Medium-sized larvae, up to 6 mm long.
Antenna 5 segmented, segment 1 about as high as wide; segment 2 subequal in height to first but narrower; segment 4 subequal to or longer than third. Blade extending no further than segment 4, usually seen to be divided with fine inner blade. Lauterborn organs weak. Small style present.
Labrum with SI plumose with 10 or more branches; SII also plumose, finer, with fewer branches; SIII and SIV normal. Weakly sclerotized labral lamellae present with several teeth. Pecten epipharyngis consisting of 3 small, basally fused scales; 1 broad chaetula lateralis present, overlying the bases of a further 4 or 5 scales. Premandible with blunt apical tooth and broad, blunt inner tooth; brush absent.
Mandible with apical tooth shorter than combined width of 3 or 4 inner teeth. Seta subdentalis present. Seta interna with 5 or 6 strongly plumose branches.
Mentum with domed median tooth 2 times the width of first of 4 pairs of lateral teeth; a weak basal tooth is present. Ventromental plate weak, not extending beyond outer lateral mental tooth.
Maxilla with 2 strong setae present on apex of palp; beard absent. Sparse simple lamellae on galea and base of palpiger; setae maxillaris weakly serrate. Distinctive setose appendix present.
Anterior parapods partially fused basally. Posterior parapods separate; both pairs of parapods with claws. Procercus absent; single strong anal setae present. Anal tubules absent. Supraanal seta present.well developed, separate and each bearing an apical crown of claws. Procercus very short, bearing 2 long thick and 2-6 shorter, thinner anal setae, without subapical setae but 1 seta arising anterior to procercus. Supra-anal setae absent. Anal tubules shorter than posterior parapods. Anal end reduced with procerci lying dorsal to anal tubules. Body setae simple.
Some 18 species of Clunio are described, of which 9 are Holarctic and 6 Japanese. The genus is worldwide except for Antarctica.
Larvae of Clunio live in the marine intertidal, with at least C. marinus found in more brackish Baltic pools to 4% salinity. Some reports claim damage to Mytilus and Ostrea, and the larvae are said to be omnivorous feeding on algae and dead animals.