Ragworm fatty acid profiles reveals habitat and trophic interactions with halophytes and with mercury

► Ragworm fatty acid (FA) profiles separate specimens from vegetated and unvegetated areas. ► Signature FAs reveal the trophic contribution of below-ground plant material to ragworms. ► In vegetated sediments ragworms display higher levels of alpha-linolenic acid. ► cis-Vaccenic/oleic acids ratio suggests higher carnivory in ragworms from unvegetated areas. ► Ragworms feeding behaviour is reflected in their accumulation of Hg.