ABSTRACT
The tube-eye, Stylephorus chordatus is a small to moderately sized (<30 cm TL) lampridiform fish, residing in meso-and bathypelagic habitats in all oceans except polar seas. The only known species in the family Stylephoridae, S. chordatus is so named because its eyes are tubular and telescopic (“Stylephorus”—tube-eye) and its caudal fin bears extremely long rays that exceed the body length (“chordatus”—rope-like). Partly due to its bizarre appearance, the species has been the subject of several classic studies of anatomy and osteology (Starks 1908, Regan 1924, Pietsch 1978). Although never directly observed, the tube-eye probably hangs vertically in the water column, searching for zooplankton prey with its highly modified eyes and slurping food items through its cylindrical mouth. When prey is detected, the head of S. chordatus rotates upward and snaps backward, the jaw protrudes forward and a loose membrane that connects the cranium to the lower jaw is stretched. The resulting fully expanded buccal cavity has a volume approximately 40 times larger than the volume of the closed mouth. The velocity of the water entering the mouth during this feeding event has been estimated at greater than 300 cm/sec (Pietsch 1978). A number of unique modifications of the jaws and vertebrae are associated with this feeding mechanism. Unlike all other lampridiform fishes, the tube-eye lacks a rostral cartilage. Instead, the upper jaws are tightly bound to the dentary to form a tube-like mouth. To permit the extreme backward rotation of the cranium, the first and second vertebral centra are extremely reduced and incompletely ossified; in addition, the second vertebra possesses an m-shaped neural arch and lacks a neural spine (Olney et al. 1993). Most authors have previously overlooked these anterior vertebrae and report a total complement of 50 vertebrae. Olney et al. (1993) reported 53 total vertebrae in a larval S. chordatus.
