ZOOLOGICAL SOCIETY BULLETIN. 



743 



METAMORPHOSIS OF THE COMMON EEL. 



ocean to breed; ('2), the sfxual orjjans cannot 

 mature their products until the eels have re- 

 turned to the depths of the ocean; (3), the pres- 

 ence of salt water is not alone sufficient, but cer- 

 tain conditions of depth and temperature are 

 necessary, viz., an approximate de))th of 1 ,000 

 meters and a tem[)erature at that depth of not 

 less than 7° Centigrade; (I), after hatching 

 tlie young gradually rise toward the surface as 

 leptocephaU, having a greatly compressed, rib- 

 bon-like form, com])letely colorless except for 

 the iris of the eye, and having the languid move- 

 ments characteristic of such pelagic animals ; 

 (5), like many otiier pelagic animals tliev are 

 negativeh' hcliotrojiic, and descend to about 100 

 meters during the day, only rising to the surface 

 at night; (6), after reaching a length of about 

 three inches, the metamorphosis, which lasts a 

 year or more, begins and the leptocephalus is 

 gradually transformed into the young elver or 

 typical eel-like form, and (7), at the close of 

 their metamorphosis the young eels gradually 

 make their way shoreward and in the course of 

 time ascend the streams into fresh water. 



Thus far the eggs of the eel have not been 

 taken nor have the young, up to the time when 



tliey assume their jielagic form of 

 It'piocephali, been observed; but 

 from the fact that A^oung leptoce- 

 phaU have been repeatedly dredged 

 from a depth of 1,000 meters or 

 more, it is safe to assume that the 

 spawning and early development 

 take place at this great depth. Such 

 being the case, it is easy to account 

 for the absence of sexually matured 

 eels in fresh or shallow waters. It 

 is not so easy, however, to explain 

 certain other events in the life his- 

 tory, and particularly tlie remark- 

 able metamorphosis of the eels, in- 

 volving as it does, tlie migration to 

 the surface of the ocean. Such a 

 migration must be very gradual to 

 permit of adaptation to changes in 

 ])ressure, for animals brought sud- 

 denly from such a depth are killed 

 before reaching the surface. The 

 migration of the adult eel to its 

 Iireeding grounds must also be slow- 

 ly accomplished for similar reasons, 

 and the time thus consumed is suffi- 

 cient for the development of the sex- 

 ual products. Dr. Peterson discov- 

 e r e d through experiments with 

 marked fish, that the rate of migra- 

 tion is aliout fifteen kilometers (ap- 

 promixatelv eight miles) a day. 



Tiie distances that many of the eels must 

 travel in order to reach the breeding grounds are 

 enormous. Not only must tliey descend from 

 the streams and lakes to the ocean, but in order 

 to reacli a sufficient depth they must in nearly 

 all cases make long journeys at sea. Thus, the 

 eels living in the fresh waters of northern 

 Europe find their nearest breeding grounds in 

 the Atlantic, south-west of the Faroe Islands ; 

 while tliose inhabiting tlie waters of the !Mis- 

 sissip]ii system must migrate out of the Gulf of 

 Mexico to the eastward and northward of the 

 Bermudas, before the jiroper temjierature of not 

 less than 7° Centigrade at the 1000 meter line 

 is reached. Not the least remarkable feature of 

 this life history is the fact, for such it seems to 

 be, that a species capable of withstanding such 

 changes of temperature and (iressure and which 

 is distributed from the tropics to the Arctic circle 

 should be so inadaptive in its breeding habits. 



The life history of the American eel, (An- 

 ^uilla chrt/siipa). has not been so thoroughly 

 investigated as that of its European relative, but 

 the facts so far as known accord well with those 

 determined by Schmidt for the European spe- 



