Both Atlantic eel species (Anguilla anguilla and A. rostrata) were collected in the Sargasso Sea during the 1979 cruise of the F. R. V. "Anton Dohrn" and R. V. "Friedrich Heincke". A total of 3,097 0-group larvae were caught during 80 hauls using the Isaacs Kidd Midwater Trawl (55 hauls) and the 9-fold opening and closing net MOCNESS (25 hauls). 11 hauls of the MOCNESS indicated that the larvae showed a preference for the 150-175 m water depth during daytime and for the 50-75 m depth during night. The northern distribution limit seems to coincide with the "18 °C-water" at the sea surface. The Antilla current characterised by higher temperatures and salinities could have been the southern distribution limit. The north-easternmost occurrence of A. anguilla was noted at 50 ° W; the eastern distribution limit of this species could be farther east. The westernmost station at 69 ° W was positive for both species. A. anguilta, therefore, is very likely to occur beyond this area. The easternmost occurrence of A. rostrata was noted at 52°W though sporadic and with increasing abundance towards the west. At the western end of the network of stations the highest concentration of larvae from both species was recorded. Oceanographic investigations reveal that the distribution of the smaller larvae (size-groups < 7 mm and < 10 ram) almost coincides with the assumed spawning area.
More than 2000 0-group larvae (5.0-30.0 mm long) of both Atlantic Anguilla species were examined taxonomically. With regard to the total number of myomeres in A. rostrata and A. anguilla, an average difference between the two species of 6 to 8 myomeres was found in all size groups. 31 specimens (i.e.t.76 %) exhibited 111 myomeres. The position of the last vertical blood vessel and the number of preanal myomeres turned out to be statistically different in both species; however, these differences cannot be used for species identification. The regression line for the position of the last vertical blood vessel according to the total number of myomeres indicates that individuals with a total of 111 myomeres may be A. anguilla. Measurements of total lengths revealed highly significant differences between the larvae of both eel species. It can be concluded that, on the average, A. rostrata (~ = 15.70 ram) hatched about two weeks before A anguilla (x = 12.32 ram). On the other hand, results obtained from the biggest A. rostrata larvae (29.5 mm) and A. anguilla larvae (23.5 ram) make a spawning of A. rostrata likely two months before A. anguilla, when findings from hatching experiments with A. japonica are taken as a basis. There is no difference in the relative length of the intestine in either Anguilla species.
Marked differences between continental samples of American and European eels have been detected electrophoretically in allele frequencies at the MDH-2 locus. Starch gel electrophoresis carried out on board F. R. V. "Anton Dohrn" during the eel expedition to the Sargasso Sea in 1979 revealed a similar clear-cut genetic difference in a sample of 0-group Anffuilla leptocephali, thus confirming the classical theory of Schmidt (1932). The MDH-2 genotypes provide an additional diagnostic character for the distinction between young A. anguilla and A. rostrata leptocephali. Species identification by biochemical genetic characters did not correspond with that by meristic characters (rnyomere numbers) in ca. 13 % of the specimens studied; this discrepancy mainly concerns leptocephali of the A. anguilla genotype. The results obtained are critically discussed.
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