THE BREEDING OF THE NORTH ATLANTIC FRESHWATER-EELS
161
gradient until they strike the 18°C isotherm a t about 300 m. From then
on several conditions become differentiated simultaneously, roughly in
relation to latitude. Eggs and larvae a t the southern limit pass through
the gradient 18-25°C during the last 250 m ascent; in particular, they
encounter an abrupt 4" rise between 200 and 100 m which may well
have the effect of ending segmentation and so fixing the number of
somites/vertebrac in thc range 103-1 1 1 rccognizcd as typifying 'Anguilla
" At the northern limit, however, the temperature gradient remains
gradual right up to the surface, where a maximum temperature of
barely 20°C is encountered. No temperature effect, certainly no shock
effect, is likely, wherefore the addition of somites presumably continues
to the genetically determined limit and results in the population with
110-119 somites/vertebrae which we recognize as 'A. anquilla '."
As pointed out above all newly hatched larvae had the full number
of myomeres, and they were caught between 200-500 m, never in nets
fishing closer to the surface ; I may add that even older larvae are never
caught close to the surface, but always a t about 25 m or deeper.
Tucker's remark that closing nets were not used does not disqualify
these statements because the newly hatchcd larvae were only caught
when the nets had been down between 200 and 500 m, and older larvae
were only caught quite rarely if the nets had not been down at 25 m or
deeper. Finally I may add that there is no indication of any sort of a
secondary thermocline anywhere which could possibly cause some
shock effect even of the type mentioned by T&ning (1962, p. 15); here
TBning had two control samples, one kept at 4°C averaging 88-94 vertetmc, arid one at 10°C nvcrfigi~ig 59.60 vc!rt.ol.)rlu~
Ily 1mLtidiwiiig IL H I L ~ U I ~ ,
firHt r w r w l i d , do(!, it1 tho wlnitivct IwriCrcl
to 10" aid kocying it Lhcre hc obtairwl i ~ i i
avcrago of litC.liD, or only 0.26
vcrtebrao lowcr than thc sample kept fiw the whole time at 4'C.
Apart from this slight lowering of thc average number of vertebrae,
fair too small to satifify Tucker's idea, it i8 impossiblc to imagine that
thorc would be II sharp limitation betwccii tho conditions in tho sea
crwtitig 'I'ucIwr'H iniaginary shock arid where tliey do not exist. The
distance betwcw 'LOON. and 25"N. is 300 m:a-miJes, so the slope of the
isotherms is in fact very gradual, and if 'I'ucker were right we would
have very many intermediate figures, mabing it impossible to distinguish between the two species of leptocephali.
Therefore I fail to see any support for Tucker's main assumption,
that the difference between the American and the European eel is only
eco-phenotypical. Field studies and experiments are both against it.
r08h'dU *.
161
gradient until they strike the 18°C isotherm a t about 300 m. From then
on several conditions become differentiated simultaneously, roughly in
relation to latitude. Eggs and larvae a t the southern limit pass through
the gradient 18-25°C during the last 250 m ascent; in particular, they
encounter an abrupt 4" rise between 200 and 100 m which may well
have the effect of ending segmentation and so fixing the number of
somites/vertebrac in thc range 103-1 1 1 rccognizcd as typifying 'Anguilla
" At the northern limit, however, the temperature gradient remains
gradual right up to the surface, where a maximum temperature of
barely 20°C is encountered. No temperature effect, certainly no shock
effect, is likely, wherefore the addition of somites presumably continues
to the genetically determined limit and results in the population with
110-119 somites/vertebrae which we recognize as 'A. anquilla '."
As pointed out above all newly hatched larvae had the full number
of myomeres, and they were caught between 200-500 m, never in nets
fishing closer to the surface ; I may add that even older larvae are never
caught close to the surface, but always a t about 25 m or deeper.
Tucker's remark that closing nets were not used does not disqualify
these statements because the newly hatchcd larvae were only caught
when the nets had been down between 200 and 500 m, and older larvae
were only caught quite rarely if the nets had not been down at 25 m or
deeper. Finally I may add that there is no indication of any sort of a
secondary thermocline anywhere which could possibly cause some
shock effect even of the type mentioned by T&ning (1962, p. 15); here
TBning had two control samples, one kept at 4°C averaging 88-94 vertetmc, arid one at 10°C nvcrfigi~ig 59.60 vc!rt.ol.)rlu~
Ily 1mLtidiwiiig IL H I L ~ U I ~ ,
firHt r w r w l i d , do(!, it1 tho wlnitivct IwriCrcl
to 10" aid kocying it Lhcre hc obtairwl i ~ i i
avcrago of litC.liD, or only 0.26
vcrtebrao lowcr than thc sample kept fiw the whole time at 4'C.
Apart from this slight lowering of thc average number of vertebrae,
fair too small to satifify Tucker's idea, it i8 impossiblc to imagine that
thorc would be II sharp limitation betwccii tho conditions in tho sea
crwtitig 'I'ucIwr'H iniaginary shock arid where tliey do not exist. The
distance betwcw 'LOON. and 25"N. is 300 m:a-miJes, so the slope of the
isotherms is in fact very gradual, and if 'I'ucker were right we would
have very many intermediate figures, mabing it impossible to distinguish between the two species of leptocephali.
Therefore I fail to see any support for Tucker's main assumption,
that the difference between the American and the European eel is only
eco-phenotypical. Field studies and experiments are both against it.
r08h'dU *.