DISTRIBUTION 1 49 



found that although there is a general resemblance in habitat 

 even very closely related species show small but distinct differences 

 in their preferences. A good example of closely related species with 

 a distinct and measurable difference in habitat is found in the 

 isopod genus Tylos. The species now known as T. sardous and T. 

 europaeus were at one time regarded as a single species, but there 

 are some small distinctive anatomical features which enable the 

 two to be separated. Both species live on sandy shores of the 

 Mediterranean, but T. sardous lives on coarser beaches than T. 

 europaeus. Samples of sand from beaches on which these isopods 

 live have been analysed by passing the sand through sieves of 

 various sizes, and then determining the percentage by weight of 

 each size of grain. All the beaches with T. europaeus had 90 per 

 cent of their particles with a diameter less than half a millimetre, 

 while the beaches with T. sardous had 65-90 per cent of their 

 particles with a diameter greater than half a millimetre. Further, 

 three out of four beaches with T. sardous had 20-30 per cent of their 

 particles with diameters greater than five millimetres, which gives 

 the beaches a gravel-like character. The difference is so striking 

 that the expert can predict which species he will find merely by 

 passing the sand through his fingers. 



Less striking, but nevertheless quite recognisable differences can 

 be found in other groups. One of the most outstanding studies on 

 the systematics and distribution of related species has been the 

 unravelling of the species of Gammarus living in British estuaries. 

 Perhaps the most important step in this process was the recognition 

 that the species known as Gammarus marinus is in fact an assemb- 

 lage of half a dozen species having sufficient differences from other 

 Gammarus to be included in a separate genus, Marino gammarus. 

 We shall only consider the remaining species here. 



As one passes inland from the mouth of a typical English estuary 

 the species of Gammarus are encountered in the following order: 

 G. locusta, G. zaddachi salinus, G. zaddachi zaddachi, G. duebeni, 

 and G. pulex. The last species is confined to fresh water, while 

 G. duebeni is found in fresh water on the western side of Britain, 

 mostly in places where G. pulex does not occur. From a study of 

 the distribution of these two species it has been deduced that G. 

 pulex is a later arrival than G. duebeni; but when it becomes estab- 

 lished it is more successful than the latter species and replaces it, 

 so that G. duebeni is restricted to brackish water in estuaries which 

 have G. pulex at the fresh-water end. 



