Structural characteristics of self-excavated burrows by boring polydorid species (Polychaeta, Spionidae)

Burrow structure of three common boring spionids, Polydora pygidialis, Dipo lydora giardi, and Dipolydora bidentata, inhabiting Barkley Sound, Vancouve r Island, Canada, was examined using SEM. The surface of burrows excavated in nine mollusc shells consisting of four types of microstructure (foliated , nacreous, prismatic, and crossed lamellar) was studied to obtain informat ion about the boring mechanism of these species. Small characteristic conce ntric-edged holes were observed on the surface of the burrows of each speci es; however, the existence of concentric-edged holes depended on the differ ence of the shell microstructure. Worm-eaten crystals, bottom parallel scra tches, and lateral scratched structures were also observed on the surface o f the burrows. We speculated that the polydorid worms (1) secrete some chem ical substance which directly acts and dissolves the crystals and a part of organic matrix first and makes them weaker and (2) scratch and loosen them mechanically both in moving back and forth along the burrow and by a rotar y motion within the burrow. We further speculated that some chemical substa nce secreted by worms and organs involved in the boring activity may be the same among the polydorid species, that the boring mechanism among polydori d species may be the same, and that the existence of concentric-edged holes or worm-eaten structure provides evidence of polydorid infestation.