1 52 A BIOLOGY OF CRUSTACEA 



persal of Crustacea they are by no means the only ones. Air cur- 

 rents and strong winds play a considerable part in the dispersal of 

 small, fresh-water Crustacea which produce resting eggs capable of 

 being dried. The wide dispersal of the Branchiopoda is to be 

 accounted for in this way. 



Biological agencies also play a part. The small eggs of Cladocera 

 and ostracods can be transported very easily between the feathers 

 of water birds. The presence of the South African ostracod, 

 Potamocypris Jmmilis, in a few rock pools on the south coast of 

 Finland can be explained by its transport on the bodies of Arctic 

 and Common Terns, which overwinter in South Africa and migrate 

 to the Northern Hemisphere to breed. The evidence in this case is 

 circumstantial, but there are other direct observations of the eggs 

 of small Crustacea on birds. 



Crustacea are also transported by insects. Ostracods have been 

 found under the wings of the water boatman, Notonecta; small 

 Gammarus sometimes attach to the large water beetle, Dvtiscus, and 

 the brackish water barnacle, Balanus improvisus, has been discovered 

 attached to another large water beetle, Hydrophilus picens. There 

 is also a record of an Australian woodlouse being transported by a 

 flying beetle. 



The parthenogenetic species are the most likely to become 

 successfully established after being transported. A single female 

 hatching from a cladoceran ephippium can be the beginning of a 

 large population if the environment proves to be suitable. It is the 

 suitability of the environment which determines the distribution 

 of the easily transportable species; they are not limited by their 

 means of dispersal, but by their inability to survive in certain 

 conditions. 



Man has, both intentionally and unintentionally, aided in the 

 distribution of Crustacea. His intentional activities have usually 

 had commercial objectives. The introduction of the North 

 American crayfish Cambarus nffinis into Germany in 1890 has been 

 followed by its introduction into Poland and France. In all these 

 countries it has thrived and spread to form the basis of a profitable 

 fishery. 



Unintentional introductions into Europe include the Chinese 

 River Crab, Eriocheir sinensis, and the barnacle Elminius modestus, 

 which hails from the Antipodes. The former was probably brought 

 over in ballast tanks, the latter attached itself to ships. A recent 

 development in barnacle distribution is their attachment to the 



