Can the early stage copepad (Copepodites and Naupliies) abundance play important role on the fatty acid composition of Sagitta setosa (Chaetognatha) in the Southeastern Black Sea?

In this study, the relationship between the fatty acid composition, abundance of carnivore Sagitta setosa (S. setosa) and total copepod abundance (copepodites and nauplii) which are the main diets of S. setosa was investigated. Sampling was conducted monthly during a year. During the sampling period, S. setosa and the total the copepod abundance showed a parallel distribution until November. Average total lipid was at the highest with 3% in spring and autumn, while the lowest was 2% in winter and summer. While DHA/EPA ratio (2.23) as carnivory index in S. setosa and the total copepod (copepodites and nauplii) abundance (1375 ind/m3) were at the highest in winter, but another carnivory index, 18:1ω9/18:1ω7 ratio did not show a parallel increase with copepod abundance. However, 18:1ω9 which increases the carnivory index, is among the main fatty acids in S. setosa. This might be indicated that S. setosa does not related to predation on the copepods, but it shows that feeding in a carnivorous style. Additionally, the herbivory index ∑ n-3/∑ n-6 ratio was detected in spring and autumn at the highest level. Consequently, although there is no direct effect between the copepod abundance and S. setosa fatty acids, it is possible an indirect effect.