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Alestion rapax, a new genus and species of miniature paedomorphic characiform fish with large conical teeth from the Lower Congo basin

by Tyson R. Roberts – aqua 25 (3) pp. 82-102

Alestion rapax, an entomophagous new genus and species of Alestidae, scaleless and largely translucent in life, possibly most closely related to Hydrocynus, is described from nine reproductively inactive specimens 12.0-16.8 mm standard length collected from a single locality in the Kwilu drainage near Moerbeke in the lower Congo basin in 1973. Eye large, body elongate and laterally strongly compressed. A number of its characters, such as a midventral fin fold, partly actinotrichous pectoral and caudal-fin rays, lack of scales and adipose eyelid may be neotenic or paedomorphic, while others are highly specialized. Circumorbital bones, including antorbital and lacrimal, absent. Dorsal-fin rays 10, its base entirely posterior to vertical through end of pelvic-fin base (dorsal fin rays usually 10 in Hydrocynus, its dorsal fin base above pelvic-fin base, its dorsal fin origin in front of or above vertical through pelvic fin origin). Anal-fin rays 14 (15-18 in Hydrocynus). Vertebrae 25-26+13-14=39-40 (versus 29-36+15-18=45-54 in Hydrocynus). Mouth slightly upturned, lower jaw with a large fleshy mentum. Apparently two rows of enlarged conical teeth on premaxillary near symphysis of jaws (nearly forming a single row) and two rows of somewhat larger teeth on dentary near symphysis. Outer row of dentary with three teeth and inner row with a single tooth as in alestids with multicupsid jaw teeth generally. Tooth replacement alternating from side to side of the jaw as in alestids and many neotropical characins having complex multicuspid teeth. Premaxillary of largest specimen with a single tricuspid replacement tooth, its small secondary cusps subapical (rather than basal, as in tricuspid teeth of very small Hydrocynus). Feeds on small aquatic insect larvae of the dipteran family Chironomidae.

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Volume 25, Issue 3 – 30 July 2019

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