distribution." Schallek (1943) reported that 

 Acartia tonsa in a glass cylinder would move 

 upwards when illuminated from above and 

 downward when illuminated obliquely. He con- 

 sidered the reaction of A. tonsa to diffuse light 

 in the cylinder to be in accord with the down- 

 ward movement in the ocean during the day, 

 but that the reaction to direct light under 

 experimental conditions had no bearing on its 

 behavior in nature. 



These experiments emphasize the importance 

 of other variables that one must consider in 

 attempting to compare and evaluate observa- 

 tions during an eclipse. The time of day and 

 resultant attitude of the sun in relation to 

 water clarity are of particular concern. Holmes 

 (1957) discussed the penetration of water by 

 light and explained that "The extinction of 

 daylight in the sea is caused by absorption (by 

 the water itself, by particles, and by dissolved 

 substances) and by scattering (by the water 

 and by particles)." Ringelberg (1964) and 

 Schwassman and Hasler (1964) have recog- 

 nized the importance of absorption and scat- 

 tering on the phototactic behavior of aquatic 

 organisms ; the former paper referred to 

 Daphnia, especially the orientation of the eye 

 axis and the body axis, and the latter referred 

 to sun orientation of fishes. 



The responses of herring observed during 

 the 1963 eclipse were in general agreement 

 with reports of other observations under vary- 

 ing conditions of light intensity. Johnson 

 (1939) studied captive herring in southern 

 New Brunswick and concluded that, in the 

 absence of direct sunlight, these fish "extended 

 to the surface at all times — dawn, sunrise, 

 cloudy days, sunset, dusk, moonlight, starlight, 

 and cloudy nights." He also found that during 

 daylight, the depth of the fish was greatest 

 when the sun's altitude was highest and the 

 largest fish were in the deepest water. Blaxter 

 and Holliday (1963) summarized the work of 

 European scientists, particularly in the North 

 Sea where the diurnal migration pattern of 

 herring is well documented; and the depth of 

 herring shoals has been correlated with isolux 

 lines to estimate the optimum depth for setting 

 gill nets. 



In studying diurnal changes in behavior of 



adult herring, Blaxter and Parrish (1965) 

 found that the depth (and light intensity) at 

 which the fish occurred during the day was 

 extremely variable ; and demonstrated that fish 

 did not move towards the surface until illumi- 

 nance decreased to 10 luxes. These authors also 

 reported that "recruit fish (21/2-3 years old)" 

 remained in higher light intensities by day. 

 The eclipse study lends support to this latter 

 conclusion. Though the response of 2-year-old 

 herring at totality and at sunset was limited, 

 the light intensity was above the level that 

 Blaxter and Parrish observed as necessary to 

 elicit a surface movement by adults. Breder 

 (1951 and 1959) discussed the influence of 

 light on the social grouping of many species of 

 fish and provided a thorough summary of other 

 scientists' work in this field. He stressed the 

 differences in responses by individuals, by sex, 

 and by species. This emphasizes the need to 

 select species whose behavior patterns are well 

 known, when attempting to evaluate the eflfects 

 of a solar eclipse. The Atlantic herring, in this 

 regard, is a suitable species, except that the 

 sexes cannot be distinguished readily through 

 external examination. 



Mention should be made of the types of 

 periodic activity and their importance to the 

 observations made during the eclipse. Allee, 

 Emerson, Park, Park, and Schmidt (1949) 

 classified successive diel periods into two types : 

 exogenous, "in which the pattern is directly 

 induced and controlled by periodic environmen- 

 tal influences" and endogenous, "in which the 

 pattern is resident in the organism." Aschoff 

 (1960) elaborated on the definitions, explain- 

 ing that an environmentally controlled perio- 

 dicity (exogenous) will cease under artificially 

 constant conditions; whereas, periodic factors 

 of the environment only serve as synchronizing 

 agents (Zeitgeber) for circadian or endogenous 

 periodicity. He pointed out that a single en- 

 vironmental event can never synchronize con- 

 tinuously and therefore cannot operate as a 

 Zeitgeber. This implies that observations made 

 during an eclipse should not, of themselves, be 

 used to determine whether a response or lack 

 thereof is indicative of either an exogenous or 

 endogenous rhythm. On the other hand, these 

 observations can provide supporting evidence 



MARINE ORGANISMS DURING SOLAR ECLIPSE 



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