A population of Mytilus edulis is described whlch is numerically dominated by young ( < 2 yr old) individuals. Growth within this population is indeterminate, i.e. no maximum shell length is attained throughout the realized life span. Predation pressure, principally by Carcinus maenas and Asterias vulgaris, results in a population mortality rate of 42 % in a 6 mo period and also probably prevents growth from being determinate. The partitioning of organic matter between shell and body growth is discussed in connection with the cycling of energy within the ecosystem of the rocky intertidal zone.

Beds of molluscan shellfish are frequently located in or near estuaries which receive sewage discharged from neighbouring towns. Each shellfish passes through its system many times its own volume of water every hour (see Fox, Sverdrup & Cunningham, 1937; Galtsoff, 1946), and the organic matter and a proportion of the bacteria present in the polluting sewage pass into the digestive tract. Thus dissection of a mussel so as to reveal the rectum shows a striking contrast between the dark brown or black colour in a mussel taken from a polluted bed and the clean appearance in one which has been immersed for some time in pure sea water.Shellfish sometimes contain pathogenic bacteria of the typhoid-paratyphoid group and may thus give rise to these diseases in people consuming them, particularly as large proportions of both mussels and oysters are eaten raw. For this reason emphasis has been placed in recent years on the importance of subjecting shellfish before marketing to a period of immersion in pure sea water so that the contents of the digestive tract, including bacteria of human faecal origin, may be evacuated during normal metabolic processes.At the cleansing stations operated or supervised by the Ministry of Agriculture and Fisheries at which mussels are purified before being released for sale to the public, the process of purification adopted is that recommended by Dodgson (1928) and approved by the Ministry of Health. In this process a batch of mussels supported on wooden grids in an open tank is subjected to two successive periods of immersion, each lasting for about [16][17][18][19][20] hr., in sterilized fresh sea water. The sea water is sterilized in a separate tank by addition of a quantity of bleaching powder equivalent to three parts of chlorine per million. After standing overnight the residual chlorine is discharged by addition of a suitable quantity of a solution of sodium thiosulphate, and the dechlorinated water is then transferred to the tank containing the mussels. After each period of purification the water is allowed to flow back to the sea and a fresh batch of sea water is used for the next cycle of cleansing. As a result of several years' experience of this method of treatment Dodgson concluded that it was reasonable to expect cleansed mussels to contain not more than five lactose-fermenting bacteria per c.c. of flesh. 29-2