First Record and extended description of Dicyema platycephalum Penchaszadeh1969 (Dicyemida: Dicyemidae) in Octopus tehuelchus from northern Patagonia, SW Atlantic.

Dicyemids are highly specialized parasites found in the renal sacs of most cephalopod species. In this study, the description of Dicyemaplatycephalum is expanded from specimens infecting Octopus tehuelchus from the Patagonian coasts. The species is redescribed with histological sections, scanning electron microscopy, and molecular phylogenetic analysis of 18S rDNA sequences. This species had a distinctive disk shaped calotte with a configuration of ten cells (4 + 4 + 2), which is typical for the cephalic swelling of the genus. Histological sections revealed renal epithelial tissue with no signs of severe tissue damage, except in cases of heavy infection, where deformation of the epithelial folds and renal tubule obstruction were observed. Scanning electron micrographs showed the ciliated calotte inserted in renal crypts of host tissue. Many aspects of the dicyemid biology remain unknown, including how an infusoriform larva searches for and infects a new host. Attempts were made to infect uninfected juvenile octopuses experimentally reared, with potential infective larvae using three different exposure assays, but the attempts were unsuccessful. Nonetheless, this study broadens the description of D. platycephalum in the southern hemisphere using molecular phylogenetic data and contributes to the general understanding of the associations between dicyemid parasites and their cephalopod hosts.