Body up to 2 mm long; rounded posteriorly. Cephalothorax display marked sexual dimorphism: anteriorly truncate in female and abruptly narrowed and elongate in male. Eyes prominent in both sexes, especially in the male. Antenna 2 with a strong dorsal spine at distal corner of second and third articles. Antenna 1 of female three-articled and without a flagellum, that of male with a three-articled peduncle and a two-articled flagellum. Antenna with six articles. Mandible with a cylindrical molar process and serrated chewing surfaces. Endite of maxillule bearing nine long spines, maxillipedal palp three-articled in male, four-articled in female.
Chelipeds of female normal, those of male very large with a caudal extension at the end of the carpus and with a downwardly deflected propodus. Bases of pereiopods 4 to 6 much thicker than those of pereiopods 1 to 3. Exopodite of uropods two-articled (articulation sometimes indistinct), endopodite four-articled.
This tube-dwelling species exhibits tolerance to a wide range of salinites from freshwater to estuarine and fully marine conditions, preferring sheltered places with silt-covered vegetation and a depth not exceeding approximately 10 m. H. oerstedi builds a tube using the chelipeds and anterior three pairs of pereiopods. The tubes may be stuck to plant material, solid surfaces, or in mud.
It has been found on the coasts of Norway, Sweden, Denmark, northern France and in the Kiel Canal, Germany.