The fauna of an inier-tidal mud flat |97 



Bay by July 1946, i.e. their growth was of the same order as that given by Stephen 

 and Brady, and (in a favourable year) by Smidt. This improvement may reflect the 

 better feeding conditions in North Bay in 1945/46. 



The density of Myti/us was usually very low over the whole of North Bay, the only 

 heavy settlements being at spatfall (August), after which the animals did not come 

 into the regular samplings. Chipperfield (1953) shows that April May is the most 

 likely breeding time for Myti/us on British coasts; the period of settlement seems to be 

 May to early July. Some authors, however, have recorded settlements in August, 

 and this clearly was the main period for North Bay (p. 192). 



Following a heavy spatfall the numbers of Myti/us declined very rapidly (cf. other 

 bivalves), so that only once (July 1946) was an appreciable number of the larger 

 mussels found. (200-400 per m-). Smidt found that the Danish waddens were unsuit- 

 able for Myti/us; the numbers found were negligible, although on the banks very high 

 denshies of mussels occurred. Similarly Thamdrup recorded densities of 2,000-1 2,000 

 mussels per m" on the banks, while over the mud flats only an occasional mussel was 

 taken. 



The growth rate of young Myti/us seems to vary greatly according to conditions 

 (cf. White, 1937; Smidt, 1951). For example, Smidt quotes other workers as stating 

 that young spat may grow to 8 mm in less than two months if permanently covered 

 with water, whereas similar spat, left uncovered for a few hours daily, hardly attained 

 4 mm in three months. White (1937) states that under unfavourable conditions a 

 mussel may add only a few mm in a year, but under good conditions, at St. Andrews, 

 two-year-old mussels ranged from about 30-50 mm. This would indicate a growth 

 at least of some 15-25 mm in a year. The long exposure periods over North Bay 

 would suggest a rather unfavourable environment; nevertheless the 1945 spat grew 

 to 9 mm by November and to about 20 mm in the following July, when these mussels 

 would be one year old. This approximates to the figure mentioned by White, and 

 emphasizes that feeding conditions in 1945/46 over North Bay must have been very 

 good. 



Of the polychaetes, Pygospio was obviously dominant in North Bay. Wohlenberg 

 found Pygospio at upper tidal levels, though in relatively low densities, but Thamdrup 

 recorded very high densities, mainly near high water (Table XIV). Smidt, however, 

 found that very large concentrations of newly sQiW^d Pygospio (> 3 1,000 per m-) might 

 occur right across the shore. In North Bay Pygospio was definitely more abundant 

 at lower tidal levels, but it was so common that there was an average population for 

 the whole mud flat of 160-J80 per m^ (Fig. 4). There are few records of large popula- 

 tions of this polychaete in Britain. However, Holme (1949) found a density for the 

 Exe Estuary similar to that of North Bay, though the populations are far below those 

 of Thamdrup for Skalling (Table XIV). Brady (1943) records up to 600 Sco/op/os 

 per m- and fairly high numbers of Spio and Sco/eco/epis in Northumberland muds: 

 perhaps these polychaetes replace Pygospio. 



Heteromastus fi/iformis was the next commonest polychaete in North Bay; apart 

 from the sharp decline in July 1946, the average population was 30-290 per m- (Fig. 4). 

 Heteromastus was quite sharply restricted to mid-tide levels (higher in Transect 4), 

 and the maximal densities were considerably greater than either those recorded by 

 Spooner and Moore for the Tamar, or by Thamdrup for Skalling, Denmark (Table 

 XIV). However, Smidt (1951) found that in 1947 Heteromastus became abundant 



