A morphological and molecular reinvestigation of (Grassi, 1881) Chatton 1953 - an amoebozoan parasite of arrow-worms (Chaetognatha).

Amoebozoan parasites of arrow-worms (Chaetognatha) were isolated from their hosts living in plankton of the Bay of Villefranche (Mediterranean Sea). Based on the light microscopic characters, the amoebae were identified as (Grassi, 1881) by their limax locomotive form and due to the presence of the intracellular symbiont, , surrounded by a layer of pigment granules. Sequences of the 18S rRNA gene of both and its symbiont were obtained for the first time. The molecular phylogenetic analyses of 18S rRNA gene placed within the genus , a taxon also characterized by the presence of a symbiont, known as -like organism (PLO). The 18S rRNA gene sequence of from grouped with the sequences of 18S rRNA genes of PLOs from and . The first photo documentation of the light microscopic features of , such as locomotive form, the morphology of the nucleus and have been provided. The new results support the affinity of with the family Paramoebidae suggested previously based on the presence of PLO. In contrast to , typical members of Paramoebidae ( and ) have a flattened, dactylopodial locomotive form. This discrepancy in morphology can be explained by the obligate parasitic lifestyle of .