The e¡ects of dietary astaxanthin supplemented at 0, 40, 80 or 150 mg astaxanthin kg À1 on growth, survival, moult frequency, osmoregulatory capacity (OC) and selected metabolic and haematological variables in Litopenaeus vannamei acclimated to low-salinity water (3 g L À 1 ) were evaluated. Supplemented astaxanthin at 80 mg kg À1 improved growth, survival and moult frequency in shrimp. The lowest OC was also exhibited in shrimp fed with dietary astaxanthin at 80 mg kg À1 . Shrimp haemolymph concentrations of glucose, lactate, haemocyanin and total haemocyte count were all signi¢cantly enhanced by feeding the diet supplemented with 80 mg astaxanthin kg À1 compared with shrimp fed with the other diets. On the basis of these results, dietary astaxanthin supplementation of 80 mg kg À1 is recommended for juvenile L. vannamei cultured in low-salinity water. Physiological, metabolic and haematological responses in L. vannamei M Flores et al. r 2007 The Authors Journal Compilation r 2007 Blackwell Publishing Ltd, Aquaculture Research, 38, 740^747 Median values and con¢dence interval (95%) in parenthesis. Di¡erent letters indicate a signi¢cant di¡erence on responses (Po0.05). DGC, daily growth coe⁄cient; OC, osmoregulatory capacity; THC, total haemocyte count. Aquaculture Research, 2007, 38, 740^747 Physiological, metabolic and haematological responses in L. vannamei M Flores et al.

The aim of this study was to explore the effect of varying the stimuli dimension or the response morphology on the discrimination learning. For a group of subjects, responding to the left lever after a fixed tone or responding to the right lever after a light were followed by water. For a second group, the arrangement was the same except that the light was replaced by an intermittent tone (Experiment 1). It was found that subjects trained with different stimuli dimension (tone vs. light) had a faster acquisition. In Experiment 2, for a group of subjects the first response to the left lever after presenting a fixed tone and pulling a chain after presenting an intermittent tone resulted in the delivery of water; whereas for a second group the conditions were the same except that instead of pulling the chain after the intermittent tone, subjects need to pressed the right lever to obtain water. Subjects trained with morphologically different responses (lever press vs. chain pulling) learned the task in fewer sessions than subjects trained with morphologically similar responses. As a whole, the results are discussed based on the differentiation between discriminated operants supporting the functional equivalence hypothesis.