702 DEPTHS OF THE OCEAN 



(see p. 95). Specially striking were the fishes Astronesthes 

 and Idiacanthiis occurring at the surface only at night. It was 

 also very interesting to note the remarkable coincidence between 

 the vertical migrations of the fishes and the development of their 

 light-organs. Fig. 490 shows the vertical occurrence of five 

 black fishes, each mark denoting the capture of one individual ; 

 in the case of Gonostoma rhodade^iia and Photostoniias guei-nei, 

 a black dot denotes a specimen captured at night, while a ring- 

 shaped dot denotes a specimen taken during the day. In 

 Gastrostomus, Cyema, and Gonostoma grande only slightly 

 developed light-organs, if any, are met with. In Gonostoma 

 rhodadenia and Photostomias guernei particularly large light- 

 organs are present (see Fig. 494 and Plate II.). Specially 

 interesting is a comparison of the two species of Gojiostoma, 

 the light-organs along the side of the body in G. rhodadenia 

 having a length of 2.5 mm., while in G. grande they are only 

 0.5 mm. long. Evidently we have here a type of deep-sea 

 fishes, living in deep water, but with the power of migrating 

 towards the surface. These forms have retained their well- 

 developed light-organs, which in other black fishes of the deep 

 sea must be considered as extremely reduced, perhaps even 

 quite rudimentary, organs. A perfect analogy is found in the 

 decapod Crustacea. The deepest living species (see table on 

 p. 668, Nos. 8-11) have no light-organs and make no vertical 

 migrations. Light-organs, or organs which are believed to 

 produce light, are found only in species living between 150 and 

 500 metres with a maximum distribution at about 500 metres. 

 These species have been found much higher up in the water 

 during the night than during the day, as is brought out quite 

 clearly by the table. 



During our southern cruise we might have had a good 

 opportunity of making an exact study of vertical migrations 

 by the aid of precise closing-net hauls, but time did not permit, 

 though our isolated observations are very interesting, for 

 instance those made at Station 48. While towing our big 

 trawl all day at this station, we were continually taking hauls 

 with surface tow-nets, the catches during the day consisting 

 only of the common surface forms : lant/iina, Pterotrackea, fish 

 eggs, pteropoda, radiolaria, etc. ; but between 6 and 7 p.m. 

 the nets suddenly captured a mass of small red copepoda, which 

 during the day had been taken at about 70 metres. At 

 Station 53, during the day, we captured only radiolarians at the 

 surface ; at 30 metres there were a few copepoda, no young 



