Halicryptus spinulosus is a species of priapulid worm found in marine environments. It is a macrobenthic priapulid living in disjunct subarctic brackish seas, including the White Sea, waters off Iceland, northern Siberia and Alaska, and in an outlying range in the
Baltic Sea. Known for its distinctive spiny appearance, this worm plays a role in the benthic ecosystem, often inhabiting sandy or muddy substrates. Like other priapulids, it has a relatively simple body structure and is considered a predator, feeding on small invertebrates. Its presence provides insights into the evolutionary history and ecological roles of priapulids, linking modern forms to ancient lineages. lives in sulfide-rich, oxygen-poor sediments. During elevated sulfide levels it increasingly relies on anaerobic metabolism and turns black, due to a film of bacteria covering its cuticle. ==See also==