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Growth pattern and dynamics of a southern peripheral population of Pisidium amnicum (Müller, 1774) (Bivalvia: Sphaeriidae) in Spain

AuthorsAraujo, Rafael ; Ramos, M. Ángeles CSIC ORCID ; Molinet, R.
Issue DateJan-1999
PublisherInstitute of Malacology (Ann Arbor, Michigan)
CitationMalacologia 41(1): 119-137 (1999)
AbstractThe dynamics of a Spanish population of P. amnicum, one of the southernmost populations of the species in the world, is presented. P. amnicum is the only living European species of the subgenus Pisidium s. s. The study is based on monthly samples, from June 1990 to May 1991, from the Miño River, Galicia, northwestern Spain. Specimens from all size classes were dissected and all embryo/larval stages were counted and measured. Studies were conducted to find out the growth pattern of this population using the von Bertalanffy model. Pisidium amnicum in Spain is semelparous and univoltine. Only two cohorts coexisted from late spring to late summer. The life span of the species is about 15 months. The parental generation disappear in August. Juvenile recruitment occurs in April-May when water temperature ranges between 15-20°C. The minimum observed size (shell length) of a gravid specimen was 3-4 mm. Fertilized eggs were brooded for approximately nine months in the inner demibranches until they reached up to 2 mm. The bigger clams had, in the same month, more and bigger embryos (or larvae), than the smaller ones. Not all the initial embryos completed their development. Nevertheless, this phenomenon of intramarsupial suppression does not seem to be very important in the Spanish population. A main feature of the Spanish population of P. amnicum is the high number of larvae incubated during the months immediately before birth.
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