(Krøyer, 1842)
Description:
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.
Habitat:
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.
Distribution:
It has been found on the coasts of Norway, Sweden, Denmark, northern France and in the Kiel Canal, Germany.