392 



NATURE 



[January 19, 191 1 



munity." We have come across no fewer than twenty- 

 six species of prawns, of which we shall here refer to 

 Acanthephyra miiltispina and A. purpurea. 



A. miiltispina shared with Cyclothonc the peculiarity of 

 the largest and oldest individuals, being found in the nets 

 towed at greatest depths, say at iooa-1500 metres. 

 A. purpurea resembles Cyclothone signata in that its dis- 

 tribution is chiefly confined to an intermediary layer 

 between 500 metres and 750 metres in depth. 



The instances I have given show the utility and exact- 

 ness of our method of working. Where we have to deal 

 with catches of great numbers of individuals, our errors 

 and inaccuracies will undoubtedly be very small. The 

 catches which the Michael Sars made of such forms as 

 Cyclothone and Acanthephyra were certainly most satis- 

 factory in this respect. But when we come to the catches 

 which the expedition made of scarce forms, or forms 

 more difficult to capture, then we are bound to own that 

 the method of working even of the Michael Sars is not 

 sufficiently effective. Still, it is interesting to examine a 

 few of the results yielded by the method we employed 

 with the object of discovering some conformity, or some 

 general rule, for the peculiar distribution of the different 

 organisms at different depths. 



I will commence with the view I formed during the 

 cruise itself from the appearance of the catches on board, 

 which view I find has also, to a certain extent, forced 

 itself upon other observers, chief amongst whom I may 

 mention Prof. A. Brauer, to whom was confided the treat- 

 ment of the fishes of the Valdivia expedition. I found on 

 examining the catches from great depths that the black 

 and dark-red forms w-ere the all prevailing ones among 

 animals from the greatest depths. 



Black-coloured pelagic fishes are few in number, though 

 they might be termed numerous if we take into account 

 what was previously known concerning " scarce " forms. 

 Gastrostomus bairdii, Cyemaatrum, and Gonostoma grande 

 were only caught at depths from 750 metres downwards. 

 The two species Gonostoma elongatum and Ptotostomias 

 guernei were caught at great as well as small depths, even 

 in some cases so high up as 150 metres below the surface. 

 The rule, then, that the black forms are only to be found 

 at great depths, cannot be said to hold good universally. 



The question accordingly arises whether among the black 

 forms there may not be said to be groups or different 

 types. In common with several previous observers, I have 

 been struck by the fact that even the anatomical struc- 

 ture of the black fishes points to different modes of living. 

 When we compare, for instance, pictures of the above- 

 mentioned five species of fish, we see that, of the three 

 species which were only found at great depths, Gastro- 

 stomus bairdii and Cyemaatrum are quite without light 

 organs, and Gonostroma grande has but small ones, as is 

 also the case with Cythotlone microdon. In Gonostoma 

 elongatum and Ptotostomias guernei, the light organs are 

 much more developed (as is also the case with Cyclothone 

 signata). It is an interesting fact now to notice that 

 every single individual of these two species which was 

 captured higher up than 500 metres was caught at night, 

 which coincides with previous observations regarding black 

 forms, such as Idiacanthus and Astronesthes, v^'hich have 

 been caught at night right close up to the surface. We 

 may assume, accordingly, that among the black deep-sea 

 fishes there are several different modes of life, that is to 

 say, several different " biological types." 



With the view of a better understanding of the occur- 

 rence of these black and red types in the sea, I have 

 endeavoured to compare their vertical distribution with the 

 intensity of the sunlight in different depths and at different 

 parts of the ocean. 



We have seen that the upper limit for Cyclothone 

 microdon and the red crustaceans in the northern section 

 from Newfoundland to Ireland, or about 50° N. lat., was 

 approximately 500 metres below the surface, and we have 

 also noticed that the limit of depth for the same forms at 

 the southernmost stations, or about 33° N. lat., was some 

 200-300 metres deeper. In the Norwegian Sea I have 

 already previously investigated the intermediary pelagic 

 fauna, and found pelagic red prawns as well as the dark- 

 red fish Sebastes norvegicus at depths of about 200 metres 

 below the surface. Sebastes was taken, for instance, with 



NO. 2 15 I, VOL. 85] 



floating long lines in considerable quantities on a course 

 Jan Mayen-Lofoten — that is to say, in about 67° N. lat., 

 at a depth of 200 metres — and it was even found, though 

 in decreasing quantities, higher up. Along the Norwegian 

 coast, in the fjords and sounds, we have a particularly 

 rich fauna of red crustaceans (especially Pandalus 

 borealis) residing at depths the maximum of which in the 

 north, at any rate, may be put at about 200 metres. 

 Now, if we calculate the depth to which the rays of the 

 sun penetrate, after passing through the same distance in 

 the water, assuming always that the rays are direct, and 

 that the rate of absorption is the same, we find that the 

 rays will have passed through the same distance to rf :ii li 

 a depth of 500 metres in 50° N. lat., that they will 1 

 through to reach 650 metres in 33° N. lat., or 300 m- 

 in 67° N. lat. 



However, the transparency of the water varies greatly 

 in different regions. If we take the results of previous 

 observations during different expeditions, we may set 

 down the visible depth in the open sea as being, roughly, 

 50 metres in 33° N. lat., 40 metres in 50° N. lat., and 

 25 metres at the outside in the Norwegian Sea in 67° N. 

 lat. Taking this into consideration, we find that there 

 will be the same intensity from the retilinear rays 



In 33° N. lat., at aliout 800 metres' depth 

 .) 50° ». M 500 „ „ 

 ,, 67° „ ,, 200 ,, ,, 



The red and black animal forms, as has been found] 

 the investigations I have just described, have an up 

 limit in the different waters which corresponds evef 

 where with the same intensity of light. 



During the Atlantic cruise of the Michael Sars we 

 undertook a series of measurements of the intensitv 

 light with a photometer constructed by Dr. Hell 

 Hansen ; to determine the intensity of the different co 

 rays. Dr. Helland-Hansen made use of panchroii 

 plates and gelatin colour-filters. The observation s< 

 and west of the Azores (that is to say, at the sout 

 stations) showed that the rays of light strongly aff- 

 the plate at a depth of 100 metres. The red rays ■ 

 weakest here, while the blue and ultra-violet rays •. 

 strongest. At a depth of 500 metres the blue and ultra ^ 

 violet rays were still distinctly visible, and at a depth Oj 

 1000 metres the ultra-violet rays were yet perceptible. '' 

 1700 metres, however, there was not the faintest trac 

 light, even after the plates had been exposed for : 

 hours in broad daylight. 1 



In the above-mentioned deep, which denotes the uppp' 

 limit for the black and red forms during the daytime, 

 may after this, no doubt, assume that there are onl 

 be found chemically effective rays from the violet poi 

 of the spectrum. Now, seeing that the coefficien- 

 absorption for the red rays, as compared with the vi 

 is about in the proportion of 30 to i, and that our obs. 

 tions failed to trace any red rays at a depth of 500 me. 

 it follows that the red animals at this depth must 1 

 quite as invisible as the black ones. It is interesting t, 

 note, in this connection, that it is only at night that ^'^ 

 black fish with large light organs are found high 11; 

 the water, and that red crustaceans in the Arctic re^i' 

 as was noticed by Scoresby in the case of Hymero^ 

 glacialis, are to be found right up to the edge of th- 

 at the surface of the sea. 



Above the region I have hitherto been describing, ^ 

 its black and red forms, our parallel hauls have sb 

 us an equally characteristic, though very different, i;: 

 of pelagic fishes. Their peculiarity is that their bod 

 always more or less compressed from one side to the ot 

 In colour they are dark along the back and silver\ 

 shining, with a bluish-violet gleam along the sides, ' 

 eves are large and often telescopic, and most character 

 of all, I suppose, are their strongly developed light org. 

 characteristic forms are especially to be met with an 

 the families Sternoptychidae and Stomiatidae. Froi: 

 table showing the depth at which a number of these f<- 

 occur, it can be seen that 500 metres may be taken 

 their lower limit, and that the greatest number 

 individuals reside at a depth of 300 metres ; above 

 metres there were only a few found, and even tho>e ' 



