Morphological and Molecular Identification of Octolasmis spp. Ectoparasites Infesting Scylla serrata in Southwest Papua, Bird's Head Island Peninsula

Despite existing reports, the cryptic diversity and species-specific ecological impacts of the parasitic barnacle Octolasmis on the economically important mangrove crab S. serrata remain poorly understood, particularly in understudied regions such as Papua. This study provides the first integrative taxonomic analysis (combining morphology and mitochondrial COI gene sequencing) of Octolasmis spp. infesting S. serrata from the Bird's Head Peninsula. Two species, O. cor and O. angulata, were identified. Phylogenetic analysis confirmed their strong genetic distinctness, which correlated with key morphological divergence, most notably in the development of the capitular disc (28.3% coverage in O. cor versus 10.5% in O. angulata). These fundamental morphological differences are consistent with the contrasting ecological strategies observed. O. cor demonstrated clear ecological dominance, exhibiting the highest prevalence (85%) and the greatest single-species infestation intensity (40 individuals per infested host), whereas O. Angulata showed lower prevalence (72%) and significantly lower intensity (15 individuals per infested host). Crucially, co-infestation was associated with the highest parasitic load, showing an intensity of 62 individuals per infested host individual that exceeded the sum of individual infestations, suggesting a synergistic effect. Our findings indicate that interactions between morphologically distinct ectoparasite species may contribute synergistically to the total parasitic burden on the host.