NATURE ADRIFT 



to change to the glass eel stage as soon as the river water stimuhis is felt. 

 Those carried eastwards towards Europe will develop more slowly and so 

 prolong their leptocephalus stage until the coastal region is reached after 

 about three years. Those that for some reason do not conform to pattern 

 will die. If this theory is correct all the European eels are dependent upon 

 American stock. 



So far neither theory has been proved, and the proof is eagerly awaited as 

 at present it is one of the most interesting problems in marine biology. Both 

 theories have one major assuniption: the original Danish theory assumes 

 that the adult European eel can actually make the journey to the spawning 

 ground, though it should be mentioned here that the experiments showing 

 that environmental differences could affect vertebral count were carried out 

 after Schmidt's ideas were published. Tucker's theory assumes that the 

 environment really does have this effect. Many do not accept Tucker's 

 hypothesis because, they say, the difference in vertebral number — the average 

 difference is eight — is too big to be only an environmental difference and 

 that wc would expect more evidence of dead eels washed ashore on the 

 European coasts. They say the degeneration of the gut has been exaggerated 

 and it is not always so far gone as Tucker states, and that some at least could 

 be capable of making the 3,500 mile crossing of the Atlantic. If only some do, 

 and lay 10 million eggs each, it would be sufficient. It is going to be interesting 

 to see what the correct explanation is. The proof could lie either in finding 

 an adult European eel in the Sargasso Sea area, or in experimental evidence 

 that both kinds of larvae can come from the same parentage. 



Although the leptocephali when they reach the European coasts are only 

 some 3 inches in length, much bigger ones have been found. They are the 

 larvae of different species, those of Nciiiiclitliys scoJopactis are up to about 

 10 inches (25 centimetres), others have been found up to 20 inches (50 centi- 

 metres), and a leptocephalus has been found ofi New Zealand which was 

 34 inches (89 centimetres) in length. But the biggest of all was taken by 

 Johannes Schmidt's ship, the Dana, on her expedition round the world 

 in 1928-30, and this was 62| inches long (184 centimetres). 



Wc do not know to what species these giant eel-larvae belong, and 

 various exciting visions can be conjured up. Do they grow up continuing 

 the same proportions found in the common eel and so grow to become 

 gigantic 'sea-serpent' eels of 70 feet in length, or do they metamorphose into 

 an adult perhaps little or no bigger than the leptocephalus — or something 

 in between? The adult Nciiiiclitliys arc only three to five times as long as the 

 larvae. Could they be 'lost' leptocephali that have never found the right 

 conditions to metamorphose and yet have survived — though their vertebral 

 number and other characters show they are not just overgrown specimens 



104 



