The Sectional Counts of Vertebrae in the Anguillid Elvers

The numbers of predorsal,ano-dorsal,preanal,abdominal,caudal,and total vertebrae in and shortly after the elver stage of 9 species and subspecies of eels from the various areas in the world were counted.The numbers of vertebrae in such sections are useful for the specific identification when the ranges of variations do not overlap between the respective species. A key to the species was given based on the sectional counts of vertebrae.Moreover,the graphic representations of the correlation between the number of vertebrae in respective sections facilitate the identification of most species. Although Ege(1939)reported the number of vertebrae of 19 species and subspecies of eels around the world,he counted only prehemal and total vertebrae.The present authors examined the numbers of the predorsal,anodorsal,and preanal vertebrae,as well as those of abdominal,caudal,and total.It was found that such sectional counts of vertebrae are more useful in identifying the species than count of total vertebrae only.The correlation between each of the sectional counts and the total number of vertebrae is represented in a graphic form to facilitate the specific identification. Materials and methods The materials examined in the present investigation are about 230 specimens obtained from the various areas in Asia,Europe,Canada, and New Zealand(Table 1).The samples are in and shortly after the elver stage.Among the samples,there exist distinct differences in development of pigmentation,as indicated in Table 1 according to the definition given by Bertin(1956)and Egusa(1972).Besides these samples,the young eels from Borneo were also examined.Following observation and measurements on the external morphology of the preserved specimens,the elvers were stained with alizarin,and predorsal,ano-dorsal,preanal, abdominal,caudal,and total vertebrae were counte(Fig.1).In some specimens the vertebral number was counted from X-ray photographs.The number of ano-dorsal vertebrae is defined as the number of vertebral centra between verticals through the anus and origin Fig.1.Diagram showing measurement of total length,and sectional counts of vertebrae. A•`B,predorsal vertebrae;B•`C,anodorsal vertebrae;A•`C,preanal vertebrae; A•`D,abdominal vertebrae;D E,caudal vertebrae;A•`E,total vertebrae. of the dorsal fin,excluding the centra which were cut by the verticals,as was defined by Jespersen(1942)in the anguillid leptocephali. The centra which were cut by the verticals through origin of the dorsal fin and the anus are also excluded in counting the number of predorsal and preanal vertebrae,respectively. The first vertebral centrum with closed hemal arch or hemal spine is defined as the first caudal vertebra(Ege,1939).The terminal and preterminal centra,which are known to fuse in young stage(Bertin,1956),are counted as one vertebra,even though they are not yet fused in the elver stage.