Composition and utilization of fatty acids during the 

endogenous nutrient stage of Alosa sapidissima

Abstract: In order to understand the fatty acids composition, changes and utilization rules during the endogenous nutrient stage of American shad A. sapidissima, biochemical analysis methods were used to detect and analyze the crude fat content and fatty acids composition of A. sapidissima in different developmental stages: embryos during the gastrula stage, newly hatched larvae 0dayage, and before feeding larvae 4dayage. Results showed that the individual crude fat contents of A. sapidissima during the endogenous nutrient stage were high (41.91 μg60.04 μg), the larvae consumed 30.20% of the crude fat before feeding; with the individual development, the moisture content reduced significantly (P<0.05)(the value from 96.42% to 88.25%), the crude fat content of dry matter increased slightly (P>0.05) (the value from 16.33% to 19.13%), the crude fat content of fresh quality increased significantly (P<0.05)(the value from 0.58% to 2.25%), the individual crude fat contents decreased significantly (P<0.05) (the value from 60.04 μg to 41.91 μg) during the endogenous nutrient stage. The top 4 fatty acids relative content was C16∶0 (21.65%23.06%), C18∶1n9c (19.02%20.93%), C22∶6n3 (DHA) (15.02%18.20%), and C18∶2n6c (12.33%14.43%) during the endogenous nutrient stage of A. sapidissima. The C16∶0 relative content of 0dayage larvae was significantly higher than that of embryos and 4dayage larvae (P<0.05), while there was no significant difference between embryos and 4dayage (P>0.05); the relative contents of C18∶1n9c and C18∶2n6c significantly decreased with the individual development (P<0.05); the relative contents of C22∶6n3(DHA) significantly increased (P<0.05) with the development of ontogeny, the C22∶6n3(DHA) relative content of 4dayage larvae was significantly higher than that of embryos and 0dayage larvae (P<0.05), while there was no significant difference between embryos and 0dayage (P>0.05). Although the relative content of C20∶5n3 (EPA) was not high (1.05%1.49%), the relative content of C20∶5n3 (EPA) significantly increased with the individual development (P<0.05). This also resulted in a significant upward trend of DHA+EPA with the individual development (P<0.05), DHA+EPA relative content of 4dayage larvae (19.69%) was significantly higher than that of embryos (16.07%) and 0dayage larvae (16.63%)(P< 0.05), while there was no significant difference between embryos and 0dayage larvae (P>0.05). The SFA relative content of embryos (32.39%) was significantly lower than that of 4dayage larvae (33.53%) (P<0.05), while there was no significant difference between 0dayage larvae and embryos or 4dayage larvae (P>0.05). The relative content of MUFA was relatively low (20.92%22.82%) and significantly decreased with the individual development (P<0.05). The relative content of PUFA was rich and stable (44.76%45.55%), and there was no significant change in the PUFA content among embryos, 0dayage and 4dayage larvae (P<0.05). The ratio of ∑SFA to ∑UFA of 4dayage larvae (0.50) was significantly higher than that of embryo (0.48) (P<0.05). The ratio of ∑n3PUFA to ∑n6PUFA increased significantly with individual growth and development  (P<0.05), and the value increased from 0.69 to 0.94, the ratio of ∑n3PUFA to ∑n6PUFA of embryos and 0dayage larvae was significantly lower than that of 4dayage larvae (P<0.05), while there was no significant difference between embryo and 0dayage larvae (P>0.05).The top 4 fatty acids absolute content in individual was C16∶0, C18∶1n9c, C22∶6n3(DHA) and C18∶2n6c during the endogenous nutrient stage of A. sapidissima. With the individual development and the shedding of egg membrane, the absolute content of C24∶1n9 decreased slightly at first (P>0.05) and then increased significantly (P<0.05), and the absolute content of C15∶0 showed a significant trend of first decrease and later increase (P<0.05), the absolute content of C17∶1 increased slightly at first (P>0.05) and then decreased significantly (P<0.05), the absolute contents of C17∶0, C20∶0, C20∶4n6, C20∶5n3 (EPA) and C22∶6n3 (DHA) had no significant changes (P>0.05), the absolute contents of the other 14 fatty acids in individual showed a significant downward trend (P<0.05). The superposition effect of the decrease of most fatty acids resulted in a significant decreasing trend of absolute contents of the SFA, MUFA and PUFA in individual with individual development (P<0.05). In addition, the absolute contents of EPA+DHA and ∑n3PUFA showed no significant change with the individual development (P>0.05), while the absolute content of ∑n6PUFA showed a significant downward trend (P<0.05). During the embryo incubation period of A. sapidissima, C18∶1n9c (256.31 μg and 20.40%, respectively) and DHA (132.23 μg and 14.64%, respectively) reduced largely. From the point of utilization amount and rate, after the larvae emerged from the membrane to the opening before feeding, there was a certain priority in the use of fatty acids： n6PUFA (295.41 μg and 23.81  %, respectively) >MUFA (224.72 μg and 20.37%, respectively) > SFA (279.79 μg and 16.59%, respectively) > n3PUFA (38.67 μg and 4.19 %, respectively), especially C18∶2n6c (207.48 μg and 28.64 %, respectively) and C18∶3n6 (48.77 μg and 26.53 %, respectively) in n6PUFA, C16∶0 (256.35 μg and 22.02 %, respectively) in SFA and C18∶1n9c (201.86 μg and 20.18 %, respectively) in MUFA were used in priority. While the utilization amount and rate of n3PUFA were extremely low (38.67 μg and 4.19%, respectively)，especially EPA+DHA (18.26 μg and 2.17%, respectively). Researches showed that the EPA was preserved in priority but DHA was used largely during A. sapidissima embryo incubation period; n3PUFA (especially EPA+DHA) was preserved in priority after the larvae emerged from the membrane. ∑n3PUFA/∑n6PUFA was lower (0.690.94) during the endogenous nutrient stage of A. sapidissima, and this showed the fatty acids characteristic of freshwater fish. In the antepartum cultivation and spawning period of brood stock, it is suggested to supplement diets enriched with n3PUFA (especially DHA) in time in parents bodies to enhance the nutrition accumulation and increase the nutrient reserve of the fertilized egg, and then to improve the survival rate of A. sapidissima larvae before feeding.