Haplognathia ruberrima is a cosmopolitan gnathostomulid species found in sulfur bacterial mats in mangroves in Guadeloupe (French West Indies). Haplognathia ruberrima presents a δ 13 C value lower than all measured meiofaunal grazers and lower than the available measured food sources of this environment. This low δ 13 C value can not be due to specific ingestion of 13 C‐depleted methanogenic bacteria because abundances of thos...