Abstract
<jats:title>Abstract</jats:title> <jats:p>The Polydora complex, a group of genera within the polychaete family Spionidae, includes species that range from free-living, sediment-dwelling organisms to obligate shell-boring parasites. This chapter proposes an evolutionary trajectory for this transition, emphasizing how pre-adaptive traits may have facilitated the emergence of parasitism in this group. Traits such as tubicolous behavior, suspension feeding, and reproductive plasticity are explored as potential precursors to shell-boring and other parasitic strategies. These traits, coupled with ecological flexibility, likely enabled members of the Polydora complex to exploit diverse hosts and substrates. The chapter examines host–parasite interactions, with a focus on the significant ecological and economic impacts of shell-boring polydorids. While these parasites do not directly feed on host tissues, their burrowing behavior imposes substantial costs, including shell damage, reduced growth, and increased vulnerability to predation. These impacts are particularly critical in aquaculture settings, where infestations can severely compromise production and salability. Future research directions are outlined to address gaps in the understanding of parasitism in the group. These include investigating the genetic and molecular mechanisms underlying shell-boring, characterizing the biochemical role of mucopolysaccharides in shell penetration, and studying host–parasite co-evolution. The chapter also emphasizes the need for a robust phylogeny of the Polydora complex to test hypotheses about the origins and diversification of parasitic behaviors.</jats:p>