Life cycle and density of a newcomer population of zebra mussels in the ebro river, spain
Rafael Araujo, María Valladolid and Ignacio Gómez
The newcomer population of Dreissena polymorpha, or zebra mussel, in the Ebro River (Spain), is the southwestern most European population known of the species. The life cycle and changes in the density of this population were examined over a 3-year period (2002 to 2004) by installing larval settling plates at different depths in the river’s reservoirs and histologically examining gonads of adult individuals. Maximum density recorded was 30,000 specimens m-2. Attached juveniles appeared in July and August. Colonization was greater at a depth range of 0-3 metres than in the deeper part of the water column. The newly attached individuals grew approximately 5 mm per month, indicating they attain half their full length in the first three months of life.
These specimens had sometimes reached sexual maturity by September of the same year. However, given the lack of attached larvae beyond August, these new mature specimens do not contribute to the reproductive effort within that year. Gametogenesis occurred from February to September in both sexes. Females reach maturity from May to August, and males from April to September, when the water temperature is higher than 13ºC. From October, the gonads of all the specimens were at the spent or subsequent recovery stage. The population is dioecious and specimens are sexually mature at a length of 7-11 mm.
Differentiation of the gonads can occur in specimens with 4 mm shell length, which is the smallest size observed in all the populations of D. polymorpha examined to date. Although the common carp (Cyprinus carpio) preys on the zebra mussel in the Ebro River, its impact on this invading species is insignificant.
One of the main consequences of human actions is the increased introduction of exotic species all over the world impacting biodiversity (Wittenberg and Cock, 2001; Drake and Lodge, 2004). Although the spread of unwanted organisms to freshwater ecosystems has been continuous for decades (Claudi and Leach, 1999; Müller, 2001), this problem has been regarded only as a scientific issue without the social impact it deserves. Invasion by the freshwater species Dreissena polymorpha, known as the zebra mussel, nevertheless, had caused alarm both in political, media and public circles, as well as in the scientific world (Nalepa and Schloesser, 1993; Leung et al., 2002).
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