Live observations of endoparasitic stages of <i>Cymbasoma</i> sp. and <i>Monstrilla</i> sp. (Copepoda: Monstrilloida) utilizing sympatric sponge-associated <i>Haplosyllis</i> spp. (Polychaeta: Syllidae) in Okinawa

Members of the order Monstrilloida are enigmatic copepods, displaying a protelean life cycle that encompasses a modified nauplius acting as the infective stage, endoparasitic copepodids, and free-living, non-feeding, planktonic adults. Although adult monstrilloids can locally be very abundant in plankton samples, their larval endoparasitic stages are rarely encountered and, consequently, their host-parasite relationship remains poorly understood. During our study, two different morphotypes of larval monstrilloid copepods were found parasitizing two sympatric polychaete species (Haplosyllis spp.), which are themselves associated with a sponge host in the same location in Okinawa, Japan. Live observations of the infected worms enabled us to observe the emergence of monstrilloid copepodids from the host and their subsequent development into preadults and even adults. The release of the copepodid from the host was initiated and facilitated by the feeding tubes penetrating the host body wall. The two morphotypes, including their endoparasitic copepodid stages, differed significantly and could easily be distinguished by the number and shape of feeding tubes, color of ovaries and leg pigmentation. After molting outside the host, copepodids developed into morphologically distinct females and males which could be assigned to the genera Monstrilla Dana, 1849 and Cymbasoma Thompson, 1888, respectively. Molecular phylogenetic analyses based on both nuclear and mitochondrial markers confirmed that the morphotypes represent two different species. Sampling at several sites along the coast of Okinawa showed high infection rates in the Seragaki area throughout the year. The high prevalence of endoparasitic stages, in conjunction with the ready access to live material of the host, makes the Okinawan copepod-polychaete association a promising model for future research on the host-parasite interactions between monstrilloids and their invertebrate hosts.