A study of the South American catfish genus Tetranematichthys has revealed a second species and the revelation that the fish may use its unusual barbels as a fishing lure to catch prey fish.
Tetranematichthys, a genus of sexually dimorphic auchenipterid driftwood catfishes, was previously believed to be monotypic with a single species - T. quadrifilis - having a broad range across most of the Amazon basin and the southern tributaries of the Rio Orinoco.
However, new research on the genus by Richard Vari and Carl Ferraris Jr has shown that what was previously considered a single widespread species actually consists of two distinct species.
Vari and Ferraris, who published their findings in the latest issue of the ichthyological journal Copeia, say that Tetranematichthys quadrifilis is endemic to the Rio Guapore, part of the upper Rio Madeira system. The Tetranematichthys species found elsewhere in the Amazon basin and central Orinoco is a new species which the authors have named T. wallacei.
Tetranematichthys wallaceiThe new species, which reaches a size of around 17cm/7" in length, can be told only be told apart from quadrifilis by examining its morphology carefully.
The authors provided the following diagnosis:
"Tetranematichthys wallacei is distinguished from T. quadrifilis in the alignment of the lateral margins of the head in the region from the anterior margin of the orbit to the posterior limit of the opercle (running approximately in parallel from ventral view vs. diverging laterally, respectively); in the body width at the pectoral fin insertion as a proportion of the head width at the posterior margin of the orbit (0.99-1.10 vs. 1.17-1.25, respectively); in the distance from the tip of the snout to the anal-fin origin (0.29-0.32 of SL vs. 0.33-0.36 of SL, respectively; and in the position of the anterior termination of the gill opening."
The fish were found in acidic, slow-flowing blackwater streams (known as igarapes in Brazil) within the flooded parts of the rainforest, and in sand-bottomed blackwater streams with virtually no flow.
Like most other auchenipterid catfishes, T. wallacei is found in areas containing submerged logs and branches and fishermen report that the species is always collected from among such debris.
Fishing lureThe mandibular barbels of Tetranematichthys are unusual in that the ends have "a fleshy digitiform process" like a tassel on the tips. Vari and Ferraris believe that the specialised musculature of the barbels in these fish might indicate that Tetranematichthys uses these as a fishing lure to attract prey by wiggling the fleshy tip around to make attract smaller fish.
"Motions of the elaborate distal portions of the mandibular barbel in the region anterior to the mouth in the species of Tetranematichthys may function as a fishing lure in a manner comparable to the escae that are present in various ceratioids.
"Confirmation of this hypothesis in the case of Tetranematichthys would require the observations of live specimens of the genus in nature."
The use of barbels to catch prey fish has already been recorded in members of the Asian catfish family Chacidae, but not as yet in the unrelated South American catfishes with non-homologous barbel structures.
For more information see the paper: Vari RP and CJ Ferraris (2006) - The catfish genus Tetranematichthys (Auchenipteridae). Copeia, 2006 (2), pp. 168-180.