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The Rigakubu News
The Rigakubu News Sep. 2026
Frontiers of Science for Undergraduates >

A Few Years on the Waves: The Long Colonial Life of the Silver Jellyfish

Daiki Wakita (Project Researcher, Misaki Marine Biological Station / JSPS Research Fellow)
Kohei Oguchi (Associate Professor, Misaki Marine Biological Station)

Blue and yellow by-the-wind sailors wash ashore on beaches. Although their jellyfish-like bodies appear to be a single individual, they are actually colonies made up of tiny, specialized individuals called zooids, each with a different role.
How do these colonies grow on the ocean surface, and how long can they live?
By documenting the growth of these notoriously difficult-to-keep creatures for the first time, we discovered that their colonies grow by gradually adding new, coin-like floats around the outer edge. This led to a surprising prediction: they may drift across the harsh ocean surface for years.

The silver coin jellyfish (scientific name: Porpita porpita), which floats on the ocean’s surface, is an animal that spreads its tentacles around a small disk. Although its name includes the word “jellyfish,” it differs from typical jellyfish. Its body is a “colony” composed of small individuals called zoids.At the center is a single large individual that feeds; around the outer edge are many slender individuals that capture prey; and in between are many short individuals that produce the next generation, all suspended from a “float” resembling a silver coin.In other words, while each silver coin jellyfish performs a distinct role, the colony as a whole behaves as if it were “a single organism” (Figure, left).

It was not previously known how such colonies grow or how much time they spend on the ocean’s surface. This is because silver-coin jellyfish are carried by ocean currents and winds and appear suddenly, making it impossible to predict when they will wash ashore. Furthermore, since they deteriorate quickly after washing ashore, it is difficult to keep them alive for long.It was impossible to determine by appearance alone whether a colony found on the shore had been born a few days earlier or had been drifting for several months.

Therefore, we collected Silver Moon Jellyfish that had washed ashore along the coast of Kanagawa Prefecture before they became damaged or dried out. In collaboration with the New Enoshima Aquarium and the Kamo Aquarium, we successfully kept 10 colonies alive for up to three weeks by adjusting lighting and feeding based on each institution’s experience.When we measured the size of the floats before and after rearing (Figure, right), we found that while the floats in small colonies grew rapidly, growth was significantly slower in large colonies.When this characteristic was expressed mathematically and growth curves were estimated using statistical tools, the results suggested that it may take about one year for the bell diameter to exceed 2 cm and about three years to exceed 4 cm—the maximum size. Although this age estimate is based on a short captivity period, it appears that the Silver Jellyfish may drift for longer than previously expected.

Furthermore, microscopic observation of the cross-section of the float revealed how it grows. Apparently, the entire float is not stretched like a balloon. It is thought that the float consists of many small air chambers arranged in concentric circles, and its diameter increases gradually as new structures are added to the outer edge.The rim of the float serves as a new foundation where new individual polyps proliferate. The growth of a silver-coin jellyfish colony can be described as a process in which individual polyps with different roles are added to appropriate positions, building up the colony while maintaining its overall function.

This study is the first to quantify the previously unknown age of the silver-coin jellyfish and to focus on its body structure. The silver-coin-like float, which is expanded while withstanding rough waves and ultraviolet rays, is like a ship that supports numerous individual jellyfish for many years. An animal that challenges us to ask, “What is an individual?” remains, even at this very moment, at the boundary between sky and sea.

(Left) A single colony of silver-coin jellyfish. (Right) Growth of the floats during cultivation.

　This study was published in D. Wakita et al., Scientific Reports 16, 13766 (2026).
(May 20, 2026 press release)