1. Areas VII–VIII with a single row of denticles (rarely 2–5 supernumerary paragnaths)...............2
– Areas VII–VIII with two rows of denticles; palpophores continuous, 3–4 × as long as wide; area II with about 15 paragnaths per side...............................................................................................7
2. Palpophores with a transverse depression (looking bisegmented); longest tentacular cirri reaching chaetigers 2–3; area II with about 15 medium-sized paragnaths per side................................................................................................................................ H. limnicola (Johnson, 1903) (California)
– Palpophores continuous, 2 × as long as wide...................................................................................3
3. Area II with 10 or fewer large paragnaths per side; longest tentacular cirri reaching chaetigers 6–9; areas VII–VIII with 19–26 paragnaths.............................. H. japonica (Izuka, 1908) (Japan)
– Area II with more than 20 tiny paragnaths per side; longest tentacular cirri reaching chaetigers 8–9; areas VII–VIII with 9–35 paragnaths............... H. atoka Sato & Nakashima, 2003 (Japan)
– Area II with 10–20 paragnaths........................................................................................................4
4. Areas VII–VIII with 8 paragnaths; longest tentacular cirri reaching chaetiger 5................................................................................. H. sakhalinensis (Okuda, 1935) (Sakhalin Island, Japan Sea)
– Areas VII–VIII with 11–41 paragnaths..........................................................................................5
5. Posterior parapodia with median ligule as long as dorsal one; longest tentacular cirri reaching chaetigers 6–12...................................................... H. diadroma Sato & Nakashima, 2003 (Japan)
– Posterior parapodia with median ligule markedly smaller than dorsal one.................................6
6. Longest tentacular cirri reaching chaetiger 6.................. H. lighti (Hartman, 1938) (California)
– Longest tentacular cirri reaching chaetiger 2...... H. rabatensis (Mohammad, 1989) (Morocco)
7. Neuropodial anchylosed falcigers subdistally constricted; tentacular cirri reaching chaetigers 5–7.................................... H. diversicolor (Müller, 1776) sensu Hartmann-Schröder 1996 (Baltic Sea)
– Neuropodial anchylosed falcigers subdistally swollen; tentacular cirri reaching chaetigers 5–9.......................................................................................................................................... H. diversicolor (Müller, 1776) sensu Khlebovich 1996 (Black Sea; matching N. falsa (de Quatrefages, 1866))
Smith (1959), after comparing the types, concluded that H. lighti was a junior synonym of H. limnicola. They differ in pigmentation in living specimens, H. limnicola being reddish, whereas H. lighti is transparent. A more detailed comparison is needed to confirm this synonymy. Day (1967) described Nereis gilchristi based on a single small specimen (8 mm long, 40 chaetigers) collected in South Africa. He indicated that it has only two notopodial ligules in anterior chaetigers, which is an important difference and deserves a separate assessment when assigning it to a genus. The key above should be used cautiously because the status of the type species must be clarified, since there are two cryptic species in the Baltic Sea (Röhner et al. 1997; Virgilio et al. 2006; Audzijonyte et al. 2008), and some Mediterranean and Black Sea forms (Virgilio et al. 2009; Cossu et al. 2012) might necessitate the reestablishment of some names currently regarded as junior synonyms.