Rapidly changing abiotic conditions, particularly temperature variations, pose adaptation challenges to many animal communities across the globe. Lanice conchilega, a key polychaete in the ecosystem of the German Wadden Sea, faces an increasing threat hampered by its limited mobility. The microbiome can alleviate stress, alter local environmental conditions, and provide new metabolic capabilities, contributing to host adaptation. To understand the potential roles of the microbiome of L. conchilega, we analysed a broad sample set via 16 S rRNA gene amplicon sequencing. We compared the bacterial composition of the microbiome of L. conchilega with that of the surrounding sediment, and the effect of seasonality, zonation, tidal, and diurnal cycles. Our findings reveal a set of ASVs exclusive to the microbiome of L. conchilega. Community composition was mostly shaped by seasonality, slightly influenced by zonation, day/night cycles, and tides. Notably, Endozoicomonas ASVs were found only in the L. conchilega microbiome and were particularly abundant during summer. These bacteria, known for their symbiotic relationships, broad metabolic capabilities, and linked to heat resistance in corals, are predicted to contribute unique metabolic functions in L. conchilega. This work provides novel insights into the host-microbiome relationship of L. conchilega and highlights the potential role of symbionts in the environmental adaptation of these and similar animals.