Prolongation of somitogenesis in two anguilliform species, the<scp>J</scp>apanese eel<i><scp>A</scp>nguilla japonica</i>and pike eel<i><scp>M</scp>uraenesox cinereus</i>, with refined descriptions of their early development

<jats:p>The embryonic development of the Japanese eel<jats:italic>Anguilla japonica</jats:italic>and pike eel<jats:italic>Muraenesox cinereus</jats:italic>was morphologically investigated with laboratory‐reared specimens to clarify the characteristics of somitogenesis. In<jats:italic>A. japonica</jats:italic>, somites were first observed at 18 h post fertilization (hpf) when epiboly reached 90%. Somitogenesis progressed at a rate of 1·6 h<jats:sup>−1</jats:sup>at mean ± <jats:sc>s.d</jats:sc>. 22·6 ± 0·7° C and completed at 107 hpf (3 days post hatching; dph) when total number of somites (<jats:styled-content style="fixed-case">ST</jats:styled-content>) reached 114, which corresponds to the species' number of vertebrae (112–119). In<jats:italic>M. cinereus</jats:italic>, somites were first observed at 14 hpf when epiboly completed. Somitogenesis progressed at a rate of 1·9 h<jats:sup>−1</jats:sup>at mean ± <jats:sc>s.d</jats:sc>. 24·4 ± 0·2° C and completed at 90 hpf (2 dph) with 149 ± 4<jats:styled-content style="fixed-case">ST</jats:styled-content>, which corresponds to the species' number of vertebrae (142–158). Both species hatched before somitogenesis was completed, at 37 hpf with 47<jats:styled-content style="fixed-case">ST</jats:styled-content>and 42 hpf with 82 ± 4<jats:styled-content style="fixed-case">ST</jats:styled-content>, respectively. The formation of other organs such as the heart, mouth and pectoral fin bud occurred during somitogenesis. Comparison with the development of zebrafish<jats:italic>Danio rerio</jats:italic>indicates a prolongation of somitogenesis in<jats:italic>A. japonica</jats:italic>and<jats:italic>M. cinereus</jats:italic>. Their somitogenesis rates, however, correspond well with that of<jats:italic>D. rerio</jats:italic>estimated at the same temperature and their developmental stages at hatching are almost equivalent to other fishes having similar yolk sizes. Therefore, the prolongation of somitogenesis in<jats:italic>A. japonica</jats:italic>and<jats:italic>M. cinereus</jats:italic>may be accounted for solely by the increased numbers of somites to be formed, not by a slow somitogenesis rate or an acceleration in organogenesis.</jats:p>