Skeletal ossicles in echinoids are unreliable chronometers: Comment on “Population dynamics of Strongylocentrotus droebachiensis in kelp forests and barren grounds in Norway” (Fagerli et al. 2015)

Abstract

Sea urchins are important drivers of nearshore benthic communities. Understanding their critical life history characteristics such as age structure and mortality requires accurate and reliable estimates of growth. The green sea urchin, Strongylocentrotus droebachiensis, occurs on both sides of the northern Pacific and Atlantic oceans and can play a key role in stable state shifts from urchin barrens to kelp bed communities. A recent study of these systems in Norway used growth zones, or bands, in the skeletal ossicles (interambulacral plates) of green urchins as chronometers—estimators of age. Combined with measures of size, the age estimates were used to construct growth curves, population age structures, and estimates of site-specific mortality. However, there is much confusion surrounding the aging technique in regard to skeletal growth, ossicle selection, and misinterpretations of previous validation studies. Our comment clarifies these issues using data from the Norway study and two published validation studies. In addition, we provide criteria for assessing whether sea urchin ossicles are accurate and valid chronometers. Given the limited understanding of endogenous and exogenous causes of different types of bands, ontogenetic variation, intra- and interspecific variation, and a paucity of detailed information about their chemical composition, at least at this time, bands should not be used to determine age in sea urchins.