Burrow morphology, tube formation, and microarchitecture of shell dissolution by the spionid polychaete Polydora websteri

Burrow morphology, detrital tube formation within the burrow, and ultrastructure of the surface of the burrow of Polydora websteri Hartman, 1943, are described in the shells of Crassostrea virginica (Gmelin) and of Mytilus edulis Linné. Observations were made on live worms living normally in artificial preparations of polished shell covered with transparent plastic film. P. websteri is capable of settling wherever there are crevices in shell surfaces, and slowly penetrates the shell forming a U- or flask-shaped cavity. A detrital tube is formed within the burrow from detritus collected and pressed in place by the worm. During this process, the modified setae of the 5th setiger are thrust against the wall of the inner tube and help to keep the diameter of the inner tube constant throught its length. Individuals of P. websteri burrow into the shell when placed on preparations of polished shell of Mytilus edulis under plastic film. As they do so, they secrete a viscous fluid which dissolves interprismatic and interlamellar organic matrices and then dissolves the exposed crystals. Etched prisms and lamellae reveal a complex pattern of internal dissolution. Enlargement of the burrow by the worm thus takes place by chemical dissolution of shell and also probably by flushing of loosened partially dissolved prisms and lamellae from the tube. Some shell fragments become incorporated in the walls of the detrital tube. There is no evidence of setal abrasion on the shell walls of the burrow even at the ultrastructural level.