ASPECTS OF REPRODUCTION ASSOCIATED WITH THE USE OF A SEGMENTED REGRESSION TO DESCRIBE THE RELATIONSHIP BETWEEN BODY-WEIGHT AND SHELL LENGTHOF MYTILUS-EDULIS

Data on total dry tissue weight and shell length were obtained from an intertidal population of Mytilus edulis located in an estuary in SW E ngland. Representation of the log(10) transformed data is significantl y improved by the use of a segmented regression instead of a single li near regression. There is a significant reduction in the slope (b) of the regression, relating total dry tissue weight (y) to shell length ( x), for mussels over 50.3 mm. This pertains also to the component tiss ues: mantle (at 50.1 mm shell length), digestive gland (at 60.4 mm) an d remainder (at 52.7 mm). Weight of mantle tissue and of digestive gla nd does not change (b congruent to 0), whilst remaining tissue increas es, but at a reduced rate, as shell length increases in excess of thes e respective sizes. Stereological examination of sections of mantle ti ssue were made throughout the year. Results indicate that high levels of reproductive tissue (i.e. > 80% of mantle volume) are achieved in h alf as many large individuals (mean shell length 64.7 mm) as in 2 smal ler size groups (means of 35.1 and 50.4 mm). These results suggest tha t reproductive effort increases to a maximum at a shell length. of app roximately 50 mm, thereafter declining as size increases. A Segmented regression is also appropriate to describe the relationship of total t issue and mantle tissue weight to shell length of a sample from anothe r estuarine population, but not from a wave exposed, open shore popula tion. Reasons for these observations are discussed in the context of t he feeding:metabolism ratio, availability of food and age.