Effects of density on variation in individual growth and differentiation in endocrine response of Japanese sea cucumber (Apostichopus japonicus Selenka)

This article studied the differences of cortisol level in coelomic fluid, energy allocation and biochemical composition between big and small Japanese sea cucumbers (Apostichopus japonicus) in density treatments, and discussed the possible mechanism of density stress on individual growth variation. There were five treatments (stocking densities) in this experiment, i.e. 2, 4, 8, 16 and 32 individuals per 40 L glass aquarium (represented as D2, D4, D8, D16 and D32, respectively) and each treatment had four replicates. The experiment lasted for 60 days. The results showed that stocking density have adverse effects on growth of the sea cucumbers, not through water quality deterioration or food competition, but through the role of density as an environmental stress factor. The coefficient of variation (CV) of body weight increased as the stocking density increased, and the CVs in D16 and D32 were over 77%. The cortisol levels of the small individuals were significantly higher than those of the big ones in every treatment. The density had no significant effects on cortisol levels of the big individuals, however, the cortisol levels of the small ones increased significantly with the increase in stocking density. The lactate level increased, and the glucose level decreased with increase of stocking density for big and small animals. However, the lactate and the glucose levels of the small individuals changed more than those of the big ones. This study proves that crowding stress can stimulate the endocrine system of the small individuals raising their cortisol level in the coelomic fluid and, in turn, accelerate energy consumption, modify the energy budget model, and ultimately play a negative role on the growth and biochemical composition of the small animals. (C) 2012 Elsevier B. V. All rights reserved.