Habitat and density-dependent growth of the sea urchin Paracentrotus lividus in Galicia (NW Spain)

We studied the small-scale spatial variability in the growth of Paracentrotus lividus in two populations in Galicia (NW Spain) by reading growth rings. A tetracycline marking experiment was carried out to verify that the rings form annually. The growth rings were read by two independent readers in order to estimate the uncertainty involved in assigning the age. Of the six growth models evaluated (Tanaka, von Bertalanffy, Gompertz, Richards, logistic and Jolicoeur) the Tanaka function obtained the best fit to the data. This function predicts unlimited growth and a maximum growth rate of 15.00 (± 0.97 SE) mm·year− 1 at 3.09 ± 0.10 years old, which progressively decreases at older ages. However, habitat characteristics lead to intrapopulation variations in this general function. Recruitment seems to occur mainly in shallow waters (≤ 4 m) and when the sea urchins reach 50 mm (approximately 4 years old) they migrate to deeper areas. Sea urchins larger than 50 mm that stayed in shallow waters grew at a rate between 0.41 and 0.43 mm·year− 1 less than the sea urchins that moved to depths of 8 and 12 m. The population density also influenced the growth, and individuals older than 4 years had higher growth rates in high-density patches than in low-density areas. This could be due to the better environmental conditions in aggregation areas, that is, better protection against waves and predators and/or more abundant food.