Sea sponges have been thought to be more resilient to ocean warming than other organisms. But earlier this year, New Zealand recorded its largest-ever sponge-bleaching event off its south coast.
While only one species, the shell sponge Cymbastella lamellata, was affected, an ongoing ocean heatwave turned millions of the normally dark brown sponges bright white.
We then reported tissue loss, decay and death of other sponge species on the north coast of New Zealand, with estimated impacts on hundreds of thousands of specimens. In contrast, despite extensive surveys, we have observed no bleaching or tissue loss in central areas of New Zealand’s coast.
Our latest research in Current Biology shows that the most severe impacts on sponges occurred in areas where the marine heatwave was most intense. The loss of sponges can have a major impact on the entire ecosystem.
Why should we care about sponges?
Sponges are among the oldest and most abundant animals found on rocky reefs around the world. In New Zealand they occupy up to 70% of the available seabed, especially in so-called mesophotic ecosystems at depths of 30-150 m.
They fulfill a number of important ecological functions. They filter large volumes of water, trap small food particles, and transport carbon from the water column to the sea floor, 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 seabed, which provides habitat for a range of other species such as crabs, shrimp and starfish.
Sponge bleaching, tissue loss and decay
Like corals, sponges contain symbiotic organisms that are 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 in the spongy tissue and exchange food for protection. As the sponge bleaches, it expels the diatoms, exposing the spongy skeleton.
Tissue loss occurs when sponges are stressed and either need to invest more energy in cell repair, or when their food source is exhausted and they reabsorb their own tissues to reduce body volume and reallocate resources.
Tissue breakdown or necrosis, on the other hand, is generally associated with changes in the microbial communities living in sponges and the growth of pathogenic bacteria.
Bleaching, tissue loss, and putrefaction in sponges have all been previously linked to heat stress, but did not necessarily lead to the death of the sponges. In other places where such effects have been observed, they have been much more localized compared to what we have seen in New Zealand.
The effects of marine heat waves
Marine heat waves are defined as unusual periods of warming lasting five consecutive days or more. Some can last weeks to several months and span hundreds or thousands of kilometers of coastline.
Sponge bleaching and tissue loss or decay in New Zealand matched the duration and intensity of the marine heatwaves in northern and southern New Zealand in the summer of 2021/2022. The Hauraki Gulf, where sponge necrosis and decay was reported, was in a continuous marine heatwave for 29 weeks from November 2021 to late May 2022 with a maximum intensity of 3.77℃ above normal.
A sustained marine heatwave developed in Fiordland in early February 2022, lasting 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) lower-intensity marine heatwaves, and we did not observe any impact on sponges.
These extreme heat events can result from a combination of changes in air-sea heat exchange, wind patterns, and ocean currents. Their likelihood is also influenced by large-scale climate patterns such as the El Niño Southern Oscillation (ENSO).
What the future may bring
Most global research on the effects of climate change has focused on experimental studies exposing organisms to temperatures predicted for 2100, which are often 2–4℃ warmer than current temperatures. But the occurrence of marine heat waves means that organisms are already experiencing these temperatures, sometimes for several weeks or months. By 2100, marine heat waves will become even more extreme.
For bleached Cymbastella, recent anecdotal reports suggest that many sponges have regained their color, which is good news. However, observations immediately after bleaching suggest that many sponges were eaten by fish, possibly because their symbionts provide chemical defenses against predators.
For bleached corals, studies have shown effects on spawning success many years after the event, probably because their energy reserves are depleted.
We do not yet know whether this is the case with sponges. For sponges with decayed tissue, the prospects are even less clear, as many likely died.
Sponges are not the only species affected by the marine heatwaves New Zealand experienced in 2021/2022. There have been reports of algae die-offs and changes in the normal distribution patterns of tuna and other ecologically and commercially important fish species.
Heatwaves in the sea should be at the forefront when thinking about climate impacts. They’re happening now, not in 50 years, and we don’t yet know enough to determine whether sponges could be the canary in the coal mine.
This is particularly important as New Zealand’s northern coasts have already been hit by almost continuous marine heatwaves, with ongoing events expected to continue into next summer.
James J. Bell et al, Marine Heat Waves Drive Bleaching and Necrosis of Temperate Sponges, Current Biology (2022). DOI: 10.1016/j.cub.2022.11.013
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Citation: Why sponges may be the ‘canary in the coal mine’ for marine heatwave effects (2022 December 5) retrieved December 5, 2022 from https://phys.org/news/2022-12-sponges-canary- coal-impact-marine.html
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