The population structure and apparently erratic distribution patterns exhibited by M. edulis on many open exposed coasts, are discussed in the light of findings relating to the breeding and settlement of this species. In the absence of previous literature concerning open coast mussels, the reproductive cycle has been described in some detail. These mussels do contribute to the spawning stock, and in the three years from October 1964–December 1967, the period of spawning was shown to be particularly extended. Spawning occurred mainly from early spring to late summer, though individuals could be found in the spawning condition more or less throughout the year. No marked or consistent differences in the spawning periods of mussels from different local habitats or amongst animals of different size (=age) were recorded. Sexual maturity was attained in the first year of life.
The appearance of larvae in plankton samples confirmed the times of major spawnings recorded from histological preparations of the gonad. Settlement of young mussels (=plantigrades) on existing beds was not direct, there being a period of temporary attachment spent especially upon filamentous substrates. This occurred after about 3–5 weeks of planktonic existence. The majority of plantigrades remained on these primary sites for a period of 4 weeks (i.e. until they measured from 1–2 mm in length), although those settling later in the year often remained there overwinter before migrating on to adult beds. The stimulus to migrate is due to changes in the thigmotactic requirements of small mussels. Whilst filamentous substrates were ideally suitable to early plantigrades, somewhat older individuals preferred the niches and crevices provided on adult beds (especially amongst the byssus threads) and by the small cracks and pits in the rock surface. Since the thigmotactic requirements of all plantigrades were not satisfied at exactly the same time, some migration to adult beds occurred throughout the year, though periods of maximum settlement occurred from 8–10 weeks after spawning. Migration from these extensive “reservoirs” of temporary attachment could account for the sporadic outbursts of settlement recorded on many shores at certain times of the year and which often could not necessarily be predicted on a knowledge of the breeding cycle alone.
Although the extended settlement period is an important contributory factor, it is not thought that this alone could account for the distinctive population structure in this species. At settlement, mussels are particularly gregarious, attraction of plantigrades to adult beds being essentially a thigmotactic response, aided by their ability to attach and detach themselves until favourable situations are encountered. Settlement is favoured by roughened, scarred or pitted surfaces and the distribution patterns on many shores could partly be attributed both to the surface texture or topography of the shore (smooth, rapidly draining shores being particularly unsuitable) or the amount of local wave splash.
In a subsequent publication, the population structure and distribution patterns will be examined in the light of growth and mortality rates of this species on exposed shores.