Variations in Numbers of Median Dorsal Thorns and Rows of Teeth in Thorny Skate (Raja radiata) of the Northwest Atlantic

Abstract

Data on the number of median dorsal thorns (anterior to the first dorsal fin) and the number of rows of teeth of the thorny skate from various areas of the Northwest Atlantic in 1949-72 were analyzed. The results showed a cline of mean number of median dorsal thorns from the Baffin Island and Ungava Bay areas southward along Labrador and eastern Newfoundland to the Grand Bank and thence westward to the Gulf of St. Lawrence, Scotian Shelf and Georges Bank, the higher numbers occurring at lower developmental temperatures and vice versa. The mean tooth-row counts showed a cline which was opposite to that for median dorsal thorns, increasing from the Baffin Island and Ungava Bay areas southward to the Scotian Shelf and Georges Bank. There was a slight tendency toward increased tooth-row numbers with fish length, but the within-area differences were not significant for lengths greater than 24 em which were used in the comparisons among areas. Males tended to have slightly more rows of teeth than females, but there was no clear evidence of sexual dimorphism. The number of tooth-rows in the lower jaw was, on the average, slightly higher than in the upper jaw, but the upper-jaw counts exceeded the lower-jaw counts in small samples from the geographical extremes of the populations studied.