Box jellyfish secret uncovered

An unexpected discovery that box jellyfish begin the jellyfish phase of their life cycle at the same time each year has provided a valuable insight into the seasonal occurrence of the lethal marine stinger.

James Cook University Marine Biology PhD student Matt Gordon and Associate Professor Jamie Seymour have been studying when box jellyfish change from their polyp phase into juvenile jellyfish, and to their surprise, have found that the process regularly occurs at the beginning of September.

Dr Seymour described the finding as a significant step towards uncovering the overall life cycle of the box jellyfish, which would help develop a predictive model to determine the risk of when box jellyfish stings can occur.

Mr Gordon said the research highlighted the need to consider box jellyfish management practices from both a stinger season perspective as well as a medusae season perspective, which is when polyps change into jellyfish.

“There are two different processes going on here. The medusae season is concerned with when polyps are changing into jellyfish, while the stinger season deals with when box jellyfish are likely to represent a risk to humans. Only by understanding both processes can effective management protocols be developed,” he said.

“This is the first data to show that the start of the medusae season occurs at a consistent time each year and that medusae are produced right throughout the season.

“Over six seasons between 2000 and 2010, the medusae season not only started in early September, but varied by only one week across the decade.”

The findings were published in the international journal PLoS ONE in March, in a paper entitled 'Growth, Development and Temporal Variation in the Onset of Six Chironex fleckeri Medusae Seasons: A Contribution to Understanding Jellyfish Ecology'.

Mr Gordon has been working on the research project since 2005 and incorporated data collected by JCU Associate Professor Dr Jamie Seymour since 2000.

Jellyfish were collected from the estuaries and coastline of Weipa by driving a boat close to shore, jumping into the water to grab a jellyfish when one was spotted and then removing its four eye sets.

“Each eye set contains a solid structure called a statolith, which has a series of rings within it. Just like you would with a tree, we were able to age each jellyfish based on the number of rings within its statoliths,” Mr Gordon said.

“Given that we knew the date of capture for each jellyfish as well as its age, it was possible to calculate when each individual had changed from a polyp into a medusa.

“These results suggest that the factor telling polyps to begin changing into jellyfish isn't water temperature or rainfall or salinity, but rather length of day.”

The research has also shown that box jellyfish grow very quickly.

“Juvenile jellyfish are only about 2mm in size when they metamorphose, but they have a phenomenal growth rate of up to 3mm per day,” Mr Gordon said.

Dr Seymour said box jellyfish were still too small to be lethal to humans at the stage when they metamorphose.

“It is not until jellyfish reach about the size of your closed fist that the number and type of stinging organelles they carry changes, as do the venom components, at which point, they become more lethal to vertebrates,” he said.

“Based on the earliest date of metamorphosis and estimated growth rates, this change in stinging organelles and venom composition could occur after only a couple of months, which is when the stinger nets traditionally begin appearing along the coastline.”

Mr Gordon said the research answered a few key questions about the life cycle of the box jellyfish, but there were still many unknowns.

“We do not know what they are doing or where they are between September and the time they tend to arrive along the tropical coastline, which varies between seasons and geographic locations, but can be up to several weeks or months later,” he said.

“This is largely because the location of the polyps is yet to be identified.

"Making this discovery is proving problematic, especially since it is widely accepted that the 2mm polyps live on the underside of rocks within estuaries.

“Such a discovery is like a needle in a haystack search, so our research has instead started with the adult stage and tried to deduce what has happened.

“The next step in the research is working towards a predictive model where data such as water temperature or time of the year can be used to determine the risk of a box jellyfish sting occurring.”

Issued April 20, 2012

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