杏吧原创

When hammer coral says, 鈥渘ew year, new me鈥�, it truly means it: many colonies of the coral species change their sex annually. The pattern seems to be a first among animals.

In 2011, at the National Taiwan Ocean University and his colleagues were surveying hammer corals (Fimbriaphyllia ancora) in the tropical West Pacific Ocean. The researchers discovered that many of the colonies had changed sex since the previous year.

At that time, the only coral group known to change sex was mushroom corals, which 鈥� unusually for corals 鈥� are generally solitary and can move around a reef. Unlike mushroom corals, F. ancora dwells permanently in single-sex colonies containing individuals that are either all male or all female. Shikina and his colleagues decided to study hammer corals during several reproductive seasons to learn more about this sex-swapping phenomenon.

Over eight years, the team kept tabs on 26 different hammer coral colonies at two sites in Taiwan鈥檚 Nanwan Bay, tracking their sex year to year. Nineteen of the colonies 鈥� about 73 per cent 鈥� shifted between sexes every year. The other seven colonies remained fixed as either female or male.

The team took samples from the sex-changing colonies and looked at their gonads under a microscope. The researchers found that, in the months following sperm and egg release, testes tissue wanes before converting into an ovary and vice versa. This is similar to a process that certain sex-changing fish also undergo.

Unlike other 鈥渂idirectional hermaphrodites鈥� that can alternate between male and female forms, hammer corals appear to be the first to do so on an annual schedule. at the University of Salford in the UK is familiar with coral species that are 鈥渟imultaneous hermaphrodites鈥�, having both male and female reproductive organs at all times, and those that change from one sex to another under certain circumstances. There is even that repeatedly trades off between male and female reproduction, although 鈥� unlike the hammer coral 鈥� it maintains both sets of sex organs and merely activates one at a time. But 鈥渢his new knowledge expands the sexual plasticity of corals beyond what I was expecting鈥�, Benvenuto says.

The hammer corals鈥� sex changing doesn鈥檛 seem to be related to the colonies鈥� size, as can be the case in other marine animals that change sex. Nor is it due to environmental factors, since sex-changing and sex-fixed colonies can be found a metre apart.

Shikina and his colleagues suggest a different explanation: having both types of colonies in the population might be a good way to ensure successful mating. Since the corals can鈥檛 relocate, they can鈥檛 seek out mating partners. Having neighbours that alternate between expelling sperm and eggs may increase the odds of finding a match at least every other year.

at the University of Akron in Ohio wonders if this type of cyclical sex switcheroo is more widespread in corals than researchers have recognised.

鈥淚f these corals can switch so readily, it may mean that there is a lot of this going on in corals that people haven鈥檛 determined yet,鈥� says Weeks. 鈥淚t would take the long-term studies that these researchers did to figure that out.鈥�