Why sponges may be the ‘canary in the coal mine’ for impacts of marine heat

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.

While only one species, the cup sponge Cymbastella lamellata, was affected, a prolonged marine heat wave turned millions of the normally dark brown sponges bright white.

Subsequently, we reported tissue loss, decay and death of other sponge species across the northern coastline of New Zealand, with an estimated impact on hundreds of thousands of specimens. In contrast, we didn’t observe any bleaching or tissue loss in central areas of New Zealand’s coastline, despite extensive surveys.

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

Why should we care about sponges?

Sponges are among the most ancient and abundant animals on rocky reefs across the world. In New Zealand, they occupy up to 70% of the available seafloor, particularly in so-called mesophotic ecosystems at depths of 30-150m.

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.


Why sponges may be the 'canary in the coal mine' for impacts of marine heatwavesCC BY-ND“/
A bleached (left) and healthy (right) cup sponge Cymbastella lamellata. Credit: Valerio Micaroni and Francesca Strano, CC BY-ND

Sponge bleaching, tissue loss and decay

Like corals, sponges contain symbiotic organisms thought to be critical to their survival. Cymbastella lamellata is unusual in that it hosts dense populations of diatoms, small single-celled photosynthetic plants that give the sponge its brown color.

These diatoms live within the sponge tissue, exchanging food for protection. When the sponge bleaches, it expels the diatoms, leaving the sponge skeleton exposed.

Tissue loss occurs when sponges are stressed and either have to invest more energy into cell repair or when their food source is depleted and they reabsorb their own tissue to reduce body volume and reallocate resources.

Tissue decay or necrosis on the other hand is generally associated with changes in the microbial communities living within sponges and growth of pathogenic bacteria.

Bleaching, tissue loss and decay in sponges have all previously been associated with heat stress, but didn’t necessarily result in sponge death. In other places where such impacts have been observed, they were much more localized, compared to what we saw in New Zealand.


Why sponges may be the 'canary in the coal mine' for impacts of marine heatwavesCC BY-SA“/
Typical sponge reefs from Northland, New Zealand. Credit: Author provided, CC BY-SA

The impact of marine heat waves

Marine heat waves are defined as unusual periods of warming that last for five consecutive days or longer. Some can last from weeks to several months and extend over hundreds or thousands of kilometers of coastline.

The sponge bleaching and tissue loss or decay in New Zealand matched the duration and intensity of marine heat waves to the north and south of New Zealand during the summer of 2021/2022. The Hauraki Gulf, where sponge necrosis and decay was reported, was in a continuous marine heat wave for 29 weeks from November 2021 to the end of May 2022, with a maximum intensity of 3.77℃ above normal.

In Fiordland, a prolonged marine heat wave developed in early February 2022 and persisted for more than 16 weeks into May, with a maximum intensity of 4.85℃ above normal temperatures. In contrast, the Wellington and Marlborough Sounds regions experienced only short (weeks) marine heat waves with a lower intensity and we did not observe any impacts on sponges.

These extreme heat events can result from a combination of changes in the heat exchange between the air and the sea, wind patterns and ocean currents. Their likelihood is also influenced by large-scale climate patterns such as the El Niño Southern Oscillation (ENSO).

Moana Project, CC BY-SA

Why sponges may be the 'canary in the coal mine' for impacts of marine heatwavesMoana Project, CC BY-SA“/

Read More: Why sponges may be the ‘canary in the coal mine’ for impacts of marine heat

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