Variation in the distribution of Corbicula species (Mollusca: Bivalvia: Corbiculidae) after 25 years of its introduction in the Río de la Plata, Argentina

ABSTRACT In 1981, Corbicula fluminea (Müller, 1774) and C. largillierti (Philippi, 1844) were recorded at the Río de la Plata River. During the 1980’s, C. largillierti was found on a continuous fringe in the shore of that river, while C. fluminea was only found North of Buenos Aires’ harbor. Ten years later, C. fluminea spread to Punta Indio, while C. largillierti remained restricted to tributary streams. The density and size frequency distribution of both species were compared with previous data from samples performed in the same area in 1985-1989. The aim of this research was to determine whether the densities of both species decreased; whether C. fluminea is still more abundant than C. largillierti; and whether the size frequency distributions indicate different population structures from those observed in the end of the 1980’s. Nine localities from the river shore were sampled, and each individual was identified and measured. The size of C. fluminea varied between 2 and 39 mm, showing the highest frequency of individuals between 20 and 25 mm; furthermore the density decreased 90% in 2015/2016. No specimens of C. largillierti were found. In conclusion, the populations of Corbicula inhabiting the Río de la Plata River are retracting.

摘要 1981年，河蚬（Corbicula fluminea，Müller, 1774）与拉氏河蚬（Corbicula largillierti，Philippi, 1844）在拉普拉塔河（Río de la Plata River）被记录。20世纪80年代，拉氏河蚬在该河沿岸形成连续分布带，而河蚬仅在布宜诺斯艾利斯港以北区域被发现。十年后，河蚬扩散至因蒂奥角（Punta Indio），拉氏河蚬则仍局限于支流溪流中。研究团队将两个物种的种群密度与体长频率分布，与1985-1989年同一区域采集的样本历史数据进行对比。本研究旨在明确三个核心问题：一是两个物种的种群密度是否出现下降；二是河蚬的种群丰度是否仍高于拉氏河蚬；三是体长频率分布是否反映出与20世纪80年代末观测到的种群结构存在差异。本次研究共对沿岸9个采样点开展采样，对每一个个体进行物种鉴定与体长测量。河蚬的体长范围为2至39毫米，个体出现频率最高的区间为20至25毫米；2015/2016年其种群密度下降了90%，且未采集到拉氏河蚬的标本。综上，栖息于拉普拉塔河的河蚬种群正呈现收缩态势。