Black-winged hatchetfish: Not one but three species!

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Scientists from Australia and Brazil have found through a genetic study published in the most recent issue of the Biological Journal of the Linnean Society that the Black-winged hatchetfish (Carnegiella marthae) actually consists of three species.

The Black-winged hatchetfish is a species restricted to the floodplain forests of the Rio Negro, and Maxine Piggott and coauthors collected 403 individuals from 21 tributary populations in the Rio Negro drainage for their study. 

The authors quantified region-wide population differentiation and tested for cryptic diversity in the hatchetfish using DNA data derived from nuclear microsatellite markers, mitochondrial DNA (adenosine triphosphatase subunits 6 and 8) sequences, and morphological information.

The authors identified three genetic lineages (A, B and C) based on their analyses of the microsatellite and mitochondrial DNA.

Two forms of the Black-winged hatchetfish were identified from their morphological analyses: a light and dark morph. 

The dark morph had a dozen dark oblique lines on the side of the body, whereas the light morph has diffuse dotted lines.

The dark morph is further distinguished from the light morph in having 23–24 (vs. 27–30) anal-fin rays, six (vs. zero to four) three-cusped teeth on the lower jaw bone, smaller distance between the nostrils and dorsal fin base length, and larger least distances between the dorsal-fin base and the upper caudal-fin base. 

The light morph hatchetfish were found to belong to lineage A, while the dark morph fish were found to belong to lineages B and C. The authors also identified hybrid offspring of matings between individuals of lineages B and C, supporting the evidence from the morphological analysis that these two lineages are more closely relatied to each other.

There are management implications for the Black-winged hatchetfish as a result of this study, state the authors. 

The hatchetfish is currently exported and sold as a single species in the aquarium trade, but the light morph hatchetfish (lineage A) appears to be the rarest of the three lineages (comprising only 14% of the material that the authors studied and found in only four of the 21 sampled tributaries). The potential for overfishing this putative species therefore needs to be addressed by the Rio Negro ornamental fishery association and fishery managers.

For more information, see the paper: Piggott, MP, NL Chao and LB Bheregarary (2011) Three fishes in one: cryptic species in an Amazonian floodplain forest specialist. Biological Journal of the Linnean Society 102, pp. 391–403.