Deoembeb 11, 1908] 



SCIENCE 



845 



SPECIAL ARTICLES 



EEOENT DKCOVERIES IN THE HISTORY OP THE 

 COMMON EEL* 



No fish is more generally and widely known 

 than the common eel and none has been in- 

 volved in more mystery. The absence of 

 developed sexual organs has given rise to many 

 strange hypotheses and fables. The true his- 

 tory has been made known only within the last 

 two years. The most satisfactory of recent 

 observations have been made by Dr. Johannea 

 Schmidt, Dr. 0. G. J. Petersen and K. J. 

 Gemzoe, of Denmark. 



It has long been known that the eel descends 

 into the sea t'o spawn, but the exact destination 

 has been unknown. For information respect- 

 ing this, we are indebted to Dr. Schmidt, who 

 has in great detail recorded the results of 

 two expeditions to ascertain the facts.^ 



None of the eels found in the ocean had 

 fully matured eggs, and the ovaries were only 

 moderately developed. Indeed, according to 

 Schmidt, as recently as 1906, " no one had yet 

 succeeded in finding [in the males] sperma- 

 tozoa, nor in the female eggs which were 

 larger than 1/3 mm. in diameter, much 

 smaller as a rule, and never clear as the eggs 

 of other species are known to become before 

 spawning. No one had yet described the eggs 

 of the eel in a condition near to spawning or 

 even approximately mature." 



The period between the descent into the sea 

 and the appearance of larva near the surface 

 has yet to be known, but larvae have been 

 found by Dr. Schmidt in regions where the 

 depth of the water was about 1,000 meters, 

 and he has concluded that " the eel spawns out 

 in places of at least nearly 1,000 meters 

 depth." In such places, in May and June 

 (especially about the middle of June), larval 

 eels about three inches long are to be found. 

 They are then mostly in the typical lepto- 

 cephalus stage, diaphanous and without pig- 



^ Presented to the Biological Society of Wash- 

 ington, October 17, 1908. 



' Schmidt ( Johs. ) , " Contributions to the Life- 

 history of the Eel {Anguilla vulgaris Flem. ), 

 Rap. et P. V. Conseil Int. Expl. Mer., V., p. 137- 

 274, pi. 7-13, 1906. 



ment cells, having very compressed and high 

 or ribbon-like bodies, anus far behind, very 

 small heads, large eyes, pointed snouts and 

 full sets of slender teeth directed forwards in 

 the jaws. During the succeeding months, a 

 transformation takes place, in some early, in 

 others delayed. 



The body becomes reduced in height, espe- 

 cially backwards, and expands sideways, " the 

 hindmost portion of the gut disappears," and 

 the anus gradually advances forwards, the 

 head at last is less disproportionate and the 

 eyes relatively small, and, above all, the slender 

 spike-like teeth, so characteristic of the typical 

 leptocephaline stage, are dropped. By Sep- 

 tember and the later autumn the leptocephali 

 of summer have mostly undergone their trans- 

 formation into a later leptocephalus stage. 



All this time the larval eels remain in the 

 ocean not very far below the surface, quite 

 near, it may be, at night, sinking downwards 

 more or less during daylight. Late in the sea- 

 son they gradually approach to the shores, con- 

 tinuing meanwhile their change from lepto- 

 cephalines into true eels. The time of their 

 entrance into the mouths of rivers depends 

 to some extent on the distance of those mouths 

 from the line of about 1,000 meters or over 

 500 fathoms. In France, as well as Ireland 

 and England, for instance, they may begin to 

 enter streams as early as January or February, 

 while in Denmark and Norway, none enter 

 earlier than March, and the chief incursion is 

 during April. 



During the entire time from the appearance 

 of the leptocephalines near the surface of the 

 ocean to their entry into the rivers, the hyaline 

 condition of the little fishes continues. The 

 body is so diaphanous that only by the closest 

 attention can it be detected, and then chiefly 

 because the eyes are prominent and distinctly 

 colored. Indeed, the first thought on seeing a 

 school of leptocephalines may be wonder that 

 so many eyes are floating about. 



The diaphanous condition undoubtedly is an 

 efEcient protection against many of the 

 dangers the larval eels encounter. Neverthe- 

 less it is only a partial protection. The course 

 of the young fishes riverward is beset with 

 dangers, and these increase as the coast is ap- 



