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

Burrow structure of three common boring spionids, Polydora pygidialis, Dipolydora giardi, and Dipolydora bidentata, inhabiting Barkley Sound, Vancouver 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 information about the boring mechanism of these species. Small characteristic concentric-edged holes were observed on the surface of the burrows of each species; however, the existence of concentric-edged holes depended on the difference of the shell microstructure. Worm-eaten crystals, bottom parallel scratches, and lateral scratched structures were also observed on the surface of the burrows. We speculated that the polydorid worms (1) secrete some chemical 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 rotary motion within the burrow. We further speculated that some chemical substance secreted by worms and organs involved in the boring activity may be the same among the polydorid species, that the boring mechanism among polydorid species may be the same, and that the existence of concentric-edged holes or worm-eaten structure provides evidence of polydorid infestation.