ИСТИНА

Monstrilloid copepods are widely distributed in the world’s oceans and can be fairly common locally within restricted depth horizons. The planktonic adults can be collected in substantial numbers by deploying light traps in the epipelagic zone. Monstrilloids display a protelean life cycle, combining a larval endoparasitic phase which is completed inside an invertebrate host (typically a polychaete or a mollusc) and free-living, nonfeeding adults which mate in the water column. While their abundance and diversity in the marine plankton has been well documented, the parasitic life cycle stages, including the infective first nauplius stage and the endoparasitic nauplii and copepodids, remain poorly studied. The endoparasitic stages are typically located in the body cavity of their host where they presumably feed on fluids or pre-digested tissues using up to three pairs of root-like absorptive processes (feeding tubes). The last copepodid leaves the host by rupturing its body wall after which it undertakes a final moult into the free-living adult stage. Most of our current knowledge on the life cycle of monstrilloids originates from Malaquin’s (1901) observations, which were based on very few species, and virtually no new data have arrived since then. Unique material, including several specimens of endoparasitic copepodids, was obtained from subtidal localities in Bergen, Norway and Okinawa, Japan. One late copepodid stage (possibly copepodid V), containing a fully developed female, was encountered in a benthic sample collected with a Beyer sledge at 140 m depth off the Espegrend Marine Biological Station. Three early copepodid stages were removed from a single individual of the polychaete Terebellides sp. (Trichobranchidae) obtained from the same sample. Several specimens of another polychaete species, Haplosyllus sp. (Syllidae), containing endoparasitic monstrilloid stages, were obtained from washings of the intertidal sponge Haliclona (Gellius) cymaeformis (Esper, 1806) from Okinawa. The morphology of the endoparasitic copepodids at different stages of development was studied for the first time using scanning and transmission electron microscopy and micro-computed tomography (mCT). These studies revealed unique adaptations to an endoparasitic mode of life. The position of the copepodids inside the host was revealed with light microscopy and mCT. Both 3-D reconstructions as well as SEM observations showed that there is a close interaction between the copepod and the polychaete. The copepodid is located between the gut and the body wall of the host, with its head directed towards the anterior end of the worm. The paired feeding tubes are backwardly directed, running along the ventral side of the copepodid. The surface of the feeding tubes is covered with a layer of host cells, suggesting an intimate connection. The morphology and ultrastructure of the absorptive processes were studied using TEM. The cuticle of the feeding tubes is very thin, forming numerous microvilli arranged in a dense regular pattern. The hypodermis consists of metabolically active cells forming apical micro-projections. Underneath the hypodermis, a layer of large, synthetically active cells is discernible, lining the central cavity of the feeding tube. No obvious muscles or nerve components were observed in the feeding tubes.