Warming seas caused Singapore's corals to suffer from the longest-ever bleaching incident on record last year, and observations show that the incident has far-reaching effects.
The reproductive capacity of Singapore's corals was reduced earlier this year, even though they have started to recover from bleaching since late October 2016.
Scientists have observed that some species of hard coral, including Merulina ampliata, Platygyra sinensis and Platygyra pini, spawned outside of the usual reproductive season.
In Singapore, the main mass coral spawning event happens just once a year, usually from the third night after the full moon in late March or April. This year, spawning happened in end-April.
"But after the full moon in May, we observed some corals, which missed the season in April, spawning," said Dr Karenne Tun, director of the coastal and marine division at the National Parks Board's (NParks) National Biodiversity Centre.
Spawning is an energetically expensive process for the corals. During the process, the corals release millions of egg and sperm bundles into the water column at the same time in a spectacular underwater display.
The eggs and sperms then join to form free-floating larvae, which are carried by the water until they find a suitable home - usually a hard surface they can latch on to. But less than 1 per cent of this free-floating larvae eventually settle down and get recruited onto a reef. Many end up being eaten by predators, or fail to settle on a suitable substrate.
Researchers who observed the event in April this year had found it to be more subdued compared with previous years. This is likely because Singapore's corals, which were then still recovering from the bleaching incident last year, had more critical resource concerns.
The bleaching, which stretched from June to December, was the longest coral-bleaching incident to hit local reefs.
Corals depend on symbiotic algae, called zooxanthellae, for energy. Bleaching occurs when abnormally high sea temperatures cause corals to expel the algae, turning the corals white and depriving them of a key source of nutrition.
Following the April event, NParks researchers monitoring the reefs found that egg-sperm bundles were still present in some coral colonies. This discovery led them to hypothesise that these corals may spawn later. They were proven right.
"The fact that there was spawning, even though it was delayed, is an encouraging sign," said Dr Tun.
"But if warming seas continue to affect Singapore, resulting in reduced number of individuals or species spawning, then the number of successful recruits could fall and reduce the rate of reef expansion."
Healthy coral reefs are important as they support a diverse ecosystem and function as a nursery for many species of reef organisms.
Assistant Professor Huang Danwei, from the National University of Singapore's Department of Biological Sciences, said a protracted spawning season suggests that corals are adapting to rising sea temperatures and more stressful conditions in general.
"Spreading their egg release over a longer time period is akin to not putting all their eggs in one basket - in this case, a time basket.
"This could increase the likelihood that at least some eggs will survive during stressful conditions," said Prof Huang, a marine biologist.
But he said that while this strategy increased the average survival rate of eggs, it also lowered the probability that they could be fertilised and coral larvae develop to adulthood.
Said Prof Huang: "Delayed spawning year after year could thus be detrimental to the recruitment of corals onto the reef as successful development of corals relies on the synchronised coral spawning that has been calibrated over millions of years of reef evolution."