Type species: Spio mecznikowianus Claparède, 1869 Type locality: Mediterranean Sea, Gulf of Naples
Synonym: Mesospio Gravier, 1911; type species: Mesospio moorei Gravier, 1911, by monotypy.
Diagnosis. Prostomium anteriorly rounded, bilobed or bifid; metameric dorsal ciliated organs present; branchiae present from chaetiger 2, restricted to anterior region or continuing to posterior end; bi- to multidentate hooded hooks on middle and posterior neuropodia; 2–4 anal cirri.
Description. Prostomium anteriorly rounded to deeply incised but 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 body region or continuing to posterior end, 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.