Feeding ecology of chaetognaths in the Yellow Sea and the East Sea inferred from gut content and fatty acid analyses

Abstract

Chaetognaths are known to be not only an important planktivorous predators but also prey for high trophic level species such as fish larvae. However, even though they comprise high biomass, there are few study about their feeding ecology in the Korean coastal waters (KCW). To identify the diet of the most predominant chaetognath species (Sagitta nagae, S. crassa and S. elegans), we analyzed the gut content and fatty acids (FAs) of specimens sampled in April and August 2013, December 2015. In the KCW, about 10 ~ 20% of chaetognaths contained foods, which mainly consisted with small copepods in their guts. For FA composition, all species were dominated by 16:0, 18:1(n-9), 20:5(n-3) and 22:6(n-3). Unlike S. nagaeand S. crassa, only S. elegans was characterized by significant amounts of 20:1(n-9) and 22:1(n-9), which are biosynthesized by calanoid copepods (i.e. cold water species). Based on the results, chaetognaths are carnivores and mainly feed on the copepod. Also, FA compositions of S. nagae between the YS and ES seems to reflect different prey including copepods. These results suggest that FA composition of chaetognaths are influenced by feeding environment rather than their biosynthesis. However, a lot of empty-gut species and gut content analysis could be misrepresented the prey items because different prey types may be digested at different rates. Therefore, it is possible to clearly understand the feeding ecol