Box jellyfish are often feared as dangerous animals, with some species capable of causing severe or even fatal stings. However, box jellyfish nematocysts - organelles responsible for this toxic sting - are theorized to also play an unexpected role in reproduction. While many studies focus on researching the range of toxicity levels exhibited by the over 50 species of box jellyfish, their reproductive process is poorly understood.

In this study, a research team examined reproductive traits of box jellyfish (Cnidaria: Cubozoa), by analyzing preserved samples from multiple species collected in different regions of the world. Using histology - a technique that allows researchers to study tissues at a microscopic level - they investigated how sperm and eggs develop. These results provide new insights into the reproductive biology of this poorly understood group and help explain how different species function within their ecosystems, which is important for managing their impact on coastal activities and improve predictions of when and where they will appear.

This international collaboration brings together researchers from Tohoku University, the University of São Paulo, the Smithsonian Institution, and European partners like University of Alicante. This work represents one of the most comprehensive studies of reproduction in box jellyfish to date.

The findings were published in Frontiers in Ecology and Evolution on May 29, 2026.

"It is very difficult to obtain samples of venomous box jellyfish, so we noticed a big knowledge gap about their reproductive cycle and seasonality," says first author Jimena Garcia-Rodriguez (Advanced Institute for Marine Ecosystem Change (WPI-AIMEC)). "Thanks to the help of our international collaborators and access to curated museum collections, we were able to obtain specimens to unravel the mystery behind the complicated reproductive processes of these jellyfish."

By studying how sperm and eggs develop and how fertilization occurs across different species, researchers revealed that box jellyfish have more complex and diverse reproductive strategies than other Cnidarian classes. Understanding their life cycles can ultimately help improve predictions of jellyfish occurrences (such as when they appear in large numbers, called "blooms"), which is relevant for public safety, tourism, and coastal industries.

"While focusing on learning only about the deadly aspects of these creatures is tempting, studying how they create life is equally important," says Jimena. "After all, they play an important ecological role in the environment that is often overlooked because these dangerous traits are more emphasized."

The team is looking towards future studies that investigate how these reproductive traits are linked to environmental conditions and climate change. Thanks to deep dive analyses such as these, when and where to expect dangerous jellyfish blooms doesn't have to remain a murky mystery.