Variance of Female Genital Shape in Spiny Dogfish, Squalus Acanthias, a 2-D and 3-D Approach
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This study aimed to investigate the presence and significance of morphological variation in the internal female genitalia of spiny dog sharks, Squalus acanthias. We used traditional linear morphometrics to examine the relationship between the allometry of the vagina and the allometry of the pectoral fin; however due to small sample size we did not detect an allometric trend. We used traditional 2-D and relatively new 3-D geometric morphometrics (GM) to look for variations in genital shape correlated with changing reproductive state (Visibly Pregnant vs. reproductive but Not Visibly Pregnant). Surprisingly, only the 2-D analysis detected significant shape variation associated with reproductive state, however it was only significant after two sets of error replicates were included. This information suggests that further geometric morphometric analyses should include two sets of error models to better understand and compare the different degrees of variation detected by 2-D and 3-D methods. The data from both the 2-D and 3-D GM analyses did show that vaginal size is significantly correlated with shape—supporting the idea that as vaginal size increases the shape of the vagina becomes wider and more open, likely to accommodate for birth.