Published On: Mon, Jan 14th, 2019

Roles of Keber’s valve and foot chamber for foot manipulation in the mussel Nodularia douglasiae [RESEARCH ARTICLE]

INTRODUCTION

The main functions of the circulation system are (1) convection of solutes and heat, and (2) transmission of forces (Schmidt-Nielsen, 1983). The Mollusca Bivalvia has an open circulatory system and hydrostatic pressure is considered to be the major driving force of motion of the body (Brand, 1972; Trueman, 1983). Burrowing activity has been studied for many years (Morton, 1964; Trueman, 1983; Trueman et al., 1986; Winter et al., 2012). The burrowing process involves the extension and probing of the foot into the sand, followed by adduction of the shell and retraction of the foot, drawing the shell down towards the anchored tip of the foot (Brand, 1972). Brand (1972, 1976) observed pressures in the ventricle and the interstitial space of the foot (pedal haemocoel) of the Anodonta anatine. In the foot haemolymph is supplied by the pedal artery, filling the pedal haemocoel, and then it is accumulated into the pedal sinus. At the proximal end of the pedal sinus, Keber’s valve controls venous return to the heart via the visceral sinus (Brand, 1972; Keber, 1851). During the extension of the foot, the pressure of the haemocoel increased gradually followed by an increase of the systolic ventricular pressure up to 50 mm H2O. Then, at the adduction of the shell, the pressure of the haemocoel was transiently increased to several times that of the ventricular pressure (Brand, 1972). This high pressure might be caused by the contraction of the adductor and retractor muscles, and it is necessary to keep the Keber’s valve closed. Therefore, control of the opening and closing of Keber’s valve is essential for the burrowing process. However, as far as we have found in the related literature, no one has observed the opening and closing of Keber’s valve during the foot extension or retraction. In addition, it has also been postulated that the outflow of haemolymph is allowed only via Keber’s valve (Trueman, 1966). However that is not likely the case, because the foot contains not only the haemocoel, but also the intestine, digestive glands and gonad cells. The proximal end of the foot is not sealed by the dense muscle layer, and the haemolymph could leak into the dorsal visceral mass including the stomach, heart and gills. Thus, it may be that the pressure of the pedal haemocoel could not be retained during the foot extension. Therefore, in this study, we first observed the anatomical structure of the foot to confirm the route of the outflow of the haemolymph. Then, we measured the haemolymph flow (hereafter, ‘flow’ alone) in the ventricle, pedal artery, pedal sinus, visceral sinus and Keber’s valve in the resting state, as well as during foot extension and retraction, in order to test the conventional model of extension and retraction of the foot.

Nodularia douglasiae (Gray, 1833) was selected as the experimental animal, since this is in the same family of Unionoidae, A. anatine (Linnaeus, 1758). Unfortunately, N. douglasiae was not the Unionoidae in which the presence of the Keber’s valve was previously described (Keber, 1851; Schwanecke, 1913; Willem and Minne, 1898, Brand, 1972), so we conducted a histological study on Keber’s valve and the adjacent sinuses and organs in N. douglasiae. The flow and heartbeats were detected by magnetic resonance imaging (MRI), which was employed to detect the flow of seawater in the mantle cavity and to measure the heart rate as in our previous reports (Seo et al., 2014b, 2016). Since we could not force the mussels to rest, extend or retract the foot, we conducted more than 100 sessions of MRI measurements. Then the images obtained were sorted into the resting, extension and retraction conditions of the foot. The interbeat interval (IBI) of the heart, as well as the flow in the anterior aorta, pedal artery, pedal sinus and the visceral sinus, obtained under the three conditions mentioned above, were examined to clarify the roles of Keber’s valve in the extension and retraction of the foot.

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