Abstract
A total of 24 nematode specimens belonging to the family Raphidascarididae were collected from three ecologically and economically important marine fish species inhabiting Iraqi marine waters: Saurida macrolepis Tanaka, 1917, Nemipterus japonicus (Bloch, 1791), and Platycephalus indicus (Linnus, 1758). The present study aimed to assess the degree of genetic variability and to explore the possible evolutionary relationships among the sampled nematode populations. Phylogenetic analysis based on the Neighbor-Joining method revealed that the Raphidascarididae specimens were divided into four main clades, supported by bootstrap values ranging from 48% to 87%, indicating a largely independent evolutionary history with varying degrees of genetic relatedness among lineages. Nematodes isolated from S. macrolepis exhibited the highest level of genetic divergence, suggesting pronounced differentiation that may reflect lineage-specific adaptation to the host or differences in transmission pathways. In contrast, nematodes recovered from N. japonicus formed a tightly clustered genetic group, supporting the presence of a host-specific parasite population. Similarly, specimens from P. indicus showed clear ecological and genetic structuring, indicating substantial intraspecific variability within this host-associated parasite population. The occurrence of shared gene sequences among nematodes collected from different host species suggests the possibility of host-switching events, most plausibly facilitated by co-feeding behavior among sympatric fish hosts. Overall, the findings demonstrate that parasite ecology including feeding behavior, spatial distribution, and trophic interactions plays a critical role in shaping the genetic differentiation of Raphidascarididae nematodes. The study further supports the concept of long-term coevolution between these parasites and their marine fish hosts, providing valuable insights into the evolutionary history of marine nematodes and establishing a foundation for future investigations across diverse ecological settings.