Gymnotus omarorum is described from coastal and interior drainages of Uruguay, where it is locally abundant in streams and lagoons, and is not known to occur sympatrically with congeners. This species has been used for more than 30 years as a model organism in neurophysiological research, where it has been referred to as G. carapo or G. cf. carapo. Gymnotus omarorum is a member of the G. carapo species group, with which it shares the presence of two pores in the dorsolateral portion of the preopercle, irregular (wavy) dark pigment bands which usually become broken and/or lose contrast with the ground color through growth, a clear patch at the caudal end of an otherwise darkly pigmented anal fin, and more than four arrowhead-shaped (anteroposteriorly compressed) teeth in the anterior portion of the dentary. Gymnotus omarorum is readily differentiated from other members of the G. carapo species group by the following unique combination of character states: a short distance to first ventral lateral-line ramus (39–45% TL vs. 47–58%), few pored scales to first ventral ramus (27–35 vs. 40–78), many ventral lateral line rami (16–37 vs. 0–14), and ovoid (vs. elongate) scales on the posterior portion of body.
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Vol. 2009 • No. 3