The taxonomy of Diplectanum Diesing, 1858, a genus of monopisthocotylean monogeneans, remains unsettled and needs to be revised based on new morphological criteria. Recent studies in monopisthocotyleans have shown that the muscle arrangement in the posterior attachment organ (haptor) differs between congeneric species and can be used as an additional criterion in genus-level taxonomy. To explore the possibility of using the haptoral musculature and nervous system in the taxonomy of Diplectanum, we conducted a detailed confocal-microscopy study of three species of Diplectanum (D. aculeatum Parona et Perugia, 1889, D. sciaenae van Beneden et Hesse, 1863 and D. similis Bychowsky, 1957) with phalloidin staining for muscle and indirect immunostaining for 5HT and FMRFamide. A further goal was to clarify the functional mechanics of the haptor and the role of its essential components (squamodiscs and anchors) in attachment to the host. The system of connecting bars and gaffing anchors was found to have a complex musculature consisting of 23 muscles in D. aculeatum and D. sciaenae, and 21 muscles in D. similis. The squamodiscs were shown to be operated by several groups of muscles attached primarily to the area termed the squamodisc fulcrum. Most of the haptoral musculature is identical in D. aculeatum and D. sciaenae and these species differ only in the presence of a muscle sheath around the tissue strand between the squamodiscs in D. sciaenae and in the different patterns of superficial squamodisc muscles. Diplectanum similis shows more significant differences from the other two species: besides lacking two of the haptoral muscles, it also differs in the shapes and arrangement of several other muscles. The nervous system of all three species conforms to the general pattern typical for the Dactylogyroidea and shows little variation between species.
Keywords: 5HT; FMRFamide; Monopisthocotylea; anchor; haptor; musculature; phalloidin; squamodisc.