Grant: Chaetognatha from central and southern Middle Atlantic Bight 



35 



Relative strength of the asso- 

 ciation between pairs of species 

 was measured using 2x2 con- 

 tingency tables and Hurlbert's 

 (1969) coefficient of interspecific 

 association (C 8 ), as corrected by 

 Ratliff (1982) for errors resulting 

 from lack of absolute association 

 (Pielou 1977). Yates' correction 

 of chi-squared calculations was 

 applied for low expected frequen- 

 cies (Bailey 1981). 



Results 



Species composition 



Eighteen species (11 genera) of 

 chaetognaths were identified, 17 

 in both surface hyponeuston and 

 subsurface bongo net collections 

 (Tables 2 and 3). Compositional 

 differences in the two lists were 

 limited to the rarest species: 

 Sagitta bipunctata was found 

 only in hyponeuston collections, 

 while Pseudosagitta maxima was 

 restricted to subsurface collec- 

 tions. After adjustment for the 

 80 collections devoid of chaeto- 

 gnaths (79 surface and 1 subsur- 

 face collections), the percent fre- 

 quencies of the most common 

 cold-temperate species {Serrato- 

 sagitta tasmanica, Parasagitta 

 elegans, and Mesosagitta minima) 

 were found to be much lower in 

 surface collections. Warm-water 

 chaetognaths (Flaccisagitta 

 enflata, Sagitta helenae, Ptero- 

 sagitta draco, S. tenuis, Serrato- 

 sagitta serratodentata, Fero- 

 sagitta hispida, and Krohnitta pacifica) were either 

 equally frequent in the two types of collections or more 

 frequent in the hyponeuston. 



Temperature-salinity-abundance relationships 



The chaetognaths were collected in a wide range of 

 temperatures (2.2-26.6°C) and salinities 27.7-36.0%o 

 (Table 4). Weighted means of surface salinities 

 measured at the time of hyponeuston collections were 

 very similar among all common surface species, rang- 

 ing from 32.6 and 32.8%o for Sagitta tenuis and 

 Parasagitta elegans, respectively, to 34.9%o for Kroh- 



nitta pacifica (Table 4). Weighted mean temperatures, 

 on the other hand, varied widely among species, from 

 12.9°C for Mesosagitta minima to 24.2°C for K. paci- 

 fica. Species occurring infrequently at the surface and 

 excluded from Table 4 were those residing at greater 

 depths, with surface occurrences mostly restricted to 

 low winter temperatures. Relationships between sur- 

 face abundance of common species and the two physical 

 factors were further examined by plotting average 

 daily catch (8 collections at 3-hour intervals) in stan- 

 dard hyponeuston collections, defined previously (Grant 

 1988) as 1-m neuston nets towed at 2.5 knots for 20 

 minutes; the sampler used typically fished the upper 



