News release

From: The Conversation

By James Bell (Victoria University of Wellington), Nick Shears (University of Auckland), and Robert Smith (University of Otago). This is an excerpt from a longer article in The Conversation which can be republished under Creative Commons license.

Marine sponges were thought to be more resilient to ocean warming than other organisms. But earlier this year, New Zealand recorded the largest-ever sponge bleaching event off its southern coastline.

Our latest research shows the most severe impacts on sponges occurred in areas where the marine heatwave was most intense. The loss of sponges may have major repercussions for the whole ecosystem.

They serve a number of important ecological functions. They filter large quantities of water, capturing small food particles and moving carbon from the water column to the seafloor where it can be eaten by bottom-dwelling invertebrates. These invertebrates in turn are consumed by organisms further up the food chain, including commercially and culturally important fish species.

Sponges also add three-dimensional complexity to the sea floor, which provides habitat for a range of other species such as crabs, shrimps and starfish.

Most global research on climate change impacts has focused on experimental studies exposing organism to temperatures predicted for 2100, often 2-4℃ higher than current temperatures. But the occurrence of marine heatwaves means organisms are already experiencing these temperatures, sometimes for several weeks or months. By 2100, marine heatwaves will become even more extreme.

For bleached Cymbastella lamellata, recent anecdotal reports suggest many sponges have recovered their colour, which is good news. However, observations immediately after the bleaching indicate many sponges were being eaten by fish, possibly because their symbionts may provide chemical defences against predation.

For bleached corals, studies have shown impacts on spawning success for many years after the event, likely because their energy reserves have been depleted.

We don’t yet know if this is the case for sponges. For sponges with decayed tissue the outlook is even less clear, as many probably died.

Sponges are not the only species to be affected by marine heatwaves New Zealand experienced in 2021/2022. There were reports of seaweed die-offs and changes to normal distribution patterns of tuna and other ecologically and commercially important fish species.

Marine heatwaves should be front of mind when thinking about climate impacts. They are happening now, not in 50 years, and we don’t know enough yet to determine if sponges may be the canary in the coal mine.

This is especially important because New Zealand’s northern coastlines are already experiencing almost continuous marine heatwave conditions, with the ongoing event forecast to extend into the coming summer.