The accessory hearts of the articulate brachiopod Hemithyris psittacea

Abstract

Scientists have long known that the articulate brachiopods have accessory hearts in addition to the main heart. However, the critical factors that determine the location of the accessory hearts and their fine structure remained unclear. We studied the location and fine structure of the accessory hearts in the articulate brachiopod Hemithyris psittacea that is necessary for a better understanding of their function. We observed regular arrangement of the accessory hearts behind the nephridial funnels, close to the gonad vessels, but each examined specimen showed an individual pattern of the accessory heart location. The accessory heart wall is composed of outer coelomic epithelium, extracellular matrix, and inner amoebocytes. The coelomic epithelium consists of epithelio-muscle cells with the basal muscle processes, which give rise to the secondary thin non-muscular processes, interdigitating with each other and forming irregular ultrafiltration slits. The inner amoebocytes do not form a continuous layer and lack desmosomes; thus, they cannot be considered as a true epithelium. Brachiopod accessory hearts apparently fulfill two functions. The well-developed muscular net in the accessory heart wall performs the propulsion of blood to branched gonad vessels. The epithelio-muscle cells in the accessory heart wall are the podocyte-like cells; therefore, the accessory heart wall can be interpreted as the ultrafiltration site. The association of accessory hearts with nephridia suggests their involvement in excretion.