Microphthalmus Mecznikow 1865

Microphthalmus Mecznikow, 1865 Type species. Microphthalmus sczelkowii Mecznikow, 1865. Diagnosis. Very small bodied. Prostomium with finger-like median antenna positioned mid-dorsum at prostomial posterior edge at level of segment I; pair of lateral antennae and palps positioned antero- ventrally. Achaetous segments I–III each with two pairs of dorsal and ventral tentacular cirri, total six pairs of cirri. Distinctly shorter ventral cirri of segment III compared to five tentacular cirri of three anterior achaetous segments. Sub-biramous parapodia. Notochaetal fascicle absent to less developed; simple notochaetae with one or two rows of subdistal serrations. Neuropodia sometimes with hooked acicula spines; compound falcigerous fascicle sometimes with simple neurochaetae. Pygidium with filamentous dorso-lateral anal cirri and ventral anal lamella lobe with or without fimbriate papillae, with or without medial notch. Remarks. The genus currently includes 35 species (Read & Fauchald, 2024b), the majority of which are very small-bodied (~ 2 mm) interstitial organisms living in subtidal habitats (Westheide, 2013; Salazar-Villejo et al., 2019). Deep-sea dwelling species include M. bifurcatus Hartmann-Schröder, 1974 from 310–500 m in the Skagerrak Strait and Microphthalmus sp. from sediment and whale bones at 4,200 m in the SW Atlantic (Sumida et al., 2016).