of which made them more likely to mature as male.

“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,” says Shima.

“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.”

This phenomenon could be happening in other marine species.

“We know many marine organisms – fishes and invertebrates – reproduce on a lunar cycle,” says Shima.

“These patterns may be adaptive, allowing either the parents or their offspring to capitalise on the beneficial effects of tides or food availability, or to avoid predators.

“But we still have a lot to learn about when marine animals reproduce, and when, where, and how their offspring develop—in short, some of the greatest mysteries in nature.”

But the researchers are concerned for the effect that artificial light, and climate change, may have on these patterns.

“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,” says Shima.

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