The Columellar Muscle of Prosobranch Gastropods: Morphological Zonation and Its Functional Implications

Abstract

The arrangement of muscle and connective tissue fibers in the columellar muscle of three prosobranch gastropods, Calliostoma euglyptum, Littorina littorea, and Ilyanassa obsoleta, was analyzed. The columellar muscle in the three snails is a flat, sheet-like muscle that originates on the columella of the shell and inserts on the operculum. The muscle was found to comprise a densely packed three-dimensional array of muscular and connective tissue fibers, supporting the hypotheses of previous authors that it functions as a muscular hydrostat. The fiber arrangement is not uniform throughout the entire muscle; instead it includes three regions that grade into one another. Nearest its origin, the muscle consists primarily of longitudinal muscle fibers with few dorsoventral or transverse fibers, and no oblique fibers. The middle portion of the columellar muscle in the region posterior to the head consists of longitudinal, dorsoventral, and transverse fibers wrapped by two opposed layers of oblique fibers. The third region of the columellar muscle extends through the foot and includes longitudinal, transverse, and dorsoventral fibers with a layer of oblique fibers on the dorsal surface. A narrow band of spherical spaces divides the columellar muscle into dorsal and ventral halves in this region. The morphological zonation within the muscle suggests that different regions perform different functions. In addition, compared to a muscle that is circular in cross section, the elliptical crosssectional shape may increase the muscular force necessary to twist the foot relative to the shell and may decrease the resistance to buckling. Additional key words: muscular antagonism, muscular hydrostat, hydrostatic skeleton, Mollusca One familiar aspect of most coiled snails is their ability to retract the soft parts into the shell when disturbed. In coiled gastropods, the muscle responsible for retraction, the columellar muscle, originates on the columella of the shell and extends into the foot, where it inserts on the operculum. Although the function of the columellar muscle in retraction is straightforward, the mechanisms of elongation of the columellar muscle and protraction of the body from the shell are less obvious. Early studies of the anatomy of the gastropod foot and visceral mass noted that the longitudinal fibers of the columellar muscle were divided into bundles by muscle fibers oriented perpendicular to the axis of the muscle (Kiittler 1913; Weber 1926; Rotarides 1941). In a study of locomotion of Bullia digitalis, Trueman & Brown (1976) observed a similar three-dimensional array of muscle fibers in the columellar muscle and a Author for correspondence. E-mail: billkier.wilson @mhs.unc.edu hypothesized that such an arrangement could provide a mechanism for muscular antagonism. Later, Brown & Trueman (1982) briefly surveyed metapodia from five prosobranchs and two pulmonates and found a three-dimensional network of muscle fibers in the columellar muscles of each. Finally, Voltzow (1990) described a three-dimensional array of muscle fibers in the columellar muscle in the foot of Busycon contrarium and in the pedal retractor muscle of Haliotis kam-