Environment Mass bleaching of sponges in New Zealand

Marine sponges have bleached along hundreds of kilometers of coastline in New Zealand. A large proportion are likely to die off.

Not only corals can bleach, but sponges are also affected. Researchers discovered a particularly large mass bleaching event in Fiordland, New Zealand, in 2022: according to their findings, around 66 million marine sponges were affected.

The population of the local sponge species Cymbastella lamellata is estimated to have been reduced by almost half along the approximately 1,000-kilometre-long coastline, according to a study by the researchers. "This was not only the largest sponge bleaching event observed worldwide, but also the largest sponge die-off ever recorded," says marine biologist James Bell from Te Herenga Waka - Victoria University of Wellington, lead author of the study.

Up to 4.4 degrees warmer than usual

According to the study, the bleaching occurred during a heatwave that lasted 259 days. This caused water temperatures to rise to up to 4.4 degrees Celsius above average. As a result, the shell-shaped animals changed their color: from a dark brown to a bright white. According to the study, around 20 percent of the sponges along the coast were completely bleached, 75 percent were partially bleached and only 5 percent were unaffected.

"Sponges form symbiotic relationships with a wide range of organisms, from diatoms to crustaceans and zooanthids to a very large variety of microorganisms," explains Bell. The relationships with microbes are particularly important, as it is assumed that they help the sponge to survive - especially under stress conditions.

Diatoms give color

Similar to coral bleaching, the sponge loses diatoms from its tissue, which give it its color under normal circumstances. Whether the bleaching also leads directly to the death of the sponges is not yet certain, explains Bell. It is possible that the sponges were eaten by fish because they became more palatable or could be seen more easily by fish.

The researchers also discovered that some sponges were able to recover from the bleaching. "This makes us optimistic that sponges may be able to adapt to the warming of the water," says Bell. This is because heatwaves in the sea are becoming more frequent and more intense as the climate warms. This has a significant impact on marine life, the scientists write.