The literature on Mytilus edulis is particularly extensive. The most comprehensive account yet published is that of Field (1922), who gives excellent descriptions of the anatomy, physiology and embryology, as well as the bionomics, of this species. White (1939) adds little additional information upon the breeding and growth of Mytilus. Pelseneer (1935) gives an extensive bibliography covering both breeding and growth, whilst of the more recent work upon the breeding behaviour, the contributions of Battle (1932), Whedon (1936) and Young (1942) are most important. The occurrence oflarvae in the plankton has been described in detail by Thorson (1946) and others;Visscher (1927), Orton (1933), Kandler (1926) and many others have given much information upon the spat-fall and settlement in several localities.In spite of this wealth of published data there still remains considerable doubt over the time of onset and the duration of the breeding period of M. edulis, especially in relation to environmental factors. In particular, few recent investigations on Mytilus from beds in the sheltered and estuarine areas of the British coasts have been published, whilst the conclusions to be drawn from the older work (Herdman & Scott, 1895;Johnstone, 1898; Scott, 1900, etc.) are conflicting and lacking in precision. Finally, no attempt has previously been made to investigate the breeding, settlement and growth of this species simultaneously in a number of distinct geographical localities. This was the aim of the investigation, described below, which was commenced in 1946 as part of a general investigation of the breeding, settlement and growth of a number of common sessile littoral animals, and which is still in progress.

The chain-forming prosobranchiate gastropod Crepidula fornicata is not indigenous to British waters, but has been introduced during the last 70 or 80 years, probably from North America, upon imported oysters, and, during this period, has spread to most of the oyster beds on the south-east and south coasts of this country (see Orton, 1950b, for a recent discussion of its present distribution). In spite of the interest shown in this species because of its detrimental effect upon oyster culture, and because of its well-known change of sex from male to female, little is known in the literature of its breeding habits here, or in North America. Conklin (1897) states that the spawning period in New England lasts from early summer to about mid-August, no larvae being found in late August. Murie (1911), in discussing the introduction of this species, elaborates upon Conklin's observations, and gives evidence that females, in samples of Crepidula taken from the River Colne in May and September 1898, contained spawn beneath their shells. Orton (1912a, p. 438) states that he is ‘informed by Professor Conklin that American Crepidulae begin to spawn in May and possibly in April, whilst English Crepidulae begin to spawn in early March’. In a footnote, he adds ‘Crepidula spawned in tanks at Plymouth in early February’.

No abstract