The fauna of an inter-tidal mud flat 1 99 



found Arenicola fairly widely distributed at Skalling, but the densities were <35 per 

 m-, except for occasional heavier populations of young worms. 



Nereis diversicolor appears to be very restricted in North Bay. Most investigators 

 (Stephen, Spooner and Moore, Brady, Wohlenberg, Smidt) have found that this 

 species occurs abundantly oniy at upper tidal levels. This was true also in North Bay, 

 but the only station where the polychaete was really abundant was Transect 2, Station 

 1, where densities of >800 per m- were recorded (Table XIV). At most stations in 

 North Bay Nereis never appeared at any time. Holme (1949) also found it occurred 

 infrequently in the Exe Estuary, and Brady's densities for Northumberland are low. 

 Stephen recorded fairly small populations for the Forth and Clyde, though he did 

 find a high density on the Isle of Barra (Table XIV). On the other hand, Wohlenberg 

 (1937) found Nereis to be abundant at Sylt, and Spooner and Moore (1940) found it 

 was a very common polychaete in the Tamar muds. Rees (1940) also recorded high 

 densities for the Bristol Channel (Table XIV). Thamdrup (1935), in contrast to most 

 workers, did not find Nereis diversicolor restricted to the upper tidal zones. His 

 maximal densities were not very large, and varied from year to year (Table XIV). 

 However, Smidt (1951) has recorded many thousands of young O-Group Nereis 

 from the same area, though adults were few. Nicol (1935) also once recorded the 

 extraordinary density of 95,700 Nereis per m^ (all young) from a salt mud pool at 

 Aberlady. 



Nephthys contrasted with Nereis over North Bay in being fairly restricted to low 

 tidal levels. There appears to be general agreement that Nephthys occurs more fre- 

 quently towards low tide mark. Although Nephthys occurred regularly in North Bay, 

 the numbers were never high (Fig. 4). Indeed, the densities recorded for other areas 

 are mostly rather low. Spooner and Moore, however, found somewhat higher 

 populations in the Tamar (Table XIV). 



It is clear that, while the fauna of North Bay is rich, comparatively few species 

 reach really high densities (Hydrobia, chironomids, Peloscolex). These animals, 

 though widespread, appear to be more typical of the upper tidal zones, and were 

 probably favoured by the long and rather irregular periods of exposure over North 

 Bay. The total average fauna varies between 8,700 and 18,700 animals per m- (Table 

 IV), although individual stations showed populations exceeding 25,000 per m- 

 (maximum 39,000). By contrast, Brady (1943) gives an average density for a North- 

 umberland mud flat of <400 animals per m^ 



Smidt (1951) gives figures (Table 21, p. 138 of his paper) for the total numbers of 

 animals per m^ for Danish shallow water areas. The extraordinary richness of the 

 Danish waddens (30,000-60,000 per m-) is outstanding, and Smidt attributes this 

 mainly to the predominance of small species (Hydrobia, Pygospio, etc.), and to the 

 young of many species. Though the average density in North Bay was not nearly so 

 high, the fauna was rich and, like the waddens, was characterized by the abundance 

 of small animals (Hydrobia, Peloscolex, chironomids, Pygospio), and by young indivi- 

 duals especially of molluscs. The fauna was also similar to that already described by 

 Raymont (1947, 1949) for Loch Craiglin, where the total average density rose with 

 fertilization to 16,000 and even 23,000 animals per m-, and where the same species 

 (Hydrobia, chironomids, young Cardium) were dominant. 



The results of the population changes over North Bay show a pronounced seasonal 

 fluctuation which is not unexpected in an inter-tidal area. Previous studies in Loch 



