Grant: Chaetognatha from central and southern Middle Atlantic Bight 



39 



abundant, to Serratosagitta tasmanica, x S = 33.7°/oo, 

 the rarest in Chesapeake Bay). Aurich (1971) includes 

 the only other known T-S-P diagram of S. tasmanica, 

 used for a display of habitat differences with 5. serra- 

 todentata. Michel and Foyo (1976) and Nagasawa and 

 Marumo (1982) provided T-S-P diagrams for both Meso- 

 sagitta minima and Pterosagitta draco. Middle Atlan- 

 tic Bight T-S-P diagrams agree with their depiction of 

 greater abundance at higher salinities for these two 

 species. Finally, Michel and Foyo's (1976) T-S-P dia- 

 gram of Ferosagitta hispida shows a few occurrences 

 at lower temperatures (13-17°C), but most at 27-28°C 

 and in a broad range of salinities. 



Association among the Middle Atlantic Bight species 

 was generally positive, both in the multispecies case 

 and between pairs of species. Parasagitta elegans pro- 

 vided an important and consistent exception, evidenced 

 by its highly significant (P<0.01) negative associations 

 with five warm-water species: Krohnitta pacifica, 

 Ferosagitta hispida, Sagitta tenuis, Flaccisagitta en- 

 flata, and Sagitta helmae. Although both P. elegans and 

 S. tenuis occur abundantly in coastal and estuarine 

 waters in the Chesapeake region, they do so in opposite 

 seasons, hence their negative association. The sole 

 significant (P<0.05) positive association of P. elegans 

 was with Serratosagitta tasmanica, and it appears to 

 occupy a niche of low temperatures and salinities in this 

 region, not unlike that of Aidanosagitta crassa^ in the 

 East China Sea (Matsuzaki 1975). Highly significant 

 positive associations were shared by (1) the endemic 

 and abundant shelf species Serratosagitta tasmanica, 

 Mesosagitta minima, and Flaccisagitta enflata, (2) the 

 warm-water species Krohnitta pacifica, Serratosagitta 

 serratodentata, Ferosagitta hispida, and Sagitta 

 tenuis, and (3) the offshore, shelf-edge species Flacci- 

 sagitta hexaptera, Eukrohnia hamata, Pseudosagitta 

 lyra, Mesosagitta decipiens, and Krohnitta subtilis. 

 Three of the latter species comprised Matsuzaki's 

 (1975) "Kuroshio water" species group. 



Pearre (1973) determined that extensive diurnal 

 migration takes place with Parasagitta elegans, per- 

 haps related to feeding. Data in the present study on 

 the diurnal distribution of the species in the hypo- 

 neuston also indicates a strong upward migration at 

 night; numbers caught were about an order of magni- 

 tude higher at night than in daylight. Several other 

 species showed similar sharp increases in abundance 

 during darkness, including Serratosagitta tasmanica, 

 Krohnitta pacifica, Sagitta tenuis, Mesosagitta mini- 

 ma, and Pterosagitta draco. Some were more abundant 

 at dawn or dusk, or, in the case of Flaccisagitta enflata 

 and Sagitta helenae, were present in considerable num- 

 bers in daylight. Among these species, Nagasawa and 

 Marumo (1982) found evidence for diurnal migration 

 in F. enflata, P. draco, and K. subtilis, none for K. 



pacifica, and mixed evidence for M. minima and F. hex- 

 aptera. However, in the present comparison of day and 

 night hyponeuston collections, only a short migration 

 would be required to populate and depopulate the sur- 

 face layer diurnally. Such fine-scale diurnal movements 

 likely do occur within the surface mixed layer, but are 

 most difficult to measure. 



Acknowledgments 



Collections and identifications serving as the basis for 

 this report were supported by the U.S. Dept. of the In- 

 terior, Bureau of Land Management Contracts No. 

 08550-CT-5-42 and AA550-CT6-62. Much appreciated 

 were early and helpful reviews of this manuscript by 

 Romuald N. Lipcius, Kenneth L. Webb, and Daniel W. 

 Sved, and the suggestions by two anonymous reviewers 

 for final alterations and adjustments. 



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