Natural growth lines in echinoid ossicles are not reliable indicators of age: A test using Strongylocentrotus droebachiensis

Natural growth lines in the ossicles of echinoids have been used to estimate age, calculate growth curves, and infer population age-structure. However, few studies evaluate whether these bands are added annually - a critical assumption of the aging technique. We tested whether the banding pattern is annual in Strongylocentrotus droebachiensis. Sea urchins were tagged with a fluorescent marker, released into tidepools, and collected 1 year later. We quantified the position of the fluorescent mark relative to subsequent growth bands. In 30 individuals ranging in test diameter from 14 to 77 mm, and in a series of ∼2 mm size intervals, we examined 3 interambulacral plates (aboral, ambital, and oral) and a rotula from Aristotle's lantern. Overall, only 7 sea urchins (23%) added a complete band to all 4 ossicles. In 6 sea urchins (20%) at least 1 ossicle added more than 1 complete band. In many sea urchins, especially those >55 mm in diameter, most ossicles added less than 1 band. The banding pattern in ossicles seriously underestimates age in S. droebachiensis and population parameters inferred from these growth lines are biased. Before using the growth-band aging method in other echinoids, it must be demonstrated that 1 band is added annually for all sizes in a population under field conditions.