The feasibility of substituting soybean meal for ¢shmeal diets for juvenile white shrimp Litopenaeus schmitti (0.35 AE 0.01g) was evaluated, and an adequate substitution level was determined. Five diets were evaluated using 46%, 59%, 75%, 88% and 100% substitution levels. Pellet water stability was signi¢cantly a¡ected by dietary soybean content (Po0.05). Increased soybean content produced lower pellet stability, ranging from a dry matter loss of 142 2% after a 2-h immersion, and 20^33% after an 8-h immersion. After 52 days, signi¢cant di¡erences (Po0.05) were found in shrimp weight, feed conversion ratio and protein e⁄ciency ratio. The values were 0.64^1.06 g, 2.8^7.9 and 0.45^1.21, respectively, for the three measurements. Overall, better results were obtained with diets where soybean meal was substituted for ¢shmeal up to 75%. The 100% soybean meal diet resulted in poor growth performance of shrimp. Survival rates were acceptable for all treatments (90% or higher) and no signi¢cant di¡erences were found in survival between treatments. Regression analysis using the broken-line methodology indicated that 76.5 AE 2% is an optimum soybean substitution level in diets that contained ¢shmeal and soybean as the major protein sources for growout of juvenile white shrimp. Ã Means of triplicate samples AE SE. Values in the same row with di¡erent superscript letters are signi¢cantly di¡erent.Aquaculture Research, 2007, 38, 689^695 Substitution of ¢shmeal with soybean meal J S Alvarez et al.

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with the results of previous digestibility studies (Ward et al., 2003; Irving and Williams, 2007;Perera et al., 2010b). Poor protein use could be related to some impairment in digestive progression when lobsters are fed on pelleted diets (Simon and Jeffs, 2008;Simon, 2009).The aim of the present work was to assess the effect of feeding P. argus with different protein sources on trypsin expression and secretion. Our results demonstrate that trypsin enzymes are differentially regulated at the transcription and secretion level by ingested proteins. Additionally, we provide evidence that intact proteins, rather than small peptides or free amino acids, are the major signal eliciting the prandial secretory response in lobster, whereas SUMMARYThe effects of pelleted diets with different protein composition (fish, squid or soybean meals as main protein sources) on trypsin secretion and expression were studied in the lobster Panulirus argus. Trypsin secretion was shown to be maximal 4h after ingestion. At this time, fish-and squid-based diets induced trypsin secretion, as well as up-regulation of the major trypsin isoform at the transcription level. While fish-and squid-based diets elicited a prandial response, soybean-based diet failed to stimulate the digestive gland to secrete trypsin into the gastric fluid or induce trypsin expression above the levels observed in fasting lobsters. In vitro assays showed that intact proteins rather than protein hydrolysates stimulate trypsin secretion in the lobster. However, the signal for trypsin transcription appears to be different to that for secretion and is probably mediated by the appearance of free amino acids in the digestive gland, suggesting a stepwise regulation of trypsin enzymes during digestion. We conclude that trypsin enzymes in P. argus are regulated at the transcription and secretion level by the quality of dietary proteins through two distinct signaling pathways. Our results indicate that protein digestion efficiency in spiny lobsters can be improved by selecting appropriated protein sources. However, other factors like the poor solubility of dietary proteins in dry diets could hamper further enhancement of digestion efficiency.

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