Abundance and Vertical Distribution of Zooplankton 



We had no absolutely quantitative means of recording the 

 abundance, sizes or distances of animals seen from the 

 submersible. However, observers recorded their visual 

 observations on audio tape. From transcriptions of these 

 recordings, I have made a preliminary summary of our observations 

 on the occurrence and distribution of the major groups of 

 zooplankton. Table 2 gives the depth range observed and the 

 approximate average number of organisms reported for each group 

 on day and night dives. These numbers only indicate the relative 

 abundance of different major groups, and are not comparable to 

 data from net tows. Observers neither saw everything present nor 

 recorded everything they saw. Estimates of abundance were 

 sometimes absent or vague, especially for small, common organisms 

 like colonial radiolarians, amphipods, and copepods. All the 

 abundance figures should be considered quite conservative. 

 Observations by Robison on dives 1419, 1422, 1425 and 1428 were 

 not available at this writing, but will be added to a later 

 analysis. 



Radiolaria and Foraminifera 



Colonial radiolarians were often abundant near the surface, 

 but rarely extended below a few hundred meters. They are 

 presumably restricted to the photic zone because of their 

 zooxanthellae. Phaeodarian radiolarians in the genus 

 Coelothamnus ( Swanberg et al . , 1986) were seen occasionally at 

 greater depths. Forams were reported throughout the water 

 column; larger individuals were usually seen deeper. They were 

 never as abundant as the colonial radiolarians. 



Medusae 



Narcomedusae were the most abundant type, mainly the genera 

 Solmissus and Solmundella , found in the deeper water column, and 

 Aeginura , seen just above the bottom. The narcomedusae were 

 seen shallower at night than during the day. Few trachymedusae 

 were reported, but small ones could easily have been overlooked. 

 Other hydromedusae were leptomedusae or unspecified types. 

 Coronates were the predominant scyphomedusae; about equal numbers 

 of Atolla and Periphylla were reported, always below 700 m (the 

 coronate at 225 m in Table 2 was a Linuche ) . Other scyphomedusae 

 included Pelagia , both deep and shallow, and a new species of 

 Deepstaria (Larson et al., submitted) seen over the bottom at 

 900 m. 



Siphonophores 



Most siphonophores were identified only as physonect or 

 calycophoran types, although species of Agalma , Apolemia, 

 Nanomia , Forskalia and Hippopodius were noted. Somewhat more 



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