Grant Chaetognatha from central and southern Middle Atlantic Bight 



37 



Discussion 



Diel distribution at the surface 



Most of the chaetognaths commonly occurring in the 

 surface layer reached peak densities near midnight 

 (Table 6). Flaccisagitta enflata, Krohnitta pacifica, 

 Sagitta tenuis, Mesosagitta minima, Ferosagitta 

 hispida, and Pterosagitta draco all were caught in 

 maximum numbers at that hour. Peak numbers of 

 Parasagitta elegans and Serratosagitta tasmanica 

 occurred somewhat earlier, but they were abundant 

 throughout hours of darkness. There are also sugges- 

 tions of dusk or dawn (or both) increases in abundance 

 for K. pacifica, F. hispida, and Serratosagitta serra- 

 todentata. 



Unlike any of the other chaetognaths, Sagitta hele- 

 nae was decidedly more abundant at the surface in 

 daylight hours, with 31.0 and 32.8% of total catches 

 occurring around 1500 and 0900 hours, respectively. 



Eighteen species of chaetognaths are found in con- 

 tinental shelf waters of the Middle Atlantic Bight, in- 

 cluding the 15 species on record after the 1960s plus 

 the present records of three species previously known 

 only from surface slope waters (Grice and Hart 1962). 

 More recent studies of chaetognaths in this region have 

 been restricted to estuaries (Grant 1977b, Sweatt 1980, 

 Canino and Grant 1985) or to oceanic waters (Cheney 

 1985a, b). All of the western North Atlantic species 

 labeled epipelagic or mesopelagic by Cheney (1985b) 

 have been collected in shelf waters, so the present list 

 appears reasonably complete. The composition of Mid- 

 dle Atlantic Bight chaetognath collections from hypo- 

 neuston and subsurface plankton tows was nearly iden- 

 tical. There were no frequent or abundant species 

 unique to the hyponeuston, and all but the rarest 

 species from subsurface shelf waters were taken at 

 least occasionally from the surface layer. However, 



