New Zealand is experiencing the largest bleaching of sea sponges, scientists say, after extreme ocean temperatures have turned millions of the aquatic creatures white.
The discovery came after researchers raised the alarm in May, when sea sponges off New Zealand's southern coastline were found bleached for the first time.
Initially, researchers estimated hundreds of thousands of sponges had been bleached, but over the past month, scientists conducted investigations at coastlines around the country, and found that millions possibly tens of millions had been transformed bone-white.
As far as we know, it is the largest scale and largest number of sponges bleached in one event that has been reported in cold waters and nowhere in the world, said James Bell, marine ecologist at Victoria University.
When members of Bell's team spotted the May bleaching event in Fiordland, they sent the word out to the department of conservation and other charter vessels around the region to see if it had been seen in other Sounds.
He said that they pretty much reported the bleaching everywhere they went. The team believes there are at least millions of sponges, maybe many millions of sponges that have undergone this bleaching Sea sponges, like coral, which rely on symbiotic organisms that photosynthesise inside them, providing food for the sponge and sometimes deterring predators.
While bleaching does not kill the sponges, it evicts those organisms that lower the chemical defences of the sponges and depriving them of food. Bell said that some species can recover from severe bleaching, but others don't.
Oceanographer Dr Robert Smith, the University of Otago oceanographer, said two marine heatwaves in New Zealand had created record ocean temperatures in some areas that were five degrees hotter than normal.
He said that the longest and strongest marine heatwave has occurred in the north and southern limits of New Zealand in 40 years since satellite based measurements of ocean temperature began in 1981.
Smith said that the marine heatwave had begun in September last year and was only just ending on 213 days, and was only just beginning to end in some areas.
The unusual aspect of seeing these unusually warm temperatures last such a long period of time is what Smith said.
Some organisms are going to be OK with a day or a week above average temperatures, but once you start accumulating that heatstroke, you're going to feel the effects. Smith said that it was difficult to attribute a single heatwave event to the human-made climate crisis, but ocean temperatures were rising around the world.
He said that projections indicated that the heatwaves were getting more extreme and longer in the future and that there has been a significant increase in the frequency, duration and intensity of marine heatwaves over the past century.
We're seeing a window into what our oceans will look like for our kids and our grandkids.