Production of Large Broods in Certain Marine Lamellibranchs with a Possible Relation to Weather Conditions

Abstract

IN many species of animals it is only in certain years that there is a very large production of young, or at least of young which survive to form later, as a dominant year-group, a large proportion of the population. The problem of the underlying causes of these productive years is one of considerable economic importance. For example, it is now well known in fishes such as the haddock and the herring that successful broods are produced only at irregular interval5, and that such broods form the basis of the fishery for some years afterwards. So, too, with the small intertidal Lamellibranch Mollusc, Tellina tenuis, it is only in certain years that a successful spatfall occurs. Sampling of the population of this species in Kames Bay, Millport, begun in 1926, has been continued regularly. In that period there have only been four successful spatfalls, in the years 1926, 1930, 1933 and 1936 (Fig. 1). It was thought possible that alterations in the physical conditions of the environment might be the chief causes of these variations in the quantity of brood. Since the annual shift of the modes has been about the same, even in the years with very large broods, it may be assumed that the food supply has always been sufficient. The species being an intertidal one, the chief variable physical factors affecting the environment are temperature, sunshine and rainfall. This paper deals principally with the fluctuations of these factors from