images have emerged from New Zealand showing millions of once-velvety brown sea sponges bleached bone white, the worst mass bleaching event of its type ever recorded, marine scientists say.

The alarming discovery comes amid a continued rise in ocean temperatures, a trend that scientists say is overwhelmingly due to planet-warming fossil fuel emissions.

New Zealand scientists discovered thousands of bleached sea sponges in May of this year, in cold waters off the country’s southwestern coast. Further findings showed the damage was far worse, with millions — possibly tens of millions — of sea sponges affected throughout the Fiordland region.

“This is one of the most abundant sponges in Fiordland, and so it’s a really wide-scale event,” said James Bell, a marine biology professor from New Zealand’s Victoria University.

Bell, who led the team responsible for spotting the initial bleaching event last month, said that despite the extensive mass bleaching, some sponges were still alive and consuming oxygen.

“This region was so abundant and rich in marine life and it was almost like a white graveyard when we discovered it, it was really devastating and traumatic,” he said. “We are able to conduct experiments on board our boat to try and understand how affected the sponges were by warmer temperatures. Unfortunately a lot of them were already very unhealthy and stressed.”

Sea sponges come in a variety of sizes, colors and textures and play a crucial role in marine ecosystems, providing food and refuge for other marine animals like crabs, algae and fish.

“They pump out large volumes of water and capture tiny particles, bacteria, plankton and algae and also recycle carbon on the sea floor,” Bell said. “They also provide shelter for marine creatures and increase habitat areas of the sea floor. They are very underappreciated creatures.”

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