Factors affecting egg-ratios in planktonic rotifer populations under culture conditions

S.S.S. Sarma, R.D. Gulati, S.Nandini-2003

Abstract:

The Edmondson’s egg ratio (number of amictic eggs to the females) is an important variable, widely used to understand and predict the patterns of population growth in planktonic rotifers under field conditions. It is also useful as an indicator of the rotifer health under culture conditions. Generally, an inverse relationship exists between the egg ratio and the density of females in a population. However, a number of biotic and abiotic factors influence the egg ratio. Temperature for example, via its influence on the frequency of egg production and the hatching times of parthenogentic eggs, causes strong changes in the egg ratio which may be unrelated to the population growth rates. Similarly, if invertebrate predators, such as Asplanchna, selectively feed on only the eggs but not on the females, the egg ratios could be much lower than expected. We have observed this, particularly onder field conditions. The easy detachment of egg from some members of Brachionidae, especially, if there is an enhanced reproduction under high food levels, may also cause an underestimation of the egg ratio. In this review, we discuss the role of diel changes in the egg deposits, influence of frequency of food addition, problems of distinguishing between different egg types and negative effect of stresses such as toxicants and diet quality on the egg ratio of selected rotifer species.

(Laboratory of Aquatic Zoology, Unit U.M.F., Division of Research and Postgraduate Studies, National Autonomous University of Mexico, Campus Iztacala, AP 314, CP 54090, Los Reyes, Tlalnepantla, State of Mexico, e-mail: sarma@servidor.unam.mx)

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