ABOVE: Computed tomography scans of female and male eels in various maturation stages shown in different colors. This process involves a successive loss of bone as they mature.

European eels, Anguilla anguilla, start out yellow in color as juveniles and over the course of their lives transform into a shiny silver color. The eels spawn in the ocean and the larvae travel to coastal water, and when those offspring return to the ocean to spawn themselves, they stop eating, shrink their guts, and undergo internal structural changes, namely, breaking down and rebuilding their skeletons. Researchers induced artificial maturation by administering hormones to juvenile eels to study the breakdown of the skeleton.

M. Freese et al., “Bone resorption and body reorganization during maturation induce maternal transfer of toxic metals in anguillid eels,” PNAS, doi:10.1073/pnas.1817738116, 2019.

The internal structure of a yellow eel...
Researchers artificially induced maturation in an eel by giving it hormones and took this image of a vertebra, showing internal structural changes in bone tissue.

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European eel Anguilla bone restructuring reabsorption rebuilding juvenile mature artificial maturation

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