Shifting Jellyfish Numbers Linked To Climate Fluctuations

Brett Smith for – Your Universe Online

Every month, 8 to 12 days after each full moon, Waikiki Beach is invaded by large numbers of box jellyfish. After witnessing the phenomenon countless times, Honolulu lifeguard Landy Blair, in conjunction with researchers at the University of Hawaii at Mānoa, started tracking the numbers of jellyfish that invade the beach with each cycle.

Over 170 full moons after they began, the team’s published findings in the latest issue of PLOS ONE reveal the cycles of jellyfish behavior and how they relate to deep climactic conditions.

“Although there have been long-term studies of jellyfish abundance and climate in recent years, none have looked at box jellyfish species,” said Luciano Chiaverano, a member of the UH Mānoa study team. “This is quite surprising, as box jellyfish are among the most venomous animals in the world. Often their habitat overlaps with human recreation, resulting in painful, sometimes even lethal, stings and causing beach closures at various locations around the world.”

“Our box jellyfish collection data is the longest continual time-series census of a cubozoan species in the world, and provides a rich data set to analyze and assess physical and biological oceanographic correlations” said Angel Yanagihara, a neurobiologist at the University of Hawaii.

The study essentially confirms Blair’s early observations: box jellyfish arrive in Waikiki with a regular, predictable timing based on the lunar cycle. However, the sizes of the monthly influx varied substantially with no predictable seasonality. In a one section of the beach, an average of nearly 400 jellyfish arrived each lunar month, with numbers ranging from 5 to almost 2,400 individuals per event.

While the total number of jellyfish coming to Waikiki did not vary during the past 14 years, it did follow an increasing-and-decreasing oscillation pattern with each phase lasting approximately four years. According to study researchers, climate fluctuations played a major role in food availability, ultimately affecting the numbers of invading jellyfish.

Study researchers Brenden Holland and Jerry Crow analyzed three major climatic indexes, 13 oceanographic variables and seven local weather parameters indexes in an attempt to make sense of the cyclical jellyfish pattern. Although they did not see a noteworthy relationship between jellyfish counts and weather parameters, the counts showed a sturdy, positive relationship with a decadal-long climatic measurement specific to the sub-tropical Pacific known as the North Pacific Gyre Oscillation index (NPGO, as well as primary production and abundance of small zooplankton.

The authors realized that the number of box jellyfish arriving at the beach is probably determined by bottom-up processes: the higher the NPGO value, the higher the transport of nutrient-richer waters from the northern Pacific into the waters around Hawaii. This boost may push regional primary production, resulting in higher zooplankton levels for jellyfish to feed on.

“Jellyfish are known to have increased growth rates and reach larger adult sizes in response to increased food availability, and because body size positively correlates with fecundity in jellyfish, more eggs and more larvae are produced when food is readily available,” Chiaverano said.

Predicting changes in jellyfish populations over time is difficult due to sampling issues, the lack of historical records, and the bizarre characteristics of the jellyfish life cycle. Some research has indicated that jellyfish populations are affected by large-scale climate changes and regional environmental conditions.

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