664 DEPTHS OF THE OCEAN 



Plates I. -VI. show certain forms found in the Sargasso 

 Sea, representing a small selection from the numerous 

 coloured drawings by Rasmussen. Plate I. shows the black 

 Cyclotkone microdon from deep water and the light coloured 

 C. signaia, which has its lower limit just at the upper limit of the 

 black fish. Other black fish and some red prawns from depths 

 beyond 500 metres are represented in Plates H. and HI. 

 The black and red colours are easily seen in strong sunlight. 

 The theory of protective colours must therefore assume that 

 these colours only appear in dark surroundings. In this con- 

 nection it is very interesting to note that the upper limit to the 

 occurrence of these black and red deep-water animals, which 

 according to latitude varies between 500 and 750 metres, is also 

 the limit within which most of the sun's rays are absorbed, and 

 it is important also to note that the red rays belong to that part 

 of the spectrum which is most rapidly absorbed by the water. 



In connection with the question of the Colouring of these 

 bathypelagic forms we may refer to some observations made 

 during the cruise regarding the vertical migrations of such 

 dark-coloured forms, as shown in Fig. 490. Three species, 

 Gastrostoinus bairdii, Cyema ati^uvi, and Gonostoma grande have 

 been taken only at 750 metres or deeper, while two species, 

 Gonostoma rhodadenia^ and P/iotostoniias gtcernei, have been 

 taken also at lesser depths, even at 150 metres. I have already 

 mentioned several instances (see p. 93) where forms like 

 Asti^onesthes and Idiacantlms have been taken at the surface, 

 but only at night. In the case oi Photostomias and Gonostovia 

 rhodadenia I have denoted the night-captures with a dark disc, 

 while a ring denotes day-captures. These catches seem explic- 

 able only by supposing vertical migrations to take place, and 

 as these occur in the darker part of the twenty-four hours they 

 probably coincide so precisely with the disappearance and re- 

 appearance of daylight that the dark colouring may be of no 

 danger to the animals in their nightly migrations towards the 

 surface of the sea. 



The occurrence of dark colours thus coincides with the 

 region where the intensity of the sunlight is greatly diminished. 

 Another circumstance seems to confirm this, viz. that in 

 different waters the upper limit to the black fish and the red 

 crustaceans seems to coincide with the same low intensity of 

 light. 



^ The specimens which in Fig. 490 are referred to Goiiosioina eloiigatwn have, on closer 

 investigation, proved to be the closely allied Gonostoma rhodadenia. 



