General morphology and ultrathin structure of the gonopericardial complex of Chaetoderma nitidulum Lovén, 1844 (Caudofoveata)

Abstract

Aplacophora is a small enigmatic group of worm-like molluscs covered by sclerites. The general morphology of the posterior end, including the gonopericardial system, in many species is well studied using light microscopy. However, knowledge of the ultrastructure of this complex is fragmentary and many questions concerning the excretion of metabolites in this group still remain. The gonopericardial system of Chaetoderma nitidulum Loven, 1844 (Caudofoveata) from the White Sea was studied using light microscopy and 3D reconstruction from a series of semi-thin sections. The ultrastructure of the gonopericardial system was described in detail for the first time. The gonopericardial complex consists of gonads connected to the pericardium by ciliated paired gonopericardioducts. The pair of gonoducts begins at the lateral sides of the pericardium and opens into the mantle cavity. Gonoducts consist of upper and lower parts that are morphologically differentiated. The walls of the upper gonoducts consist of cells that likely participate in the excretion of metabolites and reabsorption of useful substances. Lower gonoducts are bulk bags with a small cell wall, in which there is a ciliary tract, likely for the excretion of metabolites and reproductive products. Spermatozoa were found at different stages of development in different parts of the pericardium and gonoducts. On the basis of previously published and newly obtained data, the process of excretion by C. nitidulum is proposed.