Fig. 1. A. Common mola (courtesy of Factzoo.com). B. Adult male miniature tetra Iotabrycon praecox from western Ecuador. White scale bar is 10mm (from Aguirre et al., 2014a). C. Pirarucu (courtesy of Rivermonsters.com).
Fig. 2. Radiographs showing examples of variation in body form and vertebral number in fishes (courtesy of the Smithsonian Institution).
- Is there sexual dimorphism in vertebral number? Stickleback are sexually dimorphic for many traits, and males and females can experience very different selective demands. They are also known to differ significantly in body form, and Reimchen and Nelson (1987) previously reported differences between sexes in total vertebral number, with males having more vertebrae.
- Is there a significant difference in vertebral number among anadromous (oceanic), benthic/stream, and limnetic populations of threespine stickleback? If so, we expected limnetics to have more vertebrae than the other ecomorphs.
- Is there body region specificity in terms of where vertebral number changes? Fish have two major types of vertebrae, abdominal (or precaudal) vertebrae and caudal vertebrae. These have different forms and serve different purposes, so it makes sense that where you add or subtract vertebrae may matter functionally.
- Finally, is there an association between vertebral number and body shape within ecomorphs? That is,do more elongate individuals within a particular ecomorph tend to have more vertebrae than more deep-bodied individuals within the same ecomorph?
Fig. 3. Radiograph showing method for counting vertebral number.
Fig. 4. Sexual dimorphism in vertebral number among Alaskan threespine stickleback. Mean number of abdominal vertebrae is plotted against mean number of caudal vertebrae, coded by sex for the nine populations for which data are available. Note the consistent pattern of divergence between sexes. In every population males have more caudal vertebrae and fewer abdominal vertebrae than females, although total vertebral number did not differ significantly between sexes.
Fig. 5. Vertebral number variation among male Alaskan stickleback. Points are population sample means. A) Variation in mean total vertebral number. B) The mean number of abdominal vertebrae plotted against the mean number of caudal vertebrae. Notice that limnetics have more vertebrae than the other ecomorphs and that it is caudal vertebral number that specifically increases in limnetics.
Fig. 6. Association between total vertebral number and body shape within ecomorphs. Anadromous samples not included, and benthic and stream samples pooled as one ecomorph.
Fig. 7. Radiograph illustrating phenotypic variation associated with elongation in gasterosteids (Gasterosteus, Pungitius, and Spinachia) and their relatives (Aulorhynchus).
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