Eel Species, their Developmental Stages and their Distribution

Abstract

In some respects it is very difficult to distinguish between different species of eels. This is particularly true in the case of larval forms; these, as is well known, show continuous changes in body form and pigmentation, so that the characteristics of individuals at different ontogenetic stages are not comparable. Only a very few characters can be seen in all stages before and after metamorphosis. Even such basic and important features as coloration and markings can only be used after the metamorphosis of glass eels into yellow eels, and then only until the final metamorphosis into the silver eel. The so-called marbled patterning of certain species only becomes evident when the eels are over 20 cm long; in the silver eel stage it disappears again (Ege, 1939; Jubb, 1961). Three features do remain the same through all stages from larva to silver eel, namely the number of vertebrae, the number of myomeres (the latter count is always one higher than the former), and the relationship between the lengths of the dorsal and anal fins (Figs. 34 and 35). All other characteristics can only be used comparatively if the individuals are of almost the same age and are at the same stage of development.