Microspio Mesnil, 1896; type-species: Spio mecznikowianus Claparède, 1869.
Mesospio Gravier, 1911; type-species: Mesospio moorei Gravier, 1911, by monotypy.
Diagnosis. Prostomium anteriorly rounded to deeply incised, frontal or lateral horns always absent; eyes present or absent; occipital antenna present or absent. Nuchal organ with short median and long lateral ciliary bands, extending to chaetiger 2 or 3; metameric dorsal ciliated organs present. Branchiae present from chaetiger 2, restricted to anterior region or continuing to posterior end of body; partly fused at base with notopodial postchaetal lamellae. Notochaetae and anterior neurochaetae all capillaries; capillaries, hooded hooks and inferior sabre chaetae on middle and posterior chaetigers; hooks bi-, tri-, or multidentate. Pygidium with 2–4 anal cirri.