A Summer of Jellyfish

Jellyfish are ancient creatures, having been around for more than 500 million years, with their body structure composing of around 90% water, and characteristically no brain or heart (Buglife, 2013). 2015 saw huge numbers of jellyfish being sighted around Britain’s coastlines.

Barrel jellyfishThe Marine Conservation Society’s National Jellyfish Survey, a citizen science project where members of the public report their sightings, appeared to show an increase in jellyfish numbers in certain species across the United Kingdom, in particular the barrel jellyfish, Rhizostoma octopus. This jellyfish (pictured) is the largest jellyfish to be found in UK waters. It can weigh around 25 kg and grow to 1 metre in width (DWT, 2015), therefore it is no wonder they are often referred to as dustbin-lid jellyfish. Although some would consider it a rather ‘scary’ sight to come across while swimming, they do not have a strong sting which can harm people, but instead feed on plankton. Their lack of tentacles but presence of eight thick arms, with frills which comprise of tiny tentacles and mouths, and bell edge which is covered in blue organs, known as statocysts which aid balance (Buglife, 2013), makes this species easily recognisable.

Barrel jellyfish contributed to around 75% of sightings for the national survey, increasing from 40% last year, mainly in the South/South West of England (MCS, 2015). The question is though, why are these jellyfish suddenly appearing in larger numbers? There could be any number of reasons for this, but swarming often occurs when there is an abundance of food, in this case a significant plankton bloom, often influenced by heatwaves and currents which can cause a nutrient upwelling from deep waters (DWT, 2015). During the summer months these blooms may occur in shallow, warm water along the coast, therefore increasing jellyfish sightings. Come winter however the jellyfish will drift back into deeper water (Buglife, 2013). Climate change may also have an impact, as sea temperatures increase, as well as ocean acidification (MCS, 2015), who knows what effect this is having on jellyfish numbers.

This phenomenon of jellyfish blooms however has been occurring throughout history and is not a new spectacle. Reviews of Rhizostoma species blooms throughout Western Europe showed there were hotspots where these events occurred, with other areas only having these events from time to time (Lilley et al, 2009), suggesting that the varied conditions of these areas influence the chance of blooms happening. Aerial surveys can be used to study the aggregations of jellyfish, helping to assess range and abundance, with barrel jellyfish being especially suited to this technique (Houghton et al, 2006a). More research is however needed as the exact reason for the large number of jellyfish sightings this year is not known.

Not only are these swarms of jellyfish an incredible visual spectacle but they also provide a food source for ocean sunfish and leatherback turtles. Studies in the Northeast Atlantic have suggested that jellyfish bloom hotspots could explain around 22.5% of the distribution of leatherback turtles, as well as influencing long-term foraging patterns (Houghton et al, 2006b). The jellyfish themselves also support Hyperia galba, an amphipod crustacean, which makes its home in cavities on the body of barrel jellyfish (Buglife, 2013).

It is not only barrel jellyfish however which have been sighted across Britain, with the blue, compass, lion’s mane, mauve stinger and by the wind sailor jellyfish also being reported. 30 Portuguese man o’ war sightings were also recorded in Cornwall and Devon, this being one of the species which has a painful sting (MCS, 2015).

Jellyfish blooms can however sometimes have economic impacts, such as when moon jellyfish blooms caused UK nuclear power plant closures as well as mauve stinger jellyfish blooms having been responsible for a reduction in fish farm salmon stocks (MCS, 2015). As ocean conditions continue to change, whether this is from climate change causing increased sea temperatures or overfishing influencing trophic structures, jellyfish look to be able to cope with changing conditions and may therefore continue to be around for the next million years.



Buglife. (2013) Barrel Jellyfish [www document]. https://www.buglife.org.uk/bugs-and-habitats/barrel-jellyfish (Accessed 09/09/2015).

DWT. (2015) Jellyfish Sightings on Dorset Beaches [www document]. http://www.dorsetwildlifetrust.org.uk/jellyfish_sightings.html (Accessed 09/09/2015).

Houghton, J., Doyle, T., Davenport, J. and Hays, G. (2006a) ‘Developing a simple, rapid method for identifying and monitoring jellyfish aggregations from the air’, Marine Ecology Progress Series314, 159-170.

Houghton, J., Doyle, T., Wilson, M., Davenport, J. and Hays, G. (2006b) ‘Jellyfish aggregations and leatherback turtle foraging patterns in a temperate coastal environment’, Ecology87, 1967-1972.

Lilley, M., Houghton, J. and Hays, G. (2009) ‘Distribution, extent of inter-annual variability and diet of the bloom-forming jellyfish Rhizostoma in European waters’, Journal of the Marine Biological Association of the United Kingdom, 89, 39-48.

MCS. (2015) ‘Smacks’ Shouldn’t be Ignored as 2015 looks set to be another Bumper Jelly Year [www document]. http://www.mcsuk.org/what_we_do.php/Wildlife+protection/What+we+do/Another+bumper+jelly+year (Accessed 09/09/2015).


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