What is Happening to Fiordland's Sponges?
In May 2022 researchers from SFI group, who were working in Fiordland on a different project noticed something very unusual in the shallow waters of BreakSea sound, thousands of the ‘bleached’ white sponges. On closer inspection the sponges were all the same species, the cup sponge Cymastella lamellata. This bleaching event was subsequently confirmed throughout Dusky, Breaksea and Doubtbful Sounds, and is likely to have occurred throughout the wider Fiordland Marine Area (FMA). This sponge species is actually one of the most abundant species throughout the FMA, and we estimate that many millions of sponges have been effected.
Sponges live in association with many symbionts (symbiotic organisms), which are thought to be critical to their survival and ability to deal with stress. In the case of Cymbastella, it contains photosynthetic symbionts. We know very little about the process of sponge bleaching, but what we do know is that the Fiordland region experienced an extreme temperature event during May 2022, with the temperature being more than 5°C warmer than normal, and this heat wave was correlated with the bleaching event. We believe this extreme event is what caused the relationship between the sponge and its symbionts to breakdown and the sponges to bleach
Sponges are the most abundant animals living on the rocky walls across the FMA. They are suspension feeders, meaning they pumping large volumes of water through their bodies and feed on the bacteria and phytoplankton in the water. They are thought to then recycle this food material to organisms living on the seafloor. Loss of the sponges could have important impacts on other species.
In June the SFI team set out on an expedition to better understand this bleaching event. Onboard the boat we measured the respiration of the sponges to see if they were alive and assessed the health of the photosymbionts in the sponges, using PAM flurometry. We found the sponges were still alive, but the symbionts left in the bleached sponges were severely compromised. The bleached sponges also seem more susceptible to fish predation, so it might not be the bleaching that kills them, but the loss of the symbionts means the sponges become more attractive to fish as food. We conducted surveys to assess the degree of bleaching and so we can compare data with surveys we will do in the future and see if the sponges survive. We also collected specimens to see if the bleaching had caused any changes in the other symbionts in the sponges. Finally, we also conducted a short term temperature stress experiment on the boat, where we heated up non-bleached sponges to see if we could induce bleaching. After 5 days these sponges were just starting to bleach.
We will be working over the coming months and years to understand more about the mechanisms and consequences of this bleaching event. Monitoring will be a critical component of this, and this bleaching event highlights the importance of long-term monitoring, which really needs to be done more extensively and regularly in the fiords.
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Written by Prof James Bell