The fauna of an inter-tidal mud flat 



185 



from the uppermost station, was sparsely distributed towards low tide, and was quite 

 sharply limited to the middle regions of the mud flat (Fig. 2). Arenicola appeared 

 to decrease towards low tide levels, especially in Transect 4, while the very abundant 

 Pygospio, while occurring across the whole shore, was more abundant at mid- and 

 low-tide levels (Fig. 2). 



Both chironomid larvae and oligochaetes (Pe/osco/ex) were commonest over the 

 mid- and upper parts of the shore, and decreased sharply towards low-tide mark (Fig. 

 2). Perhaps they avoided the higher salinities towards low water, although in Loch 

 Craiglin chironomids occurred over the bottom where the salinity was about 30^/^^ 

 over most of the year (Raymont, 1947). The distribution of juvenile Littorina was 

 rather irregular, although on the whole they were less abundant towards low water. 



The commonest gastropod {Hydrobia ulvae) showed no very clear zonation, being 

 widely distributed with populations of several thousand individuals per square metre 

 over most of the shore (Table VII). It was clear from the analysis of individual samples 

 that Hydrobia was extremely patchy, and the averaging of samples probably gave a 

 misleading impression of its real distribution. It seems, however, that Hydrobia 

 occurred in only relatively small numbers (below 1,000 per m-) at the lowest level 

 along Transect 2, and also at the lowest station in Transect 4, where the density did 

 not exceed 450 per m^ except for one dense patch of some 10,000 individuals recorded 

 in November 1943. Similarly, the lowest level along Transect 1 showed a lower 



Table VHI 

 The changes in total population at the 14 stations. Two stations {Tr. 2, St. 4 and Tr. 4, 

 St. 3) were not properly sampled in Nov. 1945. The autumn totals show these stations 

 included (figures in brackets), and also excluded (figures unbracketed). (Populations to 



nearest hundred per m^) 



