14 



HJALMAR BROCH. 



We find here a few straggling records along the wide 

 expanse from North Cape to the western coast of Spitz- 

 bergen, and there is a single record in the far East of 

 the Murman Sea. 



A general view of the localities (see fig. 14) shows 

 that the distribution of the southern hydroids follows the 

 course of the Gulf Stream, not because other territories 

 have not been investigated, for such localities as the wa- 

 ters round Spitzbergen, Bear Island and Jan Mayen have 

 been studied very thoroughly, and B. S^mundson has 

 made known the hydroid fauna of the coasts of Iceland, 

 while the "Michael Sars" brought home much material 

 from the ridges between the Faroe Islands and Iceland. 

 The absence of southern hydroids is due to the predo- 

 minant influence of arctic waters round Jan Mayen, along 

 the northern coast of Iceland, and along the north-eastern 

 sides of the ridges between the Faroes and Iceland. 



On the other hand we see that the southern fixed 

 animals like the hydroids are not absolutely limited to the 

 warm waters of the Atlantic. Perhaps their occurrence 

 outside the strictly Atlantic layers may be due to sub- 

 marine waves at the lower boundaries of the Gulf Stream. 

 Certain forms may have fixed themselves to the bottom 

 while covered by Atlantic water, and then been submerged 

 in arctic water; but this cannot hold good in all cases, 

 especially as regards the Aglaopheniids, which occur 

 too far down in the cold layers for the warmer Atlantic 

 waves to reach them. 



The question arises: How do such exotic forms get 

 down so far into a foreign realm? I am inclined to be- 

 lieve that Appellof is right, when he says 1 ) that the 

 Norwegian Sea was mainly populated by southern ani- 

 mals (fixed or nearly stationary) through transport of the 

 larvae, and that the same process is still going on. The 

 duration of the free-swimming stages of the PI u inu- 

 la ri ids and the Aglaopheniids seems to be very 

 short, but it may be sufficiently long for transport to some 

 distance, and the larvae may sink to the bottom far from 

 the warm Atlantic waters. Most of these erratic larvae 

 soon perish in the icy bottom layers of the Norwegian 

 Sea, but the resistant power of many hydroids enables 

 them to develop even under such unfavourable circum- 

 stances. Whether they can produce young capable of 



existing there is questionable, for if so we might expect 

 that some Aglaopheniids or Plumulariids would 

 have become constant members of the strictly arctic fauna, 

 but as far as we are aware at present, this is not the case. 



The distribution of the Plumulariids must be 

 considered in connection with boundaries between the 

 Atlantic current and the coastal water. Layers of coastal 

 water seem to be no absolute obstacle for many Plumu- 

 lariids, although some species, of course, are more 

 sensitive than others. Plumularia pinnata (Lin.) thrives 

 even in the brackish waters near Drobak on the Kjistiania- 

 fjord at a depth of 5 metres, so that we must assume 

 that differences in temperature arrest its progress to the 

 North and East along the Norwegian coasts. 



Among the territories adjoining the Gulf-Stream, the 

 North Sea plays an interesting part, especially as to the 

 occurrence of the Plumulariids. On his review of 

 the results obtained during the cruises of the "Michael 

 Sars" 2 ) Appellof points out that there is "a peculiar 

 faunistic area in the central portion of the North Sea 

 which is characterized by water of a lower temperature 

 and with less distinct movements of the Atlantic water, 

 and which, in accordance with these features, possesses 

 a fauna of a different character from what is to be found 

 in the Atlantic water. In this area there is a striking 

 absence of several forms which are found in Atlantic 

 water on the northern portion of the North Sea plateau." 

 Among the exceedingly rare forms, we must include both 

 the Aglaopheniids and the Plumulariids. These 

 warm water forms are found along the eastern and western 

 sides of the North Sea, but in the southern parts their 

 occurrence is very sporadic, apparently because of the low 

 salinity and the saallowness of the water. 



Wherever the Atlantic waters enter our regions of the 

 ocean, they are accompanied by warm water hydroids; 

 thus we find them along the coast of Bohuslan, having 

 passed through the submarine channel along the western 

 and southern coast of Norway. It is of great interest to 

 notice that while the main factor in populating the Sar- 

 gasso sea, is the transport of fixed individuals attached 

 to drifting sea-weed, the progress of Atlantic animals in 

 Norwegian waters is chiefly due to transport of free-swim- 

 ming larvae. 



Trondhjem, March 11th, 1912. 



1 ) Havbundens dyreliv, p. 114. 



2 ) Review of Norwegian Fishery and Marine Investigations 1900 — 1908 (Report Norweg. Fish, and Marin. Invest. Vol. II), Bergen 1909, p. 85. 



