HOCHBERG, R; Smithsonian Marine Station at Ft. Pierce: New insights on the structure and function of gastrotrich helicoidal muscles – evolutionary biomechanics of a contractile helix

Gastrotrichs are characterized by a suite of unique morphologies (apomorphies) that set them apart from other animals. Among the more recent findings include muscles that form distinct spirals around the digestive tract. These cellular spirals in the form of crossed-helices form alternating right- and left-handed geodesic springs that make alpha angles of ca. 54 degrees, the ideal solution for the fiber angle in a thin pressurized cylinder of circular cross-section. Based on new observations with confocal scanning electron microscopy (CSLM) and TEM, the helices are constructed of obliquely-striated muscle cells and are positioned external of nearly all other muscles that contact the gut tube. The crossed-helices are hypothesized to mechanically constrain the shape of underlying muscles, muscle function, gut shape, and the rigidity of the hydrostatic digestive tract. Attachment forces maintain a constant position of the helical center with respect to the longitudinal body axis, permitting radial expansion of the gut tube without linear expansion. A preliminary biomechanical model based on this system shows that the helices function as both reinforcer and mechanical spring; they do not deflect or distort under longitudinal body constriction and they restore original body shape without the necessity of antagonistic contractive forces by circular muscles.