Population dynamics of Ferosagitta ferox (Chaetognatha) was carried out in coastal waters of Khur-e-Mussa's Channel in Khuzestan Province, northwest Persian Gulf. Monthly samples were collected with 300km mesh size plankton net in oblique tow from near bottom to surface waters. In total, 7 species of Chaetognaths were identified belonging to 1 class (Sagittoidea), 1 order (Aphragmophora), 2 families (Sagittidae and Krohnittidae) and 4 genera (Aidanosagitta, Ferosagitta, Flaccisagitta and Krohnitta). The identified species included A. Bedford, A. neglecta, F. ferox, Flaccisagitta enflata, F. hexaptera, Krohnitta pacifica and one unknown species. Due to presence and excessive frequency of F. ferox in all samples, we focused on its population variation as an indicator of Chaetognatha. We determined length groups for adult individuals in 4-21mm range with 1mm increments. The year round presence of juvenile and small individuals showed the continuous reproduction of F. ferox with two considerable peaks in March and July. The evaluated recruitment indicated that the species has a low growth rate and a long life cycle
