a1 School of Ocean Sciences, Marine Science Laboratories, Menai Bridge, Gwynedd, LL59 5EH
The periodicity of microgrowth patterns observed in thin sections and acetate peel replicas of radial sections of bivalve shells are well documented for both intertidal and subtidal species (Pannella & MacClintock, 1968; Rhoads & Pannella, 1970; Evans, 1972, 1975; Kennish, 1980; Richardson et al., 1979 et seq.; Richardson, 1987a, b, 1988, b). In many species daily or tidal lines are well preserved in the shell, for example, the hard-shell clam Mercenaria mercenaria (Pannella & MacClintock, 1968), the cockle Cerastoderma edule (Richardson et al., 1979) and the clam Tapes philippinarum (Richardson, 1987a, 1988b), while in others they are poorly preserved or lacking. Such an example is the common mussel Mytilus edulis (L.) where fine growth lines seen in acetate peel replicas have previously been reported to be poorly preserved, with long continuous sequences of such lines seldom occurring (Lutz, 1976). The angular relationship of growth lines to the growing margin and to the crystalline elements of the shell structure are presumed to be the factors influencing the resolution of growth increments (MacClintock, 1967; Pannella & MacClintock, 1968). Pannella & MacClintock (1968) suggest that the larger the angle between depositional surfaces and the structural elements of a shell layer the more evident will be the growth increments. Both oysters and mussels have an unfavourable angular relationship between growth increments and the margin of the shell (Pannella & MacClintock, 1968) and this may explain why the growth lines are difficult to observe. The present paper describes improved methods used to examine the internal micro478 growth patterns in the shell of the common mussel Mytilus edulis and reports on experiments to investigate the periodicity of the patterns.