Type species: Halacarus parvirostris Trouessart, 1889.
Adults. Female and male with GA and both with single pair of internal gac. In female pair of gac in about middle of GO (Newell 1984: fig. 573). GA with three pairs of slender pgs, genital sclerites with a pair of sgs. Ovipositor at rest hardly extending beyond GO; when everted apical 11 bi- or trifid genital spines seen (Bartsch 2008 b: fig. 7). In male gac in posterior part of GO (Bartsch 1996 d: fig. 7). GA with about 30–40 slender pgs arranged in a dense ring around GO, genital sclerites with four to five pairs of sgs; anterior pairs setiform, posterior pairs either seti- or spiniform. AE with pair of large epimeral pores.
Juveniles. A larval and a single nymphal stage present. In protonymph GP separated from AP. GP without setae but with a pair of internal gac. Larva as in nymphs and adults with epimeral pores.
Remarks. The genus is spread in tidal as well as in deep sea basins but seems to be restricted to southern seas. Ten species have been described (Bartsch 2008 b).