MICROSCOPIC FAUNA OF SANDY BEACHES 



103 



leeted during this study. "Witliiu 100 em 

 of the edge of the water tlie psamniopliile 

 species were more numerous, but beyond 

 that distance the psammobiotic forms were 

 dominant. The total numbers ranged from 

 26 per sample at the edge of tlie water to 22 

 at the 100 cm station, 33 at tlie 150 cm sta- 

 tion, and then decreasing to about 15, 6, 

 and 2 at the subsequent stations, with less 

 than one per sample at the 350 cm station. 



Numerous sand samples were taken from 

 the submerged sand near the edge of the 

 water but only 31 per cent as many rotifers 

 were found there as were found directly at 

 the edge of the water. 



Table IV shows the mean vertical dis- 

 tribution at six beaches. The beach studied 



only recently pointed out by Wilson tliat 

 the sandy beaches of lakes and oceans, and 

 brackisli areas, support a great number of 

 copepods, particuhirly harpacticoids. These 

 sand-loving copepods are very small and 

 worm-like; they swim feebly and depend 

 cliiefly on continual wriggling movements 

 to get about in the interstitial water. In 

 contrast to the situation in marine beaches 

 where many species are found, only tiiree 

 species are known from lake beaches; two 

 of tliese, Virguirella paludosus Mrazek and 

 Parastenocaris starretii Pennak were found 

 on the shores of a single lake, Starrett Lake. 

 Parastenocaris hrcvipes Kessler occurred in 

 all beaches except the one at Lake Michigan. 

 Table V shows the distribution of the 



TABLE IV 



Mean Vertical Distribution of Eotatoria in Six Different Beaches to a Depth of 8 cm. 

 Eesults are Expressed as Mean Numbers of Specimens Per 10 cc Sand Sample. 



at Lake Michigan had comparatively large 

 numbers in the deeper samples with only 

 9 per cent of all specimens in the top 

 centimeter of sand. N. Trout was inter- 

 mediate with 65 per cent in the top centi- 

 meter and Day was at the other extreme 

 witli 92 per cent in the top centimeter and 

 no rotifers below a depth of 4 cm. To .some 

 extent these differences are governed by 

 exposure to wave action, amount of organic 

 matter in the sand, and the inhabitable 

 width of the beach. 



COPEPODA 



Although copepods are known to inhabit 

 many unusual environments, it has been 



Copepoda in three different beaches, in- 

 cluding N. Trout beach witli a large popula- 

 tion, S. Trout with an intermediate popula- 

 tion, and Day with a small population. In 

 comparison with otlier groups the copepods 

 Avithin a single beach show the least numer- 

 ical variation over a period of time. The 

 differences from beach to beach, however, 

 were considerable, with some beaches hav- 

 ing large numbers (over 200 specimens in 

 some samples) and others having very few. 

 Only tlie beach studied at Lake Michigan 

 contained no copepods. 



In spite of the fact that we have little in- 

 formation concerning the environmental 

 factors which govern the distribution of 



