THE BIOLOGY AND ECOLOGY OF A BRACKISH LAGOON BIVALVE, CERASTODERMA GLAUCUM BRUGUIERE, IN LAGO LUNGO, ITALY

Population and individual parameters of an Italian populationof the brackish lagoon bivalve, Cerastoderma glaucum Bruguière,living in a mixohaline lagoon with typical summer dystrophy,were assessed over a one year period. Mortality and recruitmentwere both high, such that productivity and turnover were alsohigh. Individual production was considerable; one cohort achieved27 mm mean shell height and 375 mg mean dry flesh weight in9 months. Accompanying the summer dystrophy, however, most ofthe population had a very low "condition index". The reproductivecycle followed a polycyclic pattern with up to three spawningsin a single year. Circumstantial evidence suggests that thecycle is environmentally controlled, and that spawning is nota necessary consequence of gametogenic activity. The chemicalcomposition largely reflected the reproductive state and the"condition index", with large fluctuations in water, ash andglycogen contents. Calorific content also fluctuated, as a functionof ash content and lipid content; a formula is presented topredict such variation from the latter parameters. Finally,a simple production model is presented, indicating improvementsto exploit further the high productivity of these lagoons. ^* Present address: Institut für Physiologische Chemie,Abt. Zellbiochemie, Universität Hamburg, Martinistrasse52, 2000 Hamburg 20, FRG (Received 6 November 1978;     

THE BIOLOGY AND ECOLOGY OF A BRACKISH LAGOON BIVALVE, CERASTODERMA GLAUCUM BRUGUIERE, IN AN ENGLISH LAGOON, THE WIDEWATER, SUSSEX

Data are presented on population and individual parameters fora population of Cerastoderma glaucum Bruguière livingin a small, almost closed mixohaline lagoon in southern England.The bivalve occupies two discrete niches, a benthic one anda semi-pelagic one, where juveniles are suspended in the floatingalga Chaetomorpha linum Kutz. During the study year (1972) therewas no growth in the benthic population. This was shown to bean unusual occurrence by comparing growth data obtained by severaldifferent techniques, including the microscopic examinationof shell growth lines. Reproductive condition followed the monocyclicpattern already established for this species in northern localities,with spawning taking place in May. Ash content, water contentand calorific content all varied in correspondence with thelevel of starvation, which proceeded with insignificant mortalityuntil body weight was reduced to about a quarter of that atthe beginning of the year. At this point calorific values aslow as 3.5 kcal/g were measured. Possible causes for the anomalousnegative production are considered. The most likely factor responsiblewould appear to be the high production of the alga Chaetomorphalinum which would effectively compete out the phytoplanktonfood of the Cerastoderma population. ^* Present address: Institut für Physiologische Chemie,Abt. Zellbiochemie, Universität Hamburg. Martinistrasse52, 2000 Hamburg 20, F.R.G. (Received 6 November 1978;     

TECHNICAL NOTE: THE PREPARATION OF MOLLUSCAN TISSUE FOR PRODUCTION ESTIMATES

Oven-drying at 60°C under vacuum, and presumably also dryingat higher temperatures, leads to relatively high losses of volatile,highly-calorific components, with a resultant underestimateof dry weight, lipid content and calorific content. Freeze-dryingis to be generally recommended. Fresh material should be usedwherever possible for the estimation of tissue parameters. Freezingor formalin-fixation may be used for estimations of wet weights,but only freezing if dry weights or calorific contents are alsorequired. (Received 26 November 1981;     

A QUANTITATIVE STUDY OF A CERASTODERMA -- NEPHTHYS COMMUNITY IN THE LIMFJORD, DENMARK, WITH SPECIAL REFERENCE TO PRODUCTION OF CERASTODERMA EDULE

Cerastoderma edule is the dominant member of a non-random Cerastoderma—Nephthyscommunity, occupying a physically, homogeneous, submerged sandbankin the Limfjord, Denmark. C. edule is patchily distributed withinthis community, growth rates showing a clear density-dependence.Individual production is about 35%lower in high density patchesthan outside them; at low density annual individual flesh dryweight increment is 295 mg. Seasonal variation in water, ash,lipid, protein and glycogen contents is studied, and a sizedependent(= density-dependent) relationship is shown to exist for someparameters. Population production, estimated by a new graphicalmethod, and taking into account gametic and shell protein production,attains 150 g dry flesh weight, m^–2.yr^–1 at meanpopulation density rising to 250 g dry flesh weight. m^–2.yr^–1in the high density patches. ¹Present address: Institut für Physiologische Chemie, Abt.Zellbiochemie, Universitat Hamburg, Martinistrasse 52, 2000Hamburg 20 (Received 10 March 1980;