Shape and surface variations of syphon openings during complete tidal cycles in Mya arenaria in the intertidal zone

Since the degree of syphonal opening call be an index of the pumping activi ty, the shape and the surface of syphon openings in Mya arenaria were filme d with all underwater camera at two stations middle of the intertidal zone. The individuals were perpendicularly oriented to the main current directio n or parallel with the inhalant syphon upstream during flood tide, causing refiltration risks during ebb tide. The surface of the inhalant Opening (SI ) was strongly reduced with increasing current speeds. Its shape (XI) varie d with the individual's orientation and had a tendency to become more circu lar with time. The surface of the exhalant opening (SE) decreased and its s hape (XE) flattened with increasing current velocity and with time. However , variations of XI and XE were weak. Current direction had no significant e ffect oil SI, SE, and XE, but did cause a strong decrease of the SE/SI rati o during ebb in individuals exposed to important refiltration risks during ebb tide. Significant negative correlations between stomach content in phae opigments and Sl and SE suggest that a syphonal constriction could contribu te to more efficient feeding. For parallel oriented individuals, the decrea ses of SE, SE/SI, and XE during ebb call then be interpreted as all attempt to deviate or increase the excurrent velocity relative to the incurrent so as to limit refiltration. We suggest that decreases in opening surface and shape may serve, above all, to increase syphon current jet velocity in syp honate bivalves.