138 



LEECHES 



prevent Helohdella thriving in bodies of water which otherwise 

 appear favourable. In soft waters the same process leads to acid, 

 dystrophic conditions which may exclude all species of leech. 



The distribution of certain leeches may also be correlated with 

 the dissolved oxygen content of the water. This factor fluctuates 

 diurnally and annually, but it is obvious that on average the 

 oxygen concentration is higher in unpolluted running water and 

 on the wave-washed shores of lakes than it is in sheltered ponds 

 having accumulations of decaying organic matter. It has been 

 shown (p. 59) that Erpobdella testacea is better able to maintain 

 aerobic metabolism in poorly oxygenated water than is E. octoculata 

 and it is probable that this factor determines which of the two 



Erpobdella octoculata Erpobdella testacea Glossiphonia complanota Glossiphonia heteroclito 



7o 50 



LiL kl Li 



ABCDE ABCDE ABCDE ABODE 



Helobdella stagnolis Theromyzon tessulatum Hemiclepsis marginata Piscicola geometra 



100 



% 50 



yi UL LiL 



"ABODE ABODE ABODE ABODE 



Fig. 87. The frequency of occurrence of various leeches in the 

 diff"erent types of habitat. A, soft standing water; B, inter- 

 mediate; C, hard standing water; D, slowly running water; 

 E, fast running water; ordinates, percentage of stations within 

 each group which contains the leech in question. From Mann, 



1955. 



species is the more abundant in a particular type of habitat. 

 E. testacea builds up dense populations in shallow, swampy situa- 

 tions having a rich accumulation of decaying plant material (Mann, 

 1959, 1960). In all probability dissolved oxygen plays a part in 

 determining which of the two fish parasitic leeches is present in a 

 given habitat. Piscicola geometra has a relatively high rate of 

 oxygen uptake which is depressed by low oxygen concentration in 

 the water (p. 57) and it occurs mainly in fast flowing streams and 



