Genetic and morphological differences among the three species of the genus Rastrelliger (Perciformes: Scombridae)

Genetic and morphological differences among the three species of the genus Rastrelliger, viz. Rastrelliger kanagurta, Rastrelliger brachysoma and Rastrelliger faughni were clarified on the basis of 150 specimens collected from Panay Island, the Philippines, thereby providing fundamental information for accurate species identification. Specimens were separated into three distinct clades on the Bayesian phylogenetic tree constructed from mitochondrial DNA sequences, which largely corresponded to R. kanagurta, R. brachysoma and R. faughni according to the number of gill rakers and body depth. These results indicated that they were reproductively isolated, separate species. Further morphological differences were evident among the three clades, including separation by principal component analysis on 29 measurements. Apart from such interspecific divergences, intraspecific variations of several morphological characters relative to body size [fork length (FL)] were revealed for each species. Despite a slight increase in the number and the length of gill rakers with growth, R. faughni was invariably distinguished from the other two species by fewer and shorter gill rakers throughout the entire body size range. In stark contrast, some larger (FL > 200 mm) R. kanagurta individuals attained a proportionately deeper body that had been thought to be characteristic of R. brachysoma. Likewise, some smaller individuals of R. brachysoma (FL < 160 mm) were relatively shallow-bodied, and the body depth relative to FL was similar to that previously reported throughout the range of R. kanagurta. These results underpin the importance of considering ontogenetic variations when accurately identifying species based on morphological characters. Type specimens of six nominal species available in the present study that have been generally recognized as synonyms of R. kanagurta were examined, being shown to be identical with specimens considered here as R. kanagurta. In addition, intraspecific genetic variation of each species was briefly discussed in relation to hypothesized mechanisms that may have driven the lineage diversifications of marine organisms in the Indo-Pacific Ocean.