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Fish are the main source of long-chain polyunsaturated fatty acids (LC-PUFA), in particular docosahexaenoic acid (DHA, 22:6n-3) and eicosapentaenoic acid (EPA, 20:5n-3), which are nutritionally valuable to humans. Moreover, lipid and fatty acid distribution in fish tissues are highly diverse across species. The lipid distribution pattern across tissues determines fatty acid profile, one of the product quality indicators of fish. However, this diversity of lipid distribution and fatty acid profile is not fully known, although several studies have been conducted to compare the nutritive composition of muscle in different fish species in both freshwater and marine environments. This study aimed to comprehensively investigate the lipid distribution and fatty acid profile, as well as the somatic indexes and approximate composition of five benthic marine fish species, namely yellow anglerfish (Lophius litulon), flathead (Platycephalus indicus), eelpout (Zoarces viviparus), East Asian flatfish (Pleuronichthys cornutus), and zebra sole (Zebrias zebrinus). Fishes were purchased from the 15th Street Seafood Market of Qingdao, which were stored on ice since captured in an inshore fishery in Qingdao (located at 119°30'–121°00'E and 35°35'–37°09'N), and then delivered to the market from the fishing harbor in the same morning. After purchase, fishes were kept on ice and immediately taken to the laboratory. The experimental design consisted of nine fish of each species bulked in groups of three, comprising three replicates. First, the total weight and length of each fish were measured, and the liver and viscera weight were recorded to calculate the viscerosomatic index (VSI), hepatosomatic index (HSI), and condition factor (CF). Subsequently, samples of muscle, liver, brain, eye, skin, subcutaneous adipose tissue around fins (only for East Asian flatfish and zebra sole), and intraperitoneal adipose tissue (only for flathead) were collected for lipid and fatty acid analysis. The dorsal muscle sampling position is behind the head on the right body side. The ventral muscle samples were collected under the dorsal muscle sampling spot. The proximate composition analysis of the dorsal muscle, ventral muscle, and liver (three individual samples per group) was performed according to the Association of Official Analytical Chemists standard methods. For the moisture assay, samples were oven-dried at 105 ℃ until they achieved a constant weight. The protein content was assayed by measuring nitrogen (N × 6.25) using the Soxhlet method, the lipids were analyzed using the Soxhlet method (petroleum ether extraction), and the ash by incineration at 550 ℃. The fatty acid compositions of all tissues were analyzed with gas chromatography (GC-2010, Shimadzu, Japan). The results showed that the flat body type of flatfish determines that the gut in these fish species cannot store high lipid contents in the liver or intraperitoneal adipose tissues. Like other flatfish, East Asian flatfish and zebra sole had lower HSI and VSI. In contrast, the HSI and VSI of yellow anglerfish and flathead were associated with high lipid contents in the gut. Additionally, the lipid distribution across tissues indicated that yellow anglerfish and flathead predominantly stored lipid in the liver and intraperitoneal adipose tissue, respectively. The muscle lipid content of all the five species was low, especially for yellow anglerfish and zebra sole (0.3%–0.4%). Yellow anglerfish, flathead, and East Asian flatfish had higher liver lipid content (19%–29%) than zebra sole and eelpout (4%–5%). Yellow anglerfish and flathead store lipid predominantly in the liver and intraperitoneal adipose tissue, respectively. The n-3 LC-PUFA content, especially DHA, in the muscle of yellow anglerfish was significantly higher than that of other species. The yellow anglerfish dorsal muscle also had the highest 18:1n-9 content. The relative abundances of different monounsaturated fatty acids (MUFA) and saturated fatty acids (SFA) in different fish species indicate a preference for energy storage and lipid mobilization. The most distinct fatty acid characteristics of the dorsal muscle of eelpout were low 16:0 content and high EPA content. For liver fatty acids, flathead and East Asian flatfish had significantly high MUFA contents, such as 18:1n-9, but their DHA contents were low. High DHA, 20:4n-6, and 22:5n-3 contents were consistently observed in nearly all yellow anglerfish, eelpout, and East Asian flatfish tissues, respectively, while low EPA content was observed for all of them. Among all five fish species, eelpout had a much higher ARA content than the other species in nearly all tissues. This could be due to the high ARA level in the food sources, namely algae. Yellow anglerfish had significantly high n-3 LC-PUFA content and thus high fatty acids nutritional value. This study revealed that even though these fishes are all benthic species, there are great differences in lipid and fatty acid composition among them. |
