1 04 A BIOLOGY OF CRUSTACEA 



amplitude, arc appreciated by the rotifer, and it has evolved 

 responses to them which lead to it attaching to any object moving 

 in a manner similar to Daphnia. The association is not very specific 

 as far as the species of Dapluiia are concerned, but it is very specific 

 with regard to the rotifer. There are other species of the genus 

 Bracliionas, some of which are difficult to distinguish from B. 

 rubens, but most of them do not have the habit of attaching them- 

 selves to Daphnia. Two species which do have the habit are B. 

 variabilis, which has been found on various daphnids in North 

 America, and B. sessilis which attaches itself to another cladoceran, 

 Diaphanosoma brachyura, in Europe. 



Proales daphnicola is another rotifer associated with Daphnia. 

 It is smaller than Brachionus and does not carry its own eggs, but 

 sticks them on the surface of its host so that the emerging young 

 do not have far to seek for a host. 



Sometimes one finds that there is competition for space on the 

 surface of a popular host. The epibionts, which are the organisms 

 living on the surface, of Daphnia, show this rather well. Colacium 

 vesiculosum is a relative of Euglena; it contains chloroplasts and so 

 is capable of photosynthesis and independent existence, but it 

 spends much of its life attached to Daphnia, and other small 

 arthropods, by means of a gelatinous stalk. Peritrichs, such as 

 Vorticella and Epistylis, also attach themselves to Daphnia by 

 means of stalks, but they cannot photosynthesise; thev feed on 

 bacteria and other small organisms which they collect by means 

 of ciliary currents. As a general rule it is found that when peritrichs 

 are very abundant Colacium is scarce, and vice versa. Experiments 

 show that the success or otherwise of Colacium can be influenced 

 bv light. If cultures of Daphnia which have been inoculated with 

 swimming stages of peritrichs and Colacium are kept in the dark 

 then Colacium fails to maintain itself in the face of competition 

 from the peritrichs. which increase much more than they do in the 

 light when they have to compete with a vigorous growth of 

 Colacium. 



An odd feature was found in a study of the distribution of 

 Colacium, Vorticella octava and Epistylis helenae in small pools on 

 the rocky islands off the south-west coast of Finland. When 

 Vorticella was abundant the other two were absent, or sparse. This 

 has been attributed to the fact that Vorticella has a contractile stalk, 

 which must bring it frequently into collision with other epibionts 

 and so render co-existence uncomfortable. 



