Spinal Abnormalities in a Specimen of the Panamanian Knifefish Apteronotus rostratus (Apteronotidae: Gymnotiformes) with Comments on Gymnotiform Locomotion
Skeletal deformities among fishes are well documented in the aquaculture industry and husbandry studies. However, these deformities are noticeably less prevalent among wild caught animals, likely due to their deleterious effects on the fitness of affected individuals particularly at earlier life-history stages. Here we use micro-CT imaging to describe, for the first time, spinal abnormalities in a Neotropical electric fish (Apteronotus rostratus). We find that the deformed specimen exhibits scoliosis and kyphosis of pre-caudal vertebrae 11–14 and an additional fusion of a hemal spine into the descending blade of the anterior displaced hemal spine. We also find that the abnormal specimen was able to reach sexual maturity and forage effectively (as evidenced by ripe ovaries and the presence of insect larvae in the digestive tract) despite exhibiting several spinal deformities. We infer that a highly specialized form of gymnotiform locomotion allowed the deformed specimen to reach maturity and avoid predation in the face of these abnormalities.
Left lateral view of specimens of Apteronotus rostratus. (A) Undistorted specimen (STRI 08654) with an evenly curved dorsal margin. (B) Specimen (STRI 01573) exhibiting scoliosis. Note the strongly arched dorsal margin.
Left lateral view of μ-CT rendered skeletons of specimens of Apteronotus rostratus. (A) Undistorted specimen (STRI 08654) with an evenly curved dorsal margin. (B) Specimen (STRI 01573) exhibiting scoliosis. Note the strongly arched dorsal margin.
Dorsal view of μ-CT rendered skeletons of specimens of Apteronotus rostratus. (A) Undistorted specimen (STRI 08654) with an evenly curved dorsal margin. (B) Specimen (STRI 01573) exhibiting scoliosis. Note the severe curvature of the spinal column in the pre-caudal region.
Lateral views of the posterior-most pre-caudal vertebrae of a specimen of Apteronotus rostratus (STRI 01573) exhibiting scoliosis. Region of box in Figure 2A. Note asymmetry in the extent of fusion between vertebrae and hemal spines. (A) Left-lateral view. Note fused dorsal margins of vertebrae and fused posterior-distal portion of second hemal spine (oval). (B) Right-lateral view. Note unfused vertebral dorsal margins, and a fused dorsal margin of the first caudal vertebrae (posterior to displaced hemal spine).
Ventral view of posterior-most pre-caudal vertebrae of a specimen of Apteronotus rostratus (STRI 01573) exhibiting scoliosis. Note the unusual inward curvature and fusion of the second hemal spine with the anterior portion of the displaced hemal spine (oval).
Pre-caudal vertebrae of an undistorted specimen of A. rostratus (STRI 08654) showing no spinal arching, unfused dorsal margins of vertebrae, and a free distal portion of second hemal spine (oval).
Dissected ovaries of the deformed specimen of Apteronotus rostratus (STRI 01573). Note slight disarticulation of ovaries as a result of a previous dissection prior to loan.
Specimen of Apteronotus rostratus (STRI 01573) exhibiting scoliosis with a portion of its digestive tract highlighted in yellow, showing an abundance of high contrast items, potentially aquatic arthropod parts.
(A) Lateral view of an insect larva recovered from the gut of the deformed specimen of Apteronotus rostratus (STRI 01573). (B) Ventral view.
Contributor Notes
Associate Editor: T. Grande.