KONGL. 8V. VET. AKADKMIKNS HANDT,lN(iAl!. RAND 36. N:0^ 8. 7 



make the existeuce of ;m n.rctic fauna in South Kattegatt, Oresunrl and the southoni 

 Baltic quite ua.tural. 



Currents and indigenous forms. 



From the beginning of my study of the distribution of the plankton-organisms my 

 principal object was to determine what species characterize the different systems of ocean 

 currents, and to determine their most importa-nt biological factors, temperature and sali- 

 nity. The verv large mateiial of observations offered in my paper >,The seasonal distri- 

 bution of Atlantic Plankton-organisms» will fully prove that each curi'ent-system carries 

 its own plankton-flora and fauna. But as the currents touch each others or the coast- 

 banks their plankton becomes inodified. Enryhaline and eurytherm species pass from one 

 current to another, remain for a longer or shorter time in currents, to which they do 

 not propei'ly belong and give the impression of indigenous» species. The same is the 

 case with neritic forms, which are poured out from the fjords and mix Avith the oceanic 

 forms, so it will in many cases be extremely difficult to determine whether a species is 

 to be considered as oceanic or neritic. That the neritic are meroplanktonic, as Mr. Gran 

 points out, will hold good in most instances. But there are forms Avhich are not so 

 easily disposed of. For instance, Arachnactis albida is a meroplanktonic form, but it 

 occurs far out in the northern Atlantic. Besides a large number of neritic diatoms, e. g. 

 Eucampia zodiacus, Asterionella japonien, Leptocylindrus danicus and many more«never 

 form resting spores, as far as is knoAvn. 



When tAvo currents meet there takes place an interchange of species. Thus we 

 frequently find above the Newfoundland banks arctic and tropical forms in the same 

 haul. The least sensitive forms remain for a longer or shorter time in their ncAV me- 

 dium and seem to belong to it. That is the case for inst. Avith Coscinodiscus oculus 

 iridis. This species belongs to the arctic currents of the Avestern Atlantic, but Avhen 

 their Avaters expand eastwards in the winter and the spring and become mixed Avith 

 Avaters from the temperate Atlantic the species in question remains in the eastern Atlantic, 

 Avest of NorAvav and seems to be indigenous. The sa-me is the case Avith several other 

 west-atlantic forms as for inst. Clione limacina, Ceratium arcticum a. o. 



As to Halosphcera viridis I agree Avith Mr. Gran so far that it is indigenous in 

 the Avarm Atlantic Avater Avest of Norway, but it evidently follows this current through 

 the Färö — Shetland channel and comes from southern regions. It appears in the autumn 

 Avest of Scotland, adAances in the Avinter round Scotland towards the coast of Scandinavia. 

 In the eaidy spring Avhen Avater with ai'ctic and boreal forms fills the basin of the Ska- 

 gerak and Kattegatt it disappears and does not return before next winter. West of 

 Norway it remains for a longer time because of the warm Atlantic current. Whether it 

 propagates or not in this kind of Avater is for me of no importance. It belongs in all 

 cases to the Avarm current that comes from Färöe^Shetland channel and goes towards 

 Spitsbergen. 



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