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Jellyfish Blooms Could Increase the Risk of Bacteria-Spread Illnesses in Humans and Marine Animals

Basso, Lorena, et al. “Jellyfish summer outbreaks as bacterial vectors and potential hazards for marine animals and humans health? The case of Rhizostoma pulmo (Scyphozoa, Cnidaria).” Science of the Total Environment, vol. 692, 2019, pp. 305-318. doi:10.1016/J.SCITOTENV.2019.07.155.

Happy Thanksgiving! With Black Friday right around the corner, it might be surprising to hear that humans aren’t the only species that swarm to certain areas. 

An adult barrel jellyfish (R. pulmo) (Photo by Roberto Pillon, WoRMS)


Jellyfish blooms occur when jellyfish reproduce and proliferate rapidly in a given body of water. While researchers aren’t sure what exactly causes these blooms, they think it could be due to a combination of factors. The blooms seem to be occurring more often lately, and this could be due to “habitat modification, shipping and species translocation, overfishing, eutrophication, and by ocean warming,” according to Basso et al, who are the authors of this recent study on jellyfish blooms.


What’s the problem with too many jellyfish?  

Jellyfish outbreaks are a serious environmental problem as well as a serious economic problem. Over-predation by jellyfish can damage an ecosystem, and the resulting loss of biodiversity can be devastating to the fishing industry. Over the past six years, the barrel jellyfish (Rhizostoma pulmo) blooms in the Mediterranean have become increasingly common. Basso’s study focuses on how the increasing frequency of jellyfish blooms may contribute to illnesses spread by bacteria. These illnesses could potentially make their way to other marine animals or even humans. While this jellyfish has been targeted for pharmaceutical use due to the diverse microbes that live in parts of the jellyfish tentacles, there are also concerns about the jellyfish being vectors for disease and illness for the same reason.

How do we know which microbes are harmful? 

In July 2016, a bloom occurred in the Gulf of Taranto, and this is where Basso and her team chose to do their experiment. Samples from the mucus membrane of 25 barrel jellyfish were collected and analyzed in a clean lab, then cultured for DNA, which allowed the team to quantify the amount and species of microbes present. Figure 1 shows the abundance and microbial diversity in each part of the jellyfish, with the mucus having the highest diversity. The mucus secreted by the jellyfish has several functions, including protecting eggs and preventing the accumulation of debris on the jellyfish’s dome. The results from the experiment were compared against existing data from a gene identification library in order to determine which microbes were present in the mucus of these jellyfish and whether they may be carriers for dangerous pathogens.

Figure 1: The main parts of an adult jellyfish, with the microbial diversity and abundance of each. (From Basso et al 2019)

What did they find? 

Some of the microbes and bacteria found in the mucus were the same as previously identified ones that caused infections in salmon and were fatal to crayfish. This indicates that barrel jellyfish could function as a carrier for diseases that affect marine animals. The mucus is especially an area of concern because it is a carbon-rich substance, which could considerably speed up the microbial growth rate, since many microbes use carbon as their food source. It is currently unclear whether some of these microbes are in a symbiotic relationship with the jellyfish, but Basso and her team recommended further research on the subject. They did find one genus of pathogen called Vibrio that is known to cause gastroenteritis, wound infections, septicemia and severe necrotizing infections of soft tissues in humans. Since the number of cases of these illnesses has risen in the past few years, there may be a connection between the blooms and these conditions. Fishermen who had been fishing or trawling near jellyfish blooms also reported pain and swelling shortly after interacting with the residual seawater.

So, what’s the takeaway? 

While it might be tempting to swim through a jellyfish bloom and live out your dreams of being Dory from Finding Nemo, it’s probably better for your health if you don’t. This study found five major bacteria groups of concern to either marine animals and humans: Chryseobacterium, Flavobacterium and Tenacibaculum, Coxiella and Vibrio. However, the blooms could also indicate warming temperatures in the area, making them key for identifying the effects of climate change in a region. Experiments like the one Basso and her team have done will become increasingly important in the coming years, as warming continues to intensify and ecosystems change in unexpected ways.



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