Rotifers as environmental indicators: perspectives based on sampling strategies

Rotifers have been widely used as bioindicators for assessing water quality worldwide. This study discusses the use of rotifer samples collected from Mexico and other countries through qualitative methods (presence or absence of specific taxa, species richness, and changes in morphology/morphometry), quantitative methods (total or relative abundances, biomass, and extreme densities), or a combination of both (functional and taxonomic traits) for deriving species diversity indices. Identification of rotifers to species level is fundamental in most diversity metrics. Morphological features of formalin-preserved rotifers are generally sufficient for identification to species level. Although different diversity indices have been used in literature, for most articles only a few metrics are considered at a time. In this review, different types of indicators and their metrics of rotifers have been included. The occurrence of different rotifer taxa is discussed in relation to geographical distribution, habitat types (planktonic, benthic, and epiphytic), and physicochemical variables (such as temperature, pH, salinity, and dissolved oxygen). We present examples of different indices such as the total species index of rotifers (TSIRot), saprobic, Simpson, Shannon-Wiener, Margalef, guild ratio, and egg ratio, among others, in the samples preserved in the field. While the rotifer-based indexes presented here have practical utility in water quality evaluations, inclusion of other groups of invertebrates and physicochemical variables is still needed for an integrated assessment.