Hydrography and distribution of chaetognaths over the continental shelf off North Carolina |07 



except that it was almost invariably more numerous in samples taken near the surface than in the 

 deeper samples at any given station. This was true for samples taken during the day as well as at 

 night. 



Sagitta tenuis was restricted to the continental shelf waters both north and south of Cape Hatteras 

 (Fig. 12). It extended from within a few miles of shore to the edge of the shelf. Many more specimens 

 were caught and their range from shore was greater in the winter than in the spring. This species is 

 one of the more euryhaline forms in this area, having been found in water ranging from less than 31 

 to 36-6°/eo- It does not appear beyond the shelf where the Florida Current proper is encountered. 

 Specimens were commonly found in greatest abundance at the less inshore stations. Like many other 

 species in this area it was collected in water ranging in temperature from 9° to 25^ C and little can be 

 said about its optimum temperature. 



Krohnitta pacifica and K. subtilis were never very abundant in any of the tows. They ranged from 

 midway over the shelf outward as far as the stations extended. K. subtilis was noticeably rarer than 

 K. pacifica. No significant difference could be seen between shallow and deep hauls in terms of either 

 species or abundance. These species were almost always found in water whose salinity was 36'';oo or 

 above. They were therefore directly associated with the Florida Current water. In the winter when 

 a large incursion of Florida Current water moved into Onslow Bay a number of specimens of A. 

 pacifica were taken in this water about as far inshore as it extended. 



Pterosagitta draco was always identified in these catches with the warm saline water of the Florida 

 Current (Fig. 12). Although one of the less abundant species it was widely distributed and found at 

 almost all stations from midway over the shelf seaward. This species was closely associated with 

 K. pacifica in these samples. 



Sagitta hexaptera and S. lyra were also represented in some of the samples taken well inside the 

 Florida Current. They were markedly more abundant in the spring than in the winter. Only a few 

 hexaptera were found in the winter cruise and no lyra. Because they were present in small numbers 

 their affinities are not obvious. 



DISCUSSION 



Chaetognaths were common and important members of the plankton community 

 in all parts of the area studied. As a general rule those stations closest inshore and 

 farthest offshore had fewer species than those near the edge of the shelf. The increase 

 in number of species is the result of some of the typically shelf forms such as S. tenuis 

 and S. helenae being present with species normally found in the Florida Current 

 proper. This probably is the result of mixing which augments the total number of 



species in this area. 



A comparison of the total number of chaetognaths collected at each station together 

 with the abundance of each species shows surprisingly little difference between the 

 spring and winter collections. The more abundant species such as S. enfiata and 5. 

 senatodentata were abundant in spring as well as winter. S. tenuis was more abundant 

 and widespread during the winter than in the spring. S. hexaptera was less numerous 

 in winter, and S. lyra, an uncommon form in these collections was not found ai all 



during the winter cruise. 



Certain species of chaetognaths have been observed to exhibit diurnal vortical 

 migration (Michael, 1911; Russell, 1933). In these cruises samples were taken 

 at all hours. Position of the ship dictated when a sample would be taken rather than 

 hour of the day or night. An inspection of these samples shows no consistent evidence 

 for vertical migration or pronounced abundance of individuals in the shallow samples 

 as compared with the deeper samples (Tables I and 11). S. tninuna in the spring samples 

 is an exception and does appear to be significantly more abundant in ihc deeper 



samples. ^ , 



All species of chaetognaths collected in the area surveyed are known to occur in 

 tropical and subtropical waters. None were typically Virginian (Table HI). Published 



