other factors could also contribute to the 

 degree of response. Of particular interest is 

 the barometric pressure. As shown in figure 2, 

 the pressure declined continuously for the 2 

 days preceding the 1963 eclipse. Brown (1958) 

 showed that cycles of activity in certain organ- 

 isms fluctuate with changes in pressure. These 

 species included the fiddler crab (Uca), the 

 oyster (Ostrea), and the quahog (Venus). 

 Though these animals also exhibit daily and 

 lunar cycles of activity, mean hourly rates of 

 activity were correlated with the rates of rise 

 or fall in barometric pressure. Activity in- 

 creased with the hourly rate of fall and de- 

 creased with the rate of rise. The importance 

 of this phenomenon to the observations during 

 the 1963 eclipse is uncertain; it could be of 

 significance during an eclipse with a relatively 

 long period of totality. 



In comparing the responses of zooplankton 

 during the eclipses of 1954 and 1963, the im- 

 portance of documenting the environmental 

 differences is readily apparent. Petipa (1955) 

 reported that most species reacted by rising to 

 the upper water layer (0-5 m.) during the 

 eclipse and by descending to lower depths 

 (5-14 m.) after the eclipse. The strongest re- 

 sponse was from Sagitta. and larvae of De- 

 capoda, Lamellibranchia, and Gastropoda. His 

 work was done in the Black Sea at Sevastopol 

 Bay where surface temperatures during the 

 eclipse (June) were at least 20° C. Zenkevitch 

 (1963) described the general hydrological fea- 

 tures of the Black Sea : .salinity varied between 

 17 and 18 %^ at the surface and was only 22 

 to 23 %o in deep water; temperature at 25 m. 

 was 14° C. in summer and 6° C. in winter; 

 dissolved oxygen content ranged from 1.05 to 

 7.76 cm.r 1. at 50 m. and declined rapidly with 

 increasing depth below 50 m. ; water deeper 

 than 150 m. was contaminated with hydrogen 

 sulphide; Secchi disks di.sappeared between 18 

 and 21 m. ; and most species of zooplankton 

 were found at depths above 50 m. and were 

 concentrated between the surface and 25 m. 

 Many of these characteristics are strikingly 

 different from those in the coastal waters of 

 the Gulf of Maine: during the eclipse of July 

 20 the temperature was less than 15° C. at the 

 surface and was 10° C. at 10 m. ; .salinity was 



about 32 %„; and the Secchi disk disappeared 

 at less than 10 m. The dissolved oxygen con- 

 tent in the Gulf of Maine was reported by Gran 

 and Braarud (1935) to vary between 5.5 and 

 7.8 cm. 1. at 40 m.. and Bigelow (1926), in 

 contrast to the conditions in the Black Sea, 

 reported many species of zooplankton below 

 50 m., some of which had their densest con- 

 centrations below 100 m. 



Other differences to consider include the 

 characteristics of the eclipse and the location 

 of sampling in relation to the path of totality. 

 The sampling sites in Maine were selected be- 

 cause they lay in or near the path of totality. 

 In contrast, Sevastopol Bay was about 400 

 miles from the path of totality in 1954. (This 

 figure was estimated from eclipse data pre- 

 sented by Oppolzer, 1962.) As Petipa (1955) 

 did not provide any measure of light intensity, 

 one can only a.ssume, other things being equal, 

 that the illuminance during the eclipse was 

 higher at Sevastopol than at Bar Harbor. Yet 

 Petipa recorded more activity of zooplankton 

 than was noted in the Gulf of Maine. Differ- 

 ences in species were important, but I suspect 

 that the differences in the two environments 

 were more critical. 



Though not a species encountered in this 

 study, experimentation on Daphnia offers sev- 

 eral plausible explanations for the zooplankton 

 behavior ob.served during the eclipse. Harris 

 and Wolfe (1955) found that Daphnia re- 

 sponded rapidly to changes in light intensity, 

 moving in the direction of the original optimum 

 intensity, but this was followed by movement 

 towards an adapted optimum and resulted in 

 little change of position. In essence, a high 

 change of intensity produced an alteration of 

 photonegative and photopositive phases, and the 

 net result had relatively little effect on the 

 depth at which the animal was located. When 

 changes in light intensity were slow, however, 

 the animals simply followed the movement of 

 the original optimum zone. Ringelberg (1964) 

 disagreed with the explanation of the photo- 

 tactic response offered by earlier workers; he 

 concluded, on the basis of a very thorough 

 laboratory and field study, "that the directing 

 stimulus for the phototactic reaction is a con- 

 trast or a gradient present in the angular light 



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U.S. FISH AND WILDLIFE SERVICE 



