Functional Significance of Varices in the Muricid Gastropod Ceratostoma foliatum

Functional significance of varices in the muricid gastropod Ceratostoma foliatum was investigated from the standpoints of (1) frequency of landing in the two upside-down orientations after short vertical falls of less than five body lengths through seawater and energy costs of righting from these upside-down positions, and (2) scaling relationships of varix areas with other body dimensions. Field manipulations showed that C. foliatum occupied habitats that mostly permit short falls of less than five body lengths upon dislodgment, as might occur during predation by fish. After short vertical falls in the laboratory, animals landed 48% of the time on their aperture sides (upright), 15% on their right sides (on right and middle varices), and 37% on their left sides (on left and middle varices). These frequencies differed significantly from the expected frequencies calculated on the basis of the percentage circumference delineated by each varix pair (50%, 31%, and 19%, respectively). Righting from the right-side orientation was slower and four times more energetically costly than from the left-side orientation, underscoring the advantage conferred by animals, if not landing in the upright position after short falls, preferentially landing on their left sides. Removal of individual varices showed that the large, right varix is most influential in producing this "destabilization." Landings are biased to the side from which rightings are easiest due to a combination of the location of center of mass within the left side of the main body whorl and the broad right varix possibly acting as an upward-trailing vane. Morphometric relationships of shell length, live weight, varix areas, aperture dimensions, and labial spine (tooth) length were investigated over a wide range of body sizes in an attempt to infer varix function. Aperture area scaled allometrically with length. Right-, middle-, and left-varix areas also grew relatively larger as the animals increased in length. In contrast, combined varix areas around the aperture increased in direct proportion with aperture area, forming a broad shelf surrounding the aperture. We infer from this that, in addition to their effects on landing orientation from both long and short vertical falls, the varices of C. foliatum may function to protect the aperture, and thus protect the soft body parts that protrude from it during feeding and locomotion.