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The tissue distribution of slc15a1a , a gene that encodes an oligopeptide transporter, PepT1, and its response to fasting and refeeding were investigated in the intestinal epithelium of Mozambique tilapia for a better understanding of its role on nutrient absorption. The slc15a1a was predominantly expressed in the absorptive epithelia of the anterior part of the intestine, suggesting that digested oligopeptides are primarily absorbed in the anterior intestine. The response of slc15a1a to fasting was evaluated at 1, 2, 4, 7 and 14 days after the last feeding. Fasting revealed a biphasic effect, where short-term fasting significantly upregulated slc15a1a expression and long-term fasting resulted in downregulation. The expression level continued to decrease and fell below the pre-fasted level from day 4 to 14. Proximal (the hepatic loop, HL) and distal parts (the proximal major coil, PMC) of the anterior intestine showed different magnitudes of responses to fasting; slc15a1a expression in the PMC showed greater upregulation and downregulation than that in the HL. Refeeding significantly stimulated slc15a1a expression at day 3, although the expression did not exceed the pre-fasted level. Observed responses of slc15a1a to fasting and refeeding suggest that the expression level of this gene can serve as a sensitive indicator of the changes that may occur in altering nutritional conditions. These findings contribute to a better understanding of the role of PepT1 in nutrition and of the complex mechanisms underlying the absorption of oligopeptides and amino acids in the intestine, and may lead to development of possible means to manipulate the absorption processes for the improvement of growth and other metabolic and physiological conditions in fish. [ABSTRACT FROM AUTHOR]

We examined the annual reproductive cycle of both female and male hatchery-produced Pacific bluefin tuna (PBT, Thunnus orientalis) broodstock at 3 years of age, including the associated changes in plasma levels of sex steroid hormones and the gene expression of key endocrine factors. The females progressed to vitellogenesis in April, and approximately 50% of the females were observed in late vitellogenesis between June and July at water temperatures between 24.1 °C and 29.0 °C. Approximately 90% of the males were in spermiation for at least 7 months between April and October, which included the period when vitellogenic females were present. Furthermore, the gonadosomatic index of both females and males peaked between June and July. These results indicate that the reproductive season of hatchery-produced PBT peaks between June and July, which is in agreement with the reproductive season reported for PBT broodstock originating from wild-caught juveniles. Furthermore, the percentage of hatchery-reared females that reached late vitellogenesis in the present study was higher than that in wild-caught captive-reared females reported in previous literature. Analysis of plasma sex steroid levels and gene expression profiles suggested that oocyte and ovarian development were associated with 17β-estradiol (E2) control, whereas spermatogonial differentiation towards meiosis seemed to be mediated by 11-ketotestosterone (11-KT). Furthermore, the production of E2 and 11-KT is likely regulated by luteinizing hormone and follicle-stimulating hormone, respectively, via their related receptors. The findings of this study are expected to contribute to improvements of technologies for obtaining fertilized eggs from hatchery-produced PBT broodstock. • We examined the reproductive cycles of hatchery-produced Pacific bluefin tuna. • 50% of females in full vitellogenesis were observed between June and July. • Most males reached full spermiation between April and October. • Plasma sex steroid profiles were associated with gonad development. • Pituitary gene expression of fshb and lhb was related to sex steroid production. [ABSTRACT FROM AUTHOR]

This study examined the biochemical characteristics of key digestive enzymes in artificially produced Pacific bluefin tuna juveniles and revealed details of the digestive physiology of juveniles fed a commercial pellet diet and raw fish over long-term (225 days) and short-term (response to feeding) periods. pH and thermal profiles of key digestive enzymes of pepsin, trypsin, chymotrypsin, lipase and amylase were examined in juvenile bluefin tuna and the characteristics differed from those previously reported for larval and adult tuna. Long-term feeding (225 days) of the commercial diet markedly increased stomach and pyloric caeca size as well as storage/synthesis levels of the digestive enzymes. Highly increased gene expression levels of cholecystokinin (cck) and its receptor (cck1r) were also found in juveniles fed the commercial diet, suggesting cck might be involved in the gastrointestinal hypertrophy and digestion enhancement seen in the present study. In the short-term experiment (response to feeding), juveniles fed the commercial diet displayed faster gastric emptying, higher pH in the intestinal content and increased digestive enzymes storage/synthesis whereas the enzymes secretion was similar with those of juveniles fed raw fish. These indicate that juvenile bluefin tuna morphologically and physiologically adapted their digestive system to the artificial diet, however the adaptation was not sufficient to obtain a comparable performance as tuna fed raw fish feed. Biochemical characteristics of the digestive enzymes and the observed effects of artificial diet on digestive physiology could be good indicators to improve feeding strategies such as diet composition, feeding ration and frequency in juvenile tuna. • Biochemical characteristics of key digestive enzymes were revealed in juvenile Pacific bluefin tuna and the characteristics differed from those previously reported for larval and adult tuna. • Long-term feeding (225 days) of the commercial diet markedly increased stomach and pyloric caeca size as well as storage/synthesis levels of digestive enzymes compared with juveniles fed the raw fish feed. • Juveniles fed the commercial diet displayed faster gastric emptying, higher intestinal content pH and increased digestive enzymes storage/synthesis after feeding whereas the enzyme secretion levels were similar with juveniles fed the raw fish feed. • Juvenile tuna morphologically and physiologically adapted their digestive system to the artificial diet, while the adaptation was not sufficient to enable comparable growth with the raw fish feed. [ABSTRACT FROM AUTHOR]