The influence of body size upon whole tissue metal content has been examined for eight elements in a variety of marine and estuarine molluscs. Generally, plotting element content against body weight on a double logarithmic basis produces straight line relationships. The calculated regression slopes for 131 lines fall into two main categories: those around 0-77 and those close to 100. In the former cases (e.g. zinc in Mytilus edulis) element concentrations are greatest in the smallest individuals; in the second (e.g. cadmium in M. edulis) concentrations are independent of size. In a few cases (e.g. cadmium in the limpet Patella vulgata) highest concentrations are recorded in the largest individuals.In geographically separate populations of a species from 'clean' environments, the regression slopes relating element content to body size are consistent for each element. Comparison between populations from 'clean' and 'contaminated' environments indicates that the regression slopes may be either constant, e.g. zinc (and probably cadmium) in Ostrea edulis, cadmium and zinc in M. edulis, or variable, e.g. cadmium and zinc in P. vulgata, with increase of slopes in metal elevated environments.Curves on double logarithmic plots were obtained relating copper content to body size in the oysters Crassostrea gigas and O. edulis and the mussel M. edulis introduced to a ' contaminated' environment. Large individuals of these species require longer than 5 months to equilibrate to exceptionally high environmental concentrations of copper. (The same is true for zinc in C. gigas.) In the single case of cadmium in the scallop Pecten maximus an upward directed curve was obtained relating cadmium content to body size, resulting in exceptionally high cadmium concentrations in large scallops. Steeper slopes, and almost twice as much variability, was recorded when using wet weight data compared with dry weight.

The reproductive cycles of the two cockles Cerastoderma edule (L.) and C.glaucum (Poiret) (= Cardium lamarcki Reeve) from a mixed population in the Crouch estuary, Essex, have been investigated. Reproductive maturity was estimated by assessment of gonad condition. A difference in maturity related to shore level was noted. The reproductive cycles of the two cockles were found to be essentially similar. Rapid gametogenesis occurred in both cockles in spring, C. edule then spawned in May, whereas C. glaucum did not spawn until July. The difference in time of spawning appears to be responsible for the maintenance of the two genotypes in a sympatric association, as hybridization experiments showed an absence of isolation at the gamete level. The relationship between C. edule and C. glaucum is discussed.

Aspects of the physiology of the two cockles Cerastoderma edule (L.) and C. glaucum (Poiret) in air have been investigated. Both cockles exhibit bradycardia during exposure and are similarly tolerant of anoxic conditions, but C. edule survives longer in air than C. glaucum. This is accounted for by the fact that C. edule displays a behaviour pattern of valve movements upon emersion which allows air-breathing. Oxygen uptake rates of this cockle measured in air lie close to the lowest rates recorded in water, and are considered to approximate to a basal or quiescent level. C. glaucum does not air-breathe and is restricted to lowest shore levels. Values for 'maximal' levels of oxygen uptake in water have also been obtained together with estimates of metabolic scope for the cockle.

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