The bizarre fluid-filled sacs of bubble-eye goldfish contain an important liquid for which the fish could be farmed to produce.
Bubble-eye goldfish are a selectively-bred strain developed in China centuries ago and their sacs were believed to contain lymph which is similar in composition to blood plasma or serum.
Studies on similar fluids have shown growth-promoting effects on fish cell cultures and scientists from Nagoya University in Japan undertook a study on bubble-eyes and discovered that their fluid had similar effects.
The fish also regenerated the extracted fluid after a short time, which meant the researchers were able to create sustainable supplies using live donors without needing to sacrifice the fish.