544 



THE COMMUNITY 



recently in 1939. During this latter out- 

 break the population density was thirteen 

 nymphal cases per square meter (51,397 

 per acre) for sugar maple forests, thirty- 

 two per square meter (127,885 per acre) 

 for black oak forests, and fifty per square 

 meter (202,876 per acre) for red oak- 

 white oak-maple forest communities. The 

 adult cicadas weighed, on an average, 0.15 

 gm. when dehydrated in alcohol and then 

 dried, so that the equivalent of 31,243 gm. 

 of dried cicada per acre were available to 

 predators per acre of red oak forest (Stran- 

 dine, 1940). 



LUNAR ASPECTS OF THE COMMUNITY 



Moonlight is a poorly understood in- 

 fluence in the nocturnal period. Periodicity 

 induced or controlled by the moon, in its 

 orbit about the earth, is of relatively httle 

 consequence to the terrestrial communities, 

 so far as our present knowledge is con- 

 cerned. 



Such eflFects apparently are of only 

 slightly more consequence in the fresh- 

 water communities, but have received little 

 attention (Shelford, 1918, pp. 42-43). The 

 best-known lunar influence in these is the 

 correlation between the amovmt of river 

 plankton and the phases of the moon 

 (Thomson, 1911). 



The marine community, however, does 

 show a variety of rhythmic responses to 

 tides, which latter are chiefly direct lunar 

 effects (p. 84). 



Associated with and, in certain cases, in- 

 duced by these tidal rhythms are the so- 

 called lunar rhythms of marine animals. 

 Such activity rhythms are usually restricted 

 to the marine littoral strata. In the littoral 

 zone, it must be remembered that not only 

 is the height of tides affected, but also such 

 marine influences as salinity, water tem- 

 perature, currents, sediment, and foods are 

 indirectly influenced by lunar rhythms. 



It is not surprising, therefore, that the 

 marine littoral zone has a number of lunar 

 periodicities in the activities of its inhabit- 

 ants. The best-known of these are the 

 lunar periodicities in the reproductive 

 behavior of various polychaete annelids 

 (Grave, 1922; Lillie and Just, 1913; Mayer, 

 1908; Scott, 1909; Treadwell, 1915; see 

 also p. 84) . 



A second general type of lunar periodic- 

 ity is illustrated by a fish, the grunion 



{Leuresthes tenuis). Along the California 

 littoral, these fishes appear exactly at high 

 tide, on the second, third, and fourth 

 nights following the spring tides (Thomp- 

 son and Thompson, 1919). At these times 

 the female grunion deposit their egg pods 

 in the sand, just above the water line, and 

 the male grunion fertilize the eggs at this 

 time. The eggs are ready to hatch in two 

 weeks, but will not do so until the egg 

 pods are washed from the sand by the 

 tides at the next dark moon. This adjust- 

 ment against various adversities is summed 

 up by Pearse (1939, p. 176) : 



"If spawning occurred just before the highest 

 tides, when the high beach was being eroded, 

 instead of just after, when the beach was being 

 built up, the eggs would be washed out of the 

 sand before they had developed for a fortnight. 

 If spawning occurred at the very highest tides 

 (dark of the moon), the eggs might not be 

 exposed for a month or even two months. If 

 grunions laid their eggs during the day, they 

 would be exposed to the attacks of gulls and 

 other predaceous animals." 



In summary, then, in addition to the 

 generally important influence of the rhyth- 

 mic ebb and flow of tides (Keeble, 1910), 

 lunar rhythms, especially as they affect the 

 reproductive cycles, are of importance to 

 the httoral portions of the marine commu- 

 nity (Korringa, 1947). 



DIEL ASPECTS OF THE COMMUNITY 



Within the elastic frame of seasonal 

 rhythmicity the most important periodicity 

 is that associated with day and night. To 

 avoid confusion, in the discussion that 

 follows, diel will be used for the twenty- 

 four hour period of a day and a night, fol- 

 lowing Carpenter (1934), and day and 

 night will refer to the illuminated and 

 darkened portions of the period, respec- 

 tively. 



Diel periodicities of the physical in- 

 fluences affect the community in two major 

 ways. First, there is the variation that is 

 a consequence of latitude, and its vertical 

 equivalent, altitude, in which there is a reg- 

 ular change in the number of hours of 

 daylight from equator to poles, or from sea 

 level to the highest peaks, at a given time 

 of the year. Thus, equatorial regions have 

 day and night relatively constant through- 

 out the year, twelve hours of light and 

 twelve hours of darkness; polar regions 



