Population Studies on Three Species of Cladocera

Abstract

IN a biometrical study of three species of Cladocera taken from eight lagoons of the Province of Buenos Aires, in which several measurements were taken of about 940 specimens, some interesting events were recorded. Bosmina obtusirostris Sars offers a great variation in measurements with respect to the composition of the population (young and adult individuals taken at random). For the same date, in different years, there can be differences more accentuated than the seasonal ones. For the same date, two different lagoons have similar or distinct populations according to the measurements considered. Different parts of the same biotope have very similar populations if only females with eggs are considered. With respect to the coefficient of variation v, which for the normal cases oscillates between 4 and 10, such high numbers as 21 for the caudal height and length of the mucron have been recorded. Measurements of the antennula are very constant. During the cold months of the year an increment in the height and in the length is noted ; but this increment was not recorded in caudal height or the lengths of the mucron and antennula. Therefore, there is no relationship between seasons and proportion of females with eggs. These facts, and some others, point to a singular difference from observations made in European species of the same genus¹.