Ultrastructural differences in presumed photoreceptive organs and molecular data as a means for species discrimination in Polygordius (Annelida, Protodriliformia, Polygordiidae)

Abstract

Members of Polygordiidae Czerniavsky, 1881 are distributed worldwide and are typical inhabitants of coarse grained, marine sublittoral sediments. Species in Polygordiidae are unusual among polychaetes because they lack external segmentation, parapodia, and chaetae as well as external ciliation. Morphological characters for species distinction are thus limited to the shape and minute structures of the prostomium and pygidium. As a result, morphological identification and description of the Polygordius species is difficult. In the present study, we provide new data for species distinction using molecular and morphological methods, i.e., DNA-sequencing (COI gene), transmission electron microscopy (TEM), and confocal laser scanning microscopy (cLSM). Specifically, we investigated whether the photoreceptor-like sense organs, previously described for Polygordius appendiculatus Fraipont, 1887, could provide additional characters of taxonomic value by examining the ultrastructure of these organs in two additional species, Polygordius erythrophthalmus Giard, 1880 and Polygordius lacteus Schneider, 1868. These organs were found to occur in comparatively large numbers and comprise different types of sensory cells including one rhabdomeric and two ciliary, which are variously combined to form small sensory organs. Structural differences in sensory organs were negligible between P. erythrophthalmus and P. lacteus; however, major differences were observed when compared to P. appendiculatus. These results are in accordance with our molecular data which indicated that P. erythrophthalmus and P. lacteus represent the same species. Thus P. erythrophthalmus is an invalid species and represents a junior synonym of P. lacteus. Furthermore, this study shows that ultrastructural data can significantly contribute to species distinction, especially in morphologically similar species.