The fauna of an inter-tidal mud flat 181 



In many cases, particularly among soft-bodied animals, the methods of sampling and sieving broke 

 up the material so that identification has not been taken further than the genus. Some animals 

 (chironomid larvae) have not been identified further than family, although the dominant species has 

 been noted. The list of fauna has been constructed mainly from the identification of favourable 

 specimens preserved in good condition. 



GENERAL COMPOSITION OF THE FAUNA 



Although no account was taken of the small bottom animals (protozoans, nema- 

 todes, very small crustaceans, etc.) which were not usually retained by the sieve, more 

 than 40 species were recorded from the area. Of these (Table III) comparatively 



Table IV 

 Average population for whole North Bay area based on 14 regular stations. 



{All numbers per m'-) 



few appeared to be regular members of the fauna. Four bivalves {Scrobicularia, 

 Macoma, Cardium, Mytilus), two gastropods (Littorina and Hydrobia) and several 

 polychaetes (Arenicola, Nereis, Nephthys, Heteromastus, Pygospio, Phyllodoce and 

 Eteone), with two other groups of animals (oligochaetes and chironomid larvae) 

 were the most important numerically. Crustacea were very poorly represented 

 throughout: the average population rarely reached even 1 % (Table IV). Echinoderms 

 were practically absent altogether; only an occasional small Ophiura texturata was 



taken. 



The gastropod Hydrobia ulvae was overwhelmingly dominant in number. The 

 average population calculated for the fourteen stations in North Bay showed that at 

 times^Hydrobia formed 80% of the total population, and that it never fell below 70% 

 during the whole period of the investigation. Hydrobia, with oligochaetes (chiefly 

 Pehscolex benedeni), and chironomid larvae (mostly Chironomus salinarius)* together 

 dominated the fauna over the whole area, comprising approximately 90% of the whole 



population (Table V). . • t i lo^c 



There was a slightly lowered percentage for these three organisms in July 1945, 

 which was due to an increase in one species on\y-Pygospio. This polychaete formed 

 48% of the polychaete population in July 1945 (Fig. 4) and it occurred in 13 ol the 



sampling stations. . 



In November 1945 young bivalves made an appreciable contribution to the total 

 fauna (Table IV), but only two species were abundant— Cfl/c/////» and to a lesser extent 



* I am indebted to Professor A. Thienemann for this identification. 



