Ecological estimations of summer horizontal and vertical distributions of Chaetognatha in the Turkish Mediterranean epipelagic zone

Chaetognaths play a critical role in marine food webs as predators of copepods and, occasionally, fish larvae, thereby influencing both zooplankton and ichthyoplankton communities. To assess the summer distribution patterns of chaetognath species across the Turkish Mediterranean Sea, zooplankton samples were collected during daylight from 64 stations, ranging from the western Gulf of Mersin to the coastal waters off Marmaris. In addition, diel vertical migration (DVM) and vertical distribution patterns were examined through stratified sampling at a fixed station within the epipelagic zone in summer 2019. This study represents the first comprehensive investigation encompassing both the horizontal distribution and DVM of chaetognaths along the entire Turkish Mediterranean coastline in relation to environmental conditions. Environmental parameters—including physical, chemical, and optical properties—were measured at each station and depth layer. Six chaetognath species belonging to two families were identified, with Flaccisagitta enflata (Grassi, 1881) being the most dominant, followed by Parasagitta minima (Grassi, 1881). Distinct regional patterns were observed, with F. enflata and P. minima more abundant in western regions, while Flaccisagitta hexaptera (d'Orbigny, 1836) and Serratosagitta serratodentata (Krohn, 1853) showed higher abundance in Region 4 (R4). Canonical Correspondence Analysis (CCA) revealed two major species groups: ( F. enflata , P. minima Grassi, 1881) and ( F. hexaptera , S. serratodentata ), likely reflecting similar DVM behaviors and environmental associations. Krohnitta subtilis (Grassi, 1881) was ecologically distinct, showing strong correlation with surface tripton levels. DVM patterns of dominant species were influenced by specific environmental drivers: F. enflata by pH, Chl-a, and NO₂; F. hexaptera by Si and NH₄; P. minima by salinity, seston, and PO₄; and S. serratodentata by temperature and bioseston. Regional cluster analysis indicated a pronounced differentiation in R2 and R4, whereas R1 and R3 showed strong connectivity, possibly driven by the rim current along the coast. CCA results showed that species-environment relationships were primarily explained by sea surface tripton, nutrients (NPO₄), Secchi depth, and temperature on the first axis (explaining 40.5% of variance), and by depth, near-bottom bioseston, and tripton on the second axis. These findings provide valuable ecological insights into chaetognath distribution dynamics and underscore the influence of hydrographic conditions on mesozooplankton communities in the eastern Mediterranean epipelagic zone.