Medium sized to large larvae. Body pale red. Head capsule rather long, conical; cephalic index about 0.7. Dorsally, DP absent, S7, S8 and S5 form a diagonal near-aligned row, with S5 far retracted, posterodorsal to S8. Ventrally SSm posterior-most, VP located lateral to anterolateral to S10 and S9 which are close to each other. S5, SSm and S7 multi-branched, S8, S9 and S10 strong, simple.
Long, 2x as long as mandible. Antennal ratio about 7 (6.5-7.5). Basal segment about 9x as long as basal width, with ring organ at 0.8; segment 2 weakly curved, about 6x as long as wide; segment 3 2x as long as wide, as long as width of segment 2; distal segment 1/2 as long as segment 3. Peg sensilla extending somewhat beyond last segment, 1/2 as long as style. Blade thicker than segment 2, as long as flagellum. Accessory blade reaching to segment 3. Flagellum and blade somewhat recessed into basal segment.
Slender. Apical tooth large, strongly curved (but not hooked), about 3x as long as basal width, 1/4 length of mandible; broad, rounded, apically-directed lamella, lies dorsal to inner margin below apical tooth; mola formed into smooth, rounded dorsal lamella, from ventral part of which arises short seta subdentalis. Basal 1/2 of mandible expanded on inner side. Pecten mandibularis formed as sloping ridge, extending from base with 5-6 adjacent, large, somewhat curved spines. Ventrolateral setae simple, with Ring organ between SII and SIII, equidistant and clustered on margin
Basal segment of palp very long, sclerotized section 5x as long as wide and occupying 1/2 length of basal segment of palp; ring organ situated between 0.7 and 0.8 of sclerotized section. Base of sensory organ 1/2 as long as basal segment, strongly sclerotized along one side.
With sparse fringe of swim-setae. Dorsal anterior margin of body segment 4 (abdominal segment 1) with pair of small sclerotized hooks. Four conical anal tubules, 2x as long as wide, lying between posterior parapods. Single papilla, smaller than anal tubules, located between procerci. Procercus 1.5-2.5x as long as wide, with 16 apical setae. Claws of posterior parapod pale, about 15; long claws very slender apically, curved outwards, inner and/or outer side with small, fine points; smallest claws simple, weakly curved.
Coelotanypus is closely related Clinotanypus, occupying an isolated position within the Tanypodinae, in the tribe Coelotanypodini. These two genera may be distinguished from other Tanypodinae particularly by the structure of the hypopharyngeal and mental-prementum complexes and by the mandible, by which the two genera may be distinguished from each other. The mandible of Coelotanypus is gently curved and the large inner projection is a bluntly-rounded, broad lamella instead of the characteristically stout pointed tooth of Clinotanypus. Coelotanypus uniquely has a pair of small sclerotized hooks on the dorsal anterior margin of body segment 4 (abdominal segment 1).
Epler (2001) keys and provides notes on their ecology and distributions for three species of s.e. USA Coelotanypus.
Larvae of Coelotanypus live in benthic sediments of lakes, including artificial impoundments, slowly-flowing reaches of rivers and in old riverbeds.
Five species are known from North America but Coelotanypus is unknown from the Palaearctic. Diversity is highest in the neotropics where 8 named species are known from Central America including the Caribbean (Spies and Reiss, 1996; Andersen et al., 2000) and more than 10 species from South America. Larval Coelotanypus may attain high abundance in floodplain lakes from Yucatán (Mexico) to Cordoba (Argentina) and the wetlands of s.e. Brazil (Panatta et al., 2007). One species is known each from Africa and Australia.