UC San Diego

Juvenile mermithid nematodes are endoparasitic worms that infect and kill their arthropod hosts, typically insects. Here, we provide the first report of a mermithid infecting a marine decapod crustacean. In an initial parasitological survey in December 2018, we encountered juveniles of a new mermithid species infecting the hemocoel of Hemigrapsus oregonensis (Decapoda: Varunidae) at a single site in San Diego, CA. Prevalence at that time was 15.5% (95% CI: 8.4-26.9%), and a latter survey in April 2019 yielded a lower prevalence of 5.6% (95% CI: 1.6-18.2%). Infection probability and the abundance of mermithids peaked in smaller crabs, and all infections were multiple infections (intensity ranged from 2-10). No new infections were found in subsequent surveys at the same or three other sites in San Diego County between June 2019 and February 2020 (n = 528 crabs), precluding efforts to rear worms to adulthood for thorough description. We examined parasitic juveniles using light microscopy to provide a morphological description of this stage. Preliminary phylogenetic analysis of 18S rDNA confirmed the nematode’s placement in the family Mermithidae. Because only parasitic juveniles were available, we plan to assign this new species to the genus Agamomermis, a collective group for immature mermithids. Individuals of this new species, to be named elsewhere as Agamomermis grapsifarciens n. sp., are long (20-65mm), white, filiform worms with a discernible stichosome, a prominent trophosome, six cephalic papillae arranged hexagonally in a single plane, a terminal, centered mouth, a homocephalic head, and a bluntly rounded tail lacking a tail appendage.