Growth bands are an unreliable indicator of sea urchin age: Evidence from the laboratory and the literature

Knowledge of urchin age structure is crucial for understanding their ecosystem impacts and improving their management. In sclerochronology, translucent and opaque growth bands (TGB, OGB) in urchin ossicles are used to estimate age. An essential premise for using this technique is that one TGB and one OGB are formed every year, independent of urchin size or ossicle type. TGB and OGB addition are associated with slow and fast growth, respectively, and assumed to be added seasonally due to changes in water temperature. However, these assumptions are not unanimously supported by experiments, and validation attempts have not generated consensus. We conducted an experiment on Strongylocentrotus droebachiensis to test the validity of these assumptions and reviewed the literature to assess the use and validation of sclerochronology in urchins. The experiment demonstrated that the addition of TGB and OGB was not strictly related to temperature and was not consistent across urchin size‐classes or ossicle types. TGB were added in response to temporary stress, and no distinction on the basis of band width or pigmentation could be made between natural TGB and stress‐induced TGB. Only 52% of articles that used sclerochronology for aging urchins attempted any validation, and when the methodology was put under scrutiny, it was usually found to be wanting. More detailed studies are needed to address variability in growth bands deposition and endogenous and exogenous factors affecting this process. Sclerochronology in urchins should not be used until standardized procedures are able to provide accurate and precise interpretation of growth band addition.