Reassessment of Pigmentation Patterns used for Identifying Leptocephali (Gnathophis heterognathos and G. ginanago) Collected from the Kuroshio–Oyashio transition region
Identification of leptocephali of Gnathophis heterognathos and G. ginanago, the most abundant leptocephali in Japanese water, is usually identified based on morphological characteristics such as total number of myomeres (TM), number of myomeres of anterior to the last vertical blood vessel (VBV last) and presence or absence of melanophores on the lateral surface of the posterior spinal cord. However, because some of individuals collected in the Kuroshio–Oyashio transition region differed from the norm in the above characters, the validity of the last–mentioned was examined using the mitochondrial DNA 16S rRNA gene to provide genetic evidence. A total of 394 leptocephali were classified into following four morphotypes, Types A1 (TM: 117–128, VBV last: 36–44, pigmentation: absent, Body height (BH)/Total length (TL) ratio: 0.083–0.123), A2(TM: 119–126, VBV last: 40–44, pigmentation: present, BH/TL ratio: 0.082–0.127), B1(TM: 125–134, VBV last: 41–48, pigmentation: present, BH/TL ratio: 0.093–0.137) and B2(TM: 125–135, VBV last: 43–48, pigmentation: absent, BH/TL ratio: 0.080–0.139). Group A showed lower TM, VBV last, and BH/TL ratio than those of Group B. Molecular evidence confirmed that, Groups A (Types A1 and A2) and B (Types B1 and B2) were in fact G. heterognathos and G. ginanago, respectively. In addition, relationship of total length and pre–anal myomere (PAM)/TM ratio as an indicator of metamorphosis showed that the presence of posterior spinal cord melanophores was depended on their growth. These results indicate that Gnathophis leptocephali obtained from the Kuroshio–Oyashio transition region were comprising only the two species, and that variations in the pigmentation pattern characters rendered them unsuitable for identification purposes. Combination of above morphometric or meristic characters is effective for identified these two species
