Morphological differences in the scapula-coracoid of seawater and freshwater fish

Pectoral fins are important structures that generally reflect the lifestyles of fish species, owing to their involvement in fish movement, and the skeletal scaffold of pectoral fins includes both the scapula and coracoid. However, only a few studies have compared the morphology ofpectoral skeletons from different species. Accordingly, the present study analyzed the scapula-coracoid morphology of 102 species of seawater and freshwater from 12 orders and 45 families. The species were collected and divided into four groups, based on their habitat: seawater benthic (group 1), seawater pelagic (group 2), freshwater benthic (group 3), and freshwater pelagic (group 4). Based on the difference between groups 1 and 2, it was clear that the coracoids of pelagic species were longer than those of benthic species, thereby indicating a morphological difference between the scapula-coracoid of fish species from different habitats. Furthermore, the three-dimensional scapula-coracoid aspects and coracoids were deeper and shorter, respectively, in the freshwater species (groups 3 and 4) than in the seawater species (groups 1 and 2). Together, these results support the hypothesis that pectoral fin morphology reflects the lifestyle of the fish species. In addition, the present study also examined the relationship between the body length and scapula-coracoid length of 12 fish species and found that scapula-coracoid length increases with body length. However, the slopes of the regression lines for the seawater and freshwater species were significantly different (ANCOVA, p < 0.01), which suggests that seawater and freshwater fish species possess different skeleton growth rates