I JO A BIOLOGY OF CRUSTACEA 



Gammarus duebeni also meets competition towards the seaward 

 end of an estuary, where it is replaced by G. zaddachi. This latter 

 species occurs in two forms which occupy different parts of the 

 estuary : zaddachi is very hairy and lives in more dilute water than 

 salinus, which is less hairy. There is a third form: G. zaddachi 

 oceanicus, which is even less hairy and is more marine than the 

 other two forms, but rather surprisingly it penetrates into quite 

 dilute water in the Gulf of Finland. 



The succession of species along an estuary has been confirmed 

 several times and appears to be quite constant. We do not yet know 

 how the sequence is maintained, but it is probably due to com- 

 petition between the various species which are all very similar in 

 structure and general habits. The way in which competition acts 

 is not known; it is certainly not due to fighting or active damaging 

 of one species by the other. 



Competition can also be invoked to explain the distribution of 

 some fresh-water species of Gammarus. We have already seen how 

 G. pulex appears to have replaced G. duebeni in fresh water except 

 on the western side of Britain. It also seems likely that G. pulex 

 can replace another species, G. lacustris, which is thought to have 

 survived the last glaciation, whereas G. pulex has invaded since the 

 last glaciation. These two species do however tend to occupy slightly 

 different habitats; G. pulex prefers cleaner, less silty, water than 

 G. lacustris. 



Yet another species, G. fasciatus ( = tigrinus), has been introduced 

 into Britain from North America, and seems to have made its own 

 niche in somewhat salty waters that G. pulex cannot live in; it is 

 also found in really fresh water in Northern Ireland where G. pulex 

 has not penetrated. 



The important feature which emerges from a study of this type 

 is that the detailed distribution of allied species in a given area is 

 determined in an interdependent manner. The range of a species 

 is often limited by the ranges of its allies. 



MIGRATION AND DISPERSAL 



It has already been hinted that the distribution of Crustacea is 

 not a static affair. Many planktonic forms migrate up and down in 

 response to alterations in light intensity, and many get caught up 

 in the great ocean currents. These two examples serve to differentiate 



