Cysteine is known to be a precursor of taurine, which promotes growth rate and prevents green liver syndrome in fish, but the metabolic fate of this amino acid is not well understood. In this experiment, we investigated the enzymatic production of hydrogen sulfide (H₂S) from cysteine in fish. Liver homogenate prepared from bluegill Lepomis macrochirus was incubated with l-cysteine. H₂S produced by enzyme reaction was trapped in the form of ZnS, reacted with N,N-dimethyl-p-phenylenediamine to form methylene blue, and then determined spectrophotometrically. Under the condition we have employed, H₂S production was elevated with increasing cysteine and protein contents, and reaction time. Considerable level of H₂S formation was observed at pH 6.0-9.0, and the highest production was found at 45°C. H₂S production was increased by the addition of pyridoxal 5'-phosphate (PLP) to the reaction mixture, and it was markedly inhibited by hydroxylamine, an inhibitor of PLP-dependent enzymes. H₂S formation was also found in rainbow trout and common carp, in which the production in carp was half as much as that in bluegill. These results showed a possibility that H₂S is biosynthesized from l-cysteine probably by PLP-dependent enzymes in fish as shown in mammals.

This study aimed to elucidate the effects of dietary fish meal (FM) substitution with soybean meal (SBM) at various levels on the gene expression of two antimicrobial peptides (hepcidin and piscidin) in yellowtail (Seriola quinqueradiata). Specifically, we assessed hepcidin and piscidin expression levels in the whole brain, gill, head kidney, intestine, leukocytes, liver, skin, and spleen of yellowtail using quantitative reverse transcription PCR (RT-qPCR). Although hepcidin and piscidin were detected in all examined tissues, the liver and spleen had the highest expression of hepcidin and piscidin, respectively. To evaluate the response of hepcidin and piscidin to dietary ingredients, FM was replaced with SBM in the diet of yellowtail for 5 weeks at 0% (SBM0, FM 64.5%), 15% (SBM15), and 30% (SBM30), and the expression levels of hepcidin and piscidin were detected at 3 and 5 weeks. Compared with that in the SBM0 group, FM replacement with SBM at 15 and 30% significantly decreased piscidin expression in the spleen and head kidney at 5 weeks but did not affect hepcidin expression. Overall, this finding suggests that FM replacement with SBM at 15 and 30% can cause a decrease in antimicrobial peptide gene expression in yellowtail.

We compared the movement distances and feeding behavior of juvenile sea cucumber Apostichopus japonicus to determine the effects of behavioral changes, body size, and water temperature on the emergence of growth disparities. Juveniles (standard body length, 7.0-58.6 mm) were used in the experiments. The water temperature range was 6-18°C. The movement distance was measured while adjusting the water temperature according to changes in seawater temperature. Feeding behavior was observed in warming and cooling treatments, where the water temperature was adjusted to reach the set condition. Juveniles actively migrated at water temperatures of approximately 10°C but continued to feed without active movement at temperatures ≥ 14°C. In the warming treatment, the proportion of feeding individuals and feeding duration increased with increasing body size and water temperature in juveniles. However, no difference in feeding behavior was observed in the cooling treatment, except for the condition of body length ≥ 35 mm and water temperature of 18°C. These results suggest that the uneven growth in juveniles increased under warming conditions. Juveniles reduced their movement and increased their feeding duration to reduce energy consumption under high water temperatures when the basal metabolic rate increased.

Black soldier fly (BSF) meal is a promising alternative to fish meal (FM) in aquafeeds. This study reports the short-term effects of BSF meal on the growth and muscle quality of Japanese jack mackerel. The control diet was formulated using FM as the principal protein, and FM was substituted with defatted BSF larvae meal at a ratio of 10%. Fish (131.4 ± 4.0 g) were fed one of the experimental diets for 30 days. Although 20 fish in a single tank per dietary treatment were measured, no significant differences were observed in growth parameters, such as body weight or fork length (p > 0.05). Likewise, no significant changes were observed in serum indices (p > 0.05), except that total cholesterol and protein levels were reduced by dietary BSF inclusion (p < 0.05). The muscle quality of the Japanese jack mackerel exhibited no remarkable negative effects in the BSF diet group.

Lactosucrose (LS) is an indigestible oligosaccharide that plays a role in the immune response in mammals and teleost fishes. Here, we established an immunohistochemical (IHC) method for staining IgM-positive B cells in the intestinal tissue of common carp Cyprinus carpio by confirming the effects of LS. The effects of dietary LS at two concentrations (1.5%, LS 1.5; and 2.5%, LS 2.5) on the immune response of carp were compared with a control diet. The IgM-stained area in the intestinal cross-sections of carp in the LS 2.5 group was more significant than in the other groups. Additionally, the plasma lysozyme activities in both LS groups were higher than in the control group. LS is known to be a fermentation substrate for intestinal microbes in fish. Thus, intestinal fermentation of LS may improve disease resistance regarding systemic and innate immune responses. We have developed an IHC method to evaluate the immune response of carp. This method will allow us to confirm the immunostimulatory effects of dietary substances, such as fermentable oligosaccharides, acting in the intestinal tract.

Corbicula japonica, a brackish water bivalve, is an important inland fishery resource in Japan. However, in recent years, the commercial catch of C. japonica in Japan has declined to approximately one-fifth of its peak. The reduction in catch is believed to be caused by basin-derived substances, such as ammonia, as well as habitat degradation, including the deterioration of water quality and sediment composition. The purpose of the present study was to clarify the mechanism of ammonia toxicity and ammonia tolerance according to the growth stages of the brackish water clam, C. japonica. The ammonia median lethal concentration (LC50) value for C. japonica larvae was the lowest during the growth stages. The LC50 values for ammonia decreased as shell length increased at 0.5 mm, 3 mm, and 25 mm. The histological appearance of ctenidium in C. japonica changed with an increase in ammonia concentration. According to the results of the present study, it is considered that the ctenidium’s destructive effect was the underlying mechanism of ammonia toxicity in C. japonica.

The browsing behavior of the parrotfish, Calotomus japonicus, was examined under tank-rearing conditions to help improve the efficiency of longline fishing. The amount of a seaweed Sargassum fusiforme browsed by the fish during the four periods (morning: 5:00 - 10:00, daytime: 10:00 - 15:00, evening: 15:00 - 20:00 and nighttime: 20:00 - 5:00) was 8.6 ± 4.7 g/kg-fish/h, 8.8 ± 3.4 g/kg-fish/h, 5.5 ± 2.5 g/kg-fish/h and 0.1 ± 0.1 g/kg-fish/h, respectively. Browsed rates for morning and daytime were higher than evening. Furthermore, the amount of Ecklonia cava subsp. kurome browsed by the fish ranged 76.9 -224.9 g/kg-fish/day from June to August during the season of rising water temperature. The amounts in other months ranged 8.4 - 37.6 g/kg-fish/day. In addition, the six seaweeds species, S. fusiforme, Padina arborescens, Dictyopteris undulata, Palisada papillosa, Gelidium elegans and Codium fragile, were fed to the fish. The browsed rate of S. fusiforme was 87.2 ± 19.7%, and the rates were higher than the other seaweeds (2.4 - 72.8%). Based on these results, the time period for longline fishing to remove C. japonicus is preferable during morning to daytime in spring to early summer, and the suitable bait is S. fusiforme.

This study was conducted to investigate the availability of shark bycatch muscle meal (SM) as an alternative to fish meal (FM) in the diet of juvenile red sea bream, Pagrus major, and its effect on reducing phosphorus (P) discharge to the rearing environment. A total of 5 diets were formulated: FM was used as the protein source in the control diet (C), and in the 25% (SM25), 50% (SM50), 75% (SM75) and 100% (SM100) SM diets, the respective percentage of FM protein from the C diet was replaced by SM. At the end of 10-week rearing period, although fish fed the SM100 diet showed significantly lower growth performance (p < 0.05), there were no significant differences in any growth parameters among the fish fed the C, SM25, SM50 and SM75 diets (p > 0.05). A strong negative (R² = 0.9092) linear correlation was observed between the daily feeding rate and SM levels in the diets. Although there were no significant differences in the productive values of protein and fat (p > 0.05), a significant increasing trend in the P productive value in the SM-based diets resulted in a significant decreasing trend in P discharge to the environment (p < 0.05). Therefore, the results suggest that SM can replace approximately 75% of FM protein in the diet of juvenile red sea bream without compromising the growth or health status. Furthermore, a significant reduction in P discharge suggests that ecological benefits can be achieved from SM-based diets.