Japanese and American scientists have created a new order for the strange deep-sea tube-eye fish, Stylephorus chordatus.
The new order, named Stylephoriformes, was created based on new evidence about the phylogenetic position of S. chordatus obtained from DNA sequence data, in a study published in the most recent volume of Ichthyological Research.
Stylephorus chordatus, commonly referred to as the tube-eye or thread-tail, is an extremely rare deep-sea pelagic fish, with large forward pointing eyes, a highly protrusible mouth and an elongated tail fin that greatly exceeds its body length.
The bizarre tube-eye was previously considered to be closely related to members of the order Lampridiformes, a group that includes other strange fishes such as the opahs (family Lamprididae), velifers (Veliferidae), crestfishes (Lophotidae), tapertails (Radiicephalidae), ribbonfishes (Trachipteridae) and oarfishes (Regalecidae).
Preserved specimen of the Ribbon fish Trachipterus ishikawae (family Trachipteridae) on display at Suma Aqualife Park, Kobe, Japan. Photo by Opencage, Creative Commons.
The recent study, which utilised DNA sequence data from a portion of the recombination activating gene 1 (RAG1) and the whole mitochondrial genome, discovered a close relationship between the tube-eye and members of the order Gadiformes (cods and their relatives).
The idea of a close relationship between the tube-eye and gadiform fishes is a novel one and one that challenges the current placement of S. chordatus within the Order Lampridiformes, a relationship first proposed in the 1920s by the late Charles Tate Regan of the Natural History Museum, London.
Rarely collectedThe tube-eye is rarely collected and little is known about its biology. It appears to exhibit a circumglobal distribution, living at depths between 300-800 meters along tropical and subtropical latitudes.
Based on its bizarre body-shape, it has been suggested that the tube-eye swims in a head-up, vertical position. Swimming in this head-up position is believed to facilitate in the capture of copepods (its main prey item) from the water column, which would silhouette against lighter surface waters.
Previous DNA studies of lampridiform fishes did not have access to suitably preserved specimens of S. chordatus from which DNA could be extracted. The recent study by Japanese and American scientists was only possible due to availability of three fresh specimens of tube-eye, collected from the western North Pacific and western North Atlantic using midwater trawls.
For more details on the new order see the paper: Miya, M., Holcroft, N. I., Satoh, T. P., Yamaguchi, M., Nishida, M. and Wiley, E. O. (2007). Mitochondrial genome and a nuclear gene indicate a novel phylogenetic position of deep-sea tube-eye fish (Stylephoridae). Ichthyological Research, 54: 323-332.