ART. 8 CRUSTACEANS FROM WESTERN ATLANTIC TATTEESALL o 



I have tried to analyze the results in order to find out if they pro- 

 vided any information as to the vertical distribution of the Eupha\i- 

 sians. It was obvious from the merest glance at many of the hauls 

 that certain genera and species were characteristic of certain zones 

 but it has not been eas}^ to dem.onstrate this on paper. The explana- 

 tion of this difficulty probably lies in the fact that the nets used for 

 plankton were open nets and, therefore, in a haul from deep water, 

 a certain number of specimens of upper water forms would be caugh t 

 during the ascent of the net. It has been impossible to decide how 

 much allowance must be made for this and to eliminate this source of 

 error. One example will suffice to illustrate this point. Eupham-Aa 

 americana is clearly an upper water, if not a truly surface, species 

 yet the records reveal its occurrence in small numbers in even the 

 deepest hauls down to 1,800 meters. One other consideration has 

 complicated the question. There is a certain amount of evidence 

 available to suggest that some species of Euphausians at any rate 

 exhibit diurnal movements, rising to the upper waters during hours 

 of darkness and sinking to deeper waters by daylight. I have not 

 been able to take this consideration into account in the following 

 pages. At the same time I think it is possible to suggest with a certain 

 measure of confidence the following general conclusions on the vertical 

 distribution of the species of Euphausians found in the collection. 



The species may be classified roughly into the following groups 

 apparently characteristic of particular zones of water: 



1. Species which have their maximum of abundance in the upper 

 100 meters of the sea and are frequently taken actually at the surface: 



Euphausia americana. Euphausia tenera. 



Eriphausia brevis. Euphausia hemigibba. 



Euphausia mutica. 



2. Species which have their maximum of abundance between 

 100-200 meters and are rarely captured at the surface: 



Thysanopoda tricaspidata. Thysanopoda aequalis. 



Thysanopoda monacantha. Euphausia gibboides. 



Stylocheiron carinatum. Nematobraddon flexipea. 

 Stylocheiron suhmii. 



3. Species which are truly deep water with the maximum of abund- 

 ance at depths below 200 meters: 



Bentheuphausia amblyops. Thysanopoda cornuta. 



Thysanopoda microphlhalma. Thysanoessa gregaria. 



Nematoscelis microps. N ematobrachion boopis. 



Nematoscelis tcnella. N ematobrachion sexspinosus. 



Stylocheiron elongatum. Stylocheiron longicorne. 



Stylocheiron abbreviatum. Stylocheiron maximum. 



These tentative suggestions are based on the evidence provided 

 by the present material and are not opposed to anything that was pre- 

 viously known of the vertical distributions of the species concerned. 



