84 



ANALYSIS OF THE ENVIRONMENT 



are unstable and have varied much even in 

 recent millennia and centuries. 



Periodicities 



Many local environmental variations re- 

 cur with regular rhythms, while others are 

 arrhythmic. The most obvious of the rhyth- 

 mic variations, that of day and night, is 

 beginning to attract the attention from 

 ecologists that it deserves. The day re- 

 presents a period of increased heat and 

 convection currents, as well as of in- 

 creased light; there is also typically a de- 

 crease in relative humidity. Frequently, 

 there are associated phenomena such as the 

 local changes in wind velocity and direction 

 that occur especially near the seashore, in 

 mountains, and near forest margins. Many 

 of these daytime changes markedly increase 

 the evaporating power of the air. Important 

 consequences of diurnal rhythms will be 

 discussed later. 



Tides run on a shorter period. They are 

 periodic variations in the water level pro- 

 duced by the response of water particles 

 to the attraction of the moon and .sun. 

 Tidal streams result that may attain consid- 

 erable velocity in the shallow waters over 

 shoals such as those of the Newfoundland 

 Banks or in the neighborhood of land. The 

 tidal currents usually follow the direction 

 furnished by natural channnels, if any are 

 present; they become more rapid and the 

 tide rises higher near the head of V-shaped 

 arms of the sea. The len2;th of the ebb 

 usually equals that of the flow of the tide, 

 and the currents near land are in the oppo- 

 site direction during the two tidal phases. 

 In the open sea, the height of the tide is 

 much reduced, the rate of movement is 

 slower, and the general direction may be 

 rotary. 



The oscillatory tidal movement of the 

 water has a normal period of 12.5 hours 

 (Harvey, 1928). Longer tidal rhythms also 

 exist. The simplest of these is the occur- 

 rence of a lunar cycle in tidal amplitude 

 in which the high spring tides occur each 

 fortnight when the sun and moon are ex- 

 erting supplementary influences. Between 

 the periods of spring tides, there are the 

 lower neap tides that come when the two 

 governing bodies are working more or less 

 in opposition to each other. The grunion. 

 Laureates tenuis, a small smelt of the Cal- 

 ifornia coast, exhibits an annual breeding 

 cycle that is related to this longer tidal 



rhythm (p. 544; Thompson and Thomp- 

 son, 1919; Clark, 1925). 



Many animals of the marine littoral re- 

 gion have lunar periodicities in their breed- 

 ing activities that are less obviously related 

 to the forces operating during a lunar cycle. 

 Corals, various mollusks and marine poly- 

 chaete worms, among others, show such 

 relationship. Two types of these lunar peri- 

 odicities have been described for annelid 

 worms. In one, successive breeding periods 

 occur during the summer season, and each 

 lunar cycle shows two peaks of abun- 

 dance. Thus, Nereis limbata at Woods 

 Hole, Massachusetts, ordinarily live as elon- 

 gate worms in burrows; they emerge 

 during their breeding period as short, 

 compact, actively swimming forms that are 

 only a fraction of their usual length. Each 

 so-called run begins near the time of the 

 full moon, increases to a maximum on suc- 

 cessive nights, falls to a low point about 

 the third quarter of the moon, increases to 

 another maximum, and finally all swimming 

 worms disappear shortly after the new 

 moon. A new run starts about the time of 

 the next full moon, and this double cycle 

 is normally repeated four times during the 

 summer (Lillie and Just, 1913; Townsend. 

 1939). 



A second type of lunar periodicity occurs 

 when a single annual breeding swarm 

 makes its appearance in accordance with 

 some phase or phases of the lunar cycle. 

 The Atlantic palolo, Leodice fiicata, of Ber- 

 muda and the West Indies inhabits coral 

 reefs and spawns most abundantly during 

 late June and July at about the third quar- 

 ter of the moon, less commonly about the 

 first quarter. There is thus good evidence of 

 an internal or annual rhythm, and yet the 

 time of spawning is partially under di- 

 rect environmental control. It is delayed by 

 water turbulence and by lunar influence. 

 The cavisal factors are still obscure; neither 

 changing nutritive conditions, such as may 

 be associated with the tidal cycle, nor 

 changing hydrostatic pressures are impor- 

 tant. There seems to be a direct effect of 

 moonlight (Clark, 1941, 1941a). 



When the average duration of illumina- 

 tion is increased, spawning is hastened; it 

 is retarded when the duration of exposure 

 to moonlight is decreased. If the length of 

 exposure to moonlight were the only factor 

 involved, spawning would increase to a 

 maximum near the time of the full moon 



