Deformities in diplospondylous vertebrae in Blackmouth catshark (Galeus melastomus) in Masfjorden, Western Norway

Abstract Occurrence of skeletal deformities in wild fish populations is an important biomarker for ecosystem health. Such knowledge is, however, limited regarding elasmobranchs. Herein, adult Blackmouth catsharks ( Galeus melastomus ) caught in Masfjorden in Western Norway were radiographed and examined for vertebral meristics and deformities. The number of monospondylous vertebrae ranged between 39 and 41 (mean 40) while the total number of vertebrae ranged between 138 and 144 (mean 140.8). Two out of the nine analysed specimens had deformed vertebrae, both having the same deformity phenotype and in the same location. The deformities affected four adjacent diplospondylous vertebrae aligning with the position of the lower lobe of the caudal fin: Vertebrae nos. 99 to 102 in one specimen and vertebrae nos. 98 to 101 in the other. In both specimens, the most anterior and posterior deformed vertebra had a one-sided compression while the two vertebrae in the middle of the deformity were compressed. In one specimen, the two compressed vertebrae had started to fuse. The present deformities were mild and unlikely to depress fish survival or fitness. This is the first record of vertebral deformities in catsharks. The current observations are discussed in relation to challenging natural alterations in vertebra shape during swimming.