A new species of <i>Monoplectanum sillaginis</i> sp. nov. (Monopisthocotyla: Diplectanidae) in sillaginid fish from Thailand: the investigations of species diversity, infective situation and morphology

<i>Monoplectanum</i> monogeneans are gill parasites specific to fish in the family Sillaginidae. The sillaginid fish were collected from the middle and upper Gulf of Thailand and the Andaman Sea, Thailand, from May 2024 to January 2025. A total of 2096 fish belonging to 5 species- <i>Sillago aeolus, S. asiatica S. indica, S. ingenuua</i> and <i>S. sihama</i> were identified. Three species of <i>Monoplectanum</i> monogeneans were detected in 3 fish species, including 1 newly described species, <i>M. sillaginis</i> sp. nov. <i>Monoplectanum sillaginis</i> sp. nov. was found to infect 3 fish species, <i>S. aeolus, S. indica</i> and <i>S. sihama</i>, collected from the upper Gulf of Thailand. <i>Monoplectanum youngi</i> infected <i>S. indica</i>, representing a new host record, and <i>S. sihama</i> from both the middle and upper Gulf of Thailand. <i>Monoplectanum australe</i> was found to infect only <i>S. aeolus</i>, which was collected from the upper Gulf of Thailand. Overall prevalence and mean intensity of infections were 11.21% and 1.72, respectively. Regarding morphological features, no significant differences in key characters of both <i>M. australe</i> and <i>M. youngi</i> were observed when comparing the current and original descriptions. <i>Monoplectanum sillaginis</i> sp. nov., can be distinguished from these 2 species by the characteristics of its sclerotized male copulatory organ, particularly in the terminal part of the posterior prostatic reservoir, which is shuttle-shaped, as well as the number of rodlet rows in the squamodisc.