ATLANT. DEEP-SEA EXPED. 1910. VOL. III]. 



SCYPHOMEDUSAE. 



the Canaries, and between the Mid-Atlantic ridge and the 

 continental slope to the south of Ireland. It is at present 

 rather difficult to give a satisfactory explanation of this 

 peculiar distribution of Periphylla hyaclnthina. 



The investigations of the "Michael Sars" in the Sogne- 



fjord have given us some interesting data as to the 

 occurrence of Periphylla hyacinthina, which remind one 

 of what in a previous paper 1 ) I called "secondary centres". 

 On the 22nd of May 1911a haul of six hours duration in the 

 outer parts of the Sognefjord gave the following results: — 



Depth in 

 metres 



Gear 



Number of 

 specimens 



Remarks 



75 



Net 1 meter in diameter 



56 



\ 40 large specimens, 10 — 15 cm in diameter 

 1. 60 small „ 2 - 9 „ - 



150 



Youngfishtrawl 



90 



From 1-5 — 15 cm in diameter 



300 



Net 1 meter in diameter 



31 



From 1 — 12 cm in diameter 



500 



Youngfishtrawl 



426 



( 15 — 20 large specimens, about 400 specimens 

 \ from 1—8 cm in diameter 



650 



Net 1 meter in diameter 



72 



J 2 large specimens 



I 70 small „ 1 — 5 cm in diameter 



750 



Net 3 metres in diarr.eter 



400 



/ 100 large specimens 

 \ 300 small 



The hauls are of great interest. First of all it is 

 striking how numerous Periphylla hyacinthina may some- 

 times be in the Sognefjord; Dr. Johan Hjort pointed out 

 to me that this may in part be due to the submarine 

 barrier at the mouth of the Sognefjord, which of course, 



30' 40" JO' 



8o" 70" 60* SO* 



20 |o- 



interferes with the exchange of water between the fjord 

 and the open ocean thus causing the retention in the 

 fjord of large numbers of Medusae. But this one negative 

 factor cannot produce a "secondary centre", and we must 

 admit our ignorance regarding the positive factors which 

 render such a "pool" a favourable habitat for a 

 certain species. The biophysical factors are in 

 such places evidently combined in such a manner 

 as to enable one species to thrive abundantly, 

 whereas another species cannot exist there at 

 all, although the two species in question may 

 live side by side in the open ocean under 

 apperently identical conditions. 



It is of great interest to find out the bathy- 

 metrical distribution of the individuals of a 

 plankton species. As for Periphylla hyacinthina 

 the catches from the Sognefjord show no differ- 

 ences whatever from those in the Atlantic, the 

 upper limit occurring a little above 50 metres, 

 with a somewhat prominent maximum at a 

 depth of about 500 metres. 



© Periphylla hyacinthina "Michael Sars". 



O — „ — Previous records. 



A Periphylla r eg in a. 

 Fig. 4. Geographical distribution of Periphylla hyacinthina and P. regina. 



Periphylla regina Haeckel. 



Only nine specimens can with certainty 

 be referred to this species, besides two doubt- 

 ful specimens: one from Stat. 49 (500 m) and 

 one from Stat. 81 (1500 m). 



') Die Alcyonaceen des Kolafjordes (Travaux Soc. Imper. Natur. St. Petersbourg, Vol. XL1) 1912, p. 17. 



