Drop in sea temperature linked to jellyfish invasion

An unexpected number of jellyfish in Maltese waters over the past few weeks was caused by the fact that the sea temperature went down to 14.6°C during the first two weeks of March, marine biologist Alan Deidun told MaltaToday. 

Temperature changes in the sea affect the development of many species, including that of jellyfish. 

“In this particular case, we’re talking about the mauve stinger jellyfish, which is one of the most commonly sighted jellyfish in Malta,” Deidun explained. 

Deidun said that jellyfish only live for around a year and reproduce and shed their eggs between December and March. After that, they take about three months to develop into adults. “Usually, by around April and May, we would see a bloom of young jellyfish.” 

The marine biologist said their development is affected by the sea temperature, just like many other things happening in the sea. “Normally, the development cycle of the mauve stinger jellyfish from egg to adult takes three months. This year, it took longer because of the colder than average sea temperatures.” 

Deidun said colder-than-average sea temperatures resulted in the mauve stinger jellyfish being around for a longer period of time, rather than their usual three months, followed by a few weeks of which they bloom.  

“This year, we have had jellyfish developing at different rates; this means that we have had different stages appearing during different weeks and months of the year.” 

Deidun, who runs a crowdsource monitoring website for jellyfish, said people have been spotting the stingers since January.  

The “big bloom” in jellyfish usually happens in April and lasts for a few weeks. “They cannot sustain themselves longer than that because they don’t find enough food to eat,” Deidun says. 

But it is still too early to tell whether this could be an annual recurrence, and more research is required for Deidun to give an accurate prediction. “What I can say is that what is happening is presumably because of climate change – we have seen harsher seasons on the surface, longer winters, hotter summers, and all of this affects the sea as well. We have seen that it is taking longer for the sea to cool down even in November and December, and that will have a ripple effect.” 

Deidun said the result of this is that species such as the mauve stinger jellyfish start reproducing later in the season. This could also affect species that feed on jellyfish, such as sea turtles, since the opportunity to feed on jellyfish will come later in the season than usual.  

But he also assured that jellyfish blooms will subside over the next weeks. 

Deidun urged the general public to participate in the “Spot the jellyfish” campaign, which is supported by the International Ocean Institute and the Physical Oceanography research group. “By doing this, the public can help bring awareness about the local diversity of jellyfish species through reporting sightings that are close to Malta’s shores and beaches.” 

More information is available at https://oceania.research.um.edu.mt/jellyfish/